Syntheses and a Solid State Structure of a Dinuclear Molybdenum(V) Complex with Pyridine

A mononuclear complex [MoOCl4(H2O)]− readily forms a metal−metal bonded {Mo2O4}2+ core. A high content of pyridine in the reaction mixture prevents further aggregation of dinuclear cores into larger clusters and a neutral, dinuclear complex with the [Mo2O4Cl2(Py)4] composition is isolated as a product. Solid state structures of two compounds containing this complex, [Mo2O4Cl2(Py)4]·2.25Py (1) and [Mo2O4Cl2(Py)4]·1.5PyHCl (2), were investigated by X-ray crystallography.

Introduction
The {Mo 2 O 4 } 2+ core (shown in Scheme 1) with d electrons localized in a Mo(V)−Mo(V) single bond appears as a recurrent structural motif in many contexts of molybdenum(V) coordination chemistry [1][2][3][4][5][6][7]. A group of high-nuclearity oxomolybdenum(V) complexes whose structures may be seen as the assemblies of two or more {Mo 2 O 4 } 2+ units has emerged in the past two decades [8]. The aggregation of dinuclear units is made possible by the ability of oxide and alkoxide ions to participate in μ 2 -, μ 3 -or even μ 4 -bridging interactions. The oxomolybdenum(V) clusters are of interest because of their intermediacy between the oligomeric alkoxides and the polymeric oxides on the other side. The expectation is that their chemistry may provide some insight into the catalytic processes taking place on the surface of the solid metal oxides [9].

[MoOCl 4 (H 2 O)] 3 Cl 2 with a mononuclear [MoOCl 4 (H 2 O)]
− ion has been shown to serve as a suitable starting material in the preparation of {Mo 2 O 4 } 2+ -containing clusters [10]. Changes in the contents of reaction mixtures or reaction conditions resulted in clusters with different nuclearities or different connectivities among the dinuclear building blocks.  8 ] compositions [11][12][13]. The subtle nature of the aggregation is illustrated by the assembly of four {Mo 2 O 4 } 2+ units which was seen to produce three different architectures: (i) a cyclic {Mo 8 8 } core, and two cores with more condensed structures, (ii) {Mo 8 [12] and (iii) {Mo 8 [14]. The presence of methanol in the reaction mixture was found to be crucial for the aggregation process. The possible role that methanol or alcohols in general play in the formation of dinuclear units and their further aggregation will be discussed presently. In the absence of alcohol, (PyH) 5

Results and Discussion
The metal−metal bonded, dinuclear {Mo 2 O 4 } 2+ core is a result of the facile substitution chemistry on the mononuclear [MoOCl 4 (H 2 O)] − ion. In spite of the many known compounds possessing the dinuclear structural core, the details of its formation remain unknown. Undoubtedly, one of the key prerequisites is the presence of water. Results of our previous work show that methanol actively participates in substitution and/or dimerization reactions through the coordination of the methoxide ions formed in situ. The coordinated methoxide can in turn react with water to form an oxido ligand. A direct evidence of the latter process is the observed transformation of [Mo 2 O 3 (OMe)(OOCCH 3 )Cl 4 ] 2− , a complex with a bridging methoxide, to the {Mo 2 O 4 } 2+ -containing products [15]. The formation of {Mo 2 O 4 } 2+ species was found to be faster and more reproducible when methanol was present in the reaction mixture. Furthermore, it has also been observed that the reaction mixtures containing large amounts of methanol or other alcohols tend to produce higher-nuclearity clusters with either oxido or alkoxido ligands shared between the constituent dinuclear units. In such clusters, the relative content of the chlorido ligands was found to be rather small. Conversely, the reaction systems lacking methanol do not favour further association of dinuclear cores and as such provide a convenient environment for the isolation of the {Mo 2 O 4 } 2+ prototypes. The reaction system consisting solely of molybdenum(V) starting material and pyridine exemplifies such a medium. When pyridine occupies  4 ] crystallizes only with the incorporation of either a huge amount of pyridine solvent molecules (compound 1) or pyridinium chloride (compound 2). None of the two compounds is stable when removed from the mother liquor. Compound 1 dissolves almost instantaneously in its own solvent molecules.
As shown by the X-ray structure analysis, 1 and 2 are both [Mo 2 O 4 Cl 2 (Py) 4 ]-containing compounds ( Figure 1) which differ in the content of the solvent molecules of crystallization or counter ions. For both, the asymmetric unit contains two crystallographically independent complex molecules with very similar structural parameters (Table 1).   The overall features of the basic {Mo 2 O 4 } 2+ core are as determined previously in related compounds [8]: (i) a short distance between molybdenum atoms, 2.5502(8) and 2.5517(8) Å for 1 and 2.5427(6) and 2.5441(6) Å for 2, typical for a single metal−metal bond; (ii) a puckered Mo(μ 2 -O) 2 Mo ring with the metal atoms above and the bridging oxygen atoms below the mean plane. The rather short metal−metal bond lengths are due to the high content of pyridine ligands whose nature is electron-donating. The dihedral angle between the two Mo(μ 2 -O) 2 planes is by ca. 10° smaller than the angles observed in a series of oxalato complexes of the [Mo 2 O 4 (η 2 -C 2 O 4 ) 2 (R-Py) 2 ] 2− (R-Py = alkylsubstituted pyridine) type [16]. A distorted octahedral coordination of each metal centre of the {Mo 2 O 4 } 2+ core is completed by a chloride at 2.452(2)−2.477(2) Å for 1 and at 2.453(1)−2.466(1) Å for 2 and a pair of pyridine ligands with two distinctly different bond lengths. A short one occupies the 2.256(5)−2.276(5) Å range, whereas a long one the 2.381(4)−2.407(5) Å range. The reason lies in one of the two pyridine ligands being bound to a position trans relative to the Mo=O group and being, therefore, subject to its well-documented trans influence [17]. The lengthening of the bonds trans to the terminal oxide is a general phenomenon found throughout the {MoO} 3+ compounds. The molybdenum-to-pyridine bond lengths in 1 and 2 are shorter than the ones found in [Mo 2 O 2 (μ 2 -S) 2 {S 2 P(OEt) 2 } 2 ]·2Py [18]. The latter compound exemplifies an extreme case of trans influence of the molybdenyl group: because of the extreme bond length, 2.545(7) and 2.569(7) Å, the compound is considered as an adduct with pyridine. The extent to which the trans influence is expressed within a certain complex differs from one case to another.

X-ray crystallography
Each crystal was mounted on the tip of a glass fibre with a small amount of silicon grease and transferred to a goniometer head. Data were collected on a Bruker SMART CCD (compound 1) or Nonius Kappa CCD (compound 2) diffractometer. SHELXL-97 was employed for the structure solution and refinement [22]. The high content of pyridine solvent molecules in 1 resulted in large R1 and wR2 residuals. One of the pyridine solvent molecules was located with its centre of gravity on the inversion centre. Although the disorder was resolved using a PART −1 instruction, the displacement parameters of its atoms remained large. The structure of 2 was solved and successfully refined in a non-centrosymmetric P 2 1 space group. When refined in a centrosymmetric P 2 1 /c space group, a small fragment of the structure, a pyridinium cation with a hydrogen-bonded chloride counter anion, was located on the inversion centre. Its disorder could not be resolved. Figures depicting the structures were prepared by Ortep-3 [23] and CrystalMaker [24]. Cell parameters and refinement results are summarized in Table 2. Further details on the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif by quoting the deposition numbers CCDC-755860 (1) and 755861 (2).


extfieldsofstudy:
Chemistry
Medicine
Computer Science

text:
3-Chloroazepan-2-one

In the title compound, C6H10ClNO, an intermediate for the production of lysine, there are intramolecular C—H⋯Cl hydrogen bonds.

In the title compound, C 6 H 10 ClNO, an intermediate for the production of lysine, there are intramolecular C-HÁ Á ÁCl hydrogen bonds.
The molecular structure of (I) is shown in Fig. 1. A packing diagram of (I) is shown in Fig. 2, where the dash line indicates C-H···Cl hydrogen bond.
Experimental 18.2 g (100 mmol) 3,3-dichloro-2-oxohexamethyleneimine, 2 g. 5% palladium-on-charcoal and 18 g. (220 mmol) sodium acetate were added into 100 ml glacial acetic acid. The mixture was placed in in a shaker under hydrogen (2 atm. initial pressure) until one equivalent of hydrogen was absorbed. The catalyst and sodium chloride were removed by filtration. The filtrate was neutralized and extracted with chloroform and concentrated, and then recrystallized by n-hexane to give 18.1g white solid (87.4%). (Wineman et al., 1958) Pure compound (I) was obstained by crystallizing from acetic acid. Crystals of (I) suitable for X-ray diffraction were obstained by slow evaporation of an ethanol solution.

Refinement
All H atoms bonded to the C atoms were placed geometrically at the distances of 0.93-0.97 Å, and included in the refinement in riding motion approximation with U iso (H) = 1.2 or 1.5U eq of the carrier atom. Fig. 1. The molecular structure of (I), showing the atom-numbering scheme and displacement ellipsoids at the 30% probability level.

Figures
supplementary materials sup-2  Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.


extfieldsofstudy:
Materials Science
Physics

text:
Phonon Raman scattering of perovskite LaNiO3 thin films

We report an investigation of perovskite-type LaNiO3 thin films by Raman scattering in both various scattering configurations and as a function of temperature. The room-temperature Raman spectra and the associated phonon mode assignment provide reference data for phonon calculations and for the use of Raman scattering for structural investigations of LaNiO3, namely the effect of strain in thin films or heterostructures. The temperature-dependent Raman spectra from 80 to 900 K are characterized by the softening of the rotational A1g mode, which suggest a decreasing rhombohedral distortion towards the ideal cubic structure with increasing temperature.

Among ABO 3 perovskites, rare earth nickelates with the generic formula RENiO 3 (RE = Rare Earth) have attracted a lot of research interest since the report of a sharp metal-to-insulator (MI) transition whereof the critical temperature T MI can be tuned with the rare earth size. Further to this, most RE-nickelates exhibit in the insulating phase a complex anti-ferromagnetic ordering below the Néel temperature T N (see e.g. ref. 1,2 ). A more recent interest in nickelates stems from their potential multiferroic properties [2][3][4][5] .
Within the family of nickelate perovskites, LaNiO 3 (LNO) is the only nickelate which shows in its stoichiometric bulk form no temperature-induced MI transition but remains metallic down to the lowest temperature. This property makes LaNiO 3 one of the few possible electrodes in oxide-based electronics, namely in the field of strain-engineered perovskite thin films which attract increasing interest. Moreover, LNO recently attracts attention due to properties such as correlated Fermi liquid behavior 6 , as oxygen sensor devices 7 , predictions of superconductivity 8 or electric-field control of conductivity in LNO thin films 9 . Most of the recent work focuses on LNO thin films, while single-crystal growth of LaNiO 3 has remained unsuccessful until now. Finally, it has been shown that the intrinsic properties of LNO can be significantly modified through its incorporation in superlattices such as LaNiO 3 /AMnO 3 (ref. 10,11 ) or LaNiO 3 /LaAlO 3 (ref. 12 similarly to what is observed in other perovskite-type superlattices. In all these thin film or superlattice systems the structural characterization, namely the strain state between substrate and film and in superlattices, is essential for a better understanding of the observed physical properties. In addition to the more commonly used X-ray diffraction and electron microscopy, Raman scattering has shown to be a versatile and non-destructive probe for investigating structural properties in thin oxide films. Applications of Raman spectroscopy, which probes zone centre phonons, range from more general aspects like subtle structural distortions in perovskites to the understanding of thin films characteristics such as strain effects, texture, X-ray amorphous phases, superlattice-related features, etc. [13][14][15][16][17][18][19][20][21] To the best of our knowledge there are neither literature reports on the experimental Raman signature of LaNiO 3 nor reports of phonon calculations. Even more generally, Raman investigations of perovskite nickelates are to date limited to NdNiO 3 (ref. 3,17 ) and SmNiO 3 (ref. 3 ).
Here we present an experimental Raman scattering study of LaNiO 3 thin films with the aim (i) to increase our current understanding of its phonons spectra, namely in relation with structural distortions (ii) to provide reference data for both future Raman investigations of bulk and thin film samples and future phonon calculations and (iii) to study its temperature-dependent behavior.
LaNiO 3 thin films have been deposited on Silicium (100) substrates by injection Metal-Organic Chemical Vapour Deposition (MOCVD) "band flash" using 2,2,6,6tetramethylheptanedionato-chelates of corresponding metals as volatile precursors. Deposition conditions were 680°C using an atmosphere of argon-oxygen at 10 mbar pressure, followed by an in-situ annealing at ambient pressure under a pure oxygen flow. X-ray diffraction (XRD) pattern of the so-obtained films of 200 nm thickness illustrate well-crystallized LaNiO 3 films without traces of binary oxides such as La 2 O 3 or NiO, within the diffraction detection limit. As expected, the deposited films on Si are polycrystalline.
Micro-Raman spectra were recorded using a LabRam Jobin-Yvon spectrometer with a spectral cut-off at  120 cm -1 . Our tests with three different wavelengths as exciting lines (488.0, 514.5 and 632.8 nm) illustrate that the overall spectral signature of the film is similar for the three wavelengths, as expected from Raman scattering without resonance effects. Similarly to what is commonly observed for metals, the spectrum obtained with the 632.8 nm laser line provides the best defined Raman spectra and thus this wavelength has been retained for our study. It is wellknown that Raman spectra recorded on metallic oxides with a reduced laser penetration depth (i.e. Re-manganite 15,22 , Re-nickelates 17 or La 2 CuO 4 23 ) often show a strong dependence on the exciting laser power leading to structural modifications, phase transitions or even locally decomposed material. For LaNiO 3 we have observed that experiments with a laser power above several mW lead indeed to a modified spectral signature, characterized by a small but observable low-wavenumber shift of the LaNiO 3 modes illustrating a bond lengthening from local laser heating. As a consequence, our experiments have been carried out using laser powers of less than 1 mW under the microscope, and we have carefully verified that no structural transformations or overheating take place. Controlled temperature-dependent Raman measurements have been carried out by using a commercial LINKAM THS600 heating stage placed under the Raman microscope. After an initial cooling to liquid nitrogen temperature, the temperature-dependent spectra have been obtained by heating the sample from 80 to 900 K. The Raman spectra before and after temperature measurements are identical, attesting the reversibility of temperatureinduced changes.
At ambient conditions LaNiO 3 crystallizes in a rhombohedrally distorted perovskite structure with space group R3c. With respect to the ideal cubic Pm3m structure this rhombohedral structure is obtained by an anti-phase tilt of the adjacent NiO 6 octahedra about the [111] p pseudocubic diagonal, described by the aaatilt system in Glazer's notation 24 . The 10 atoms in the unit cell of rhombohedral LaNiO 3 give rise to 27 (k = 0) optical modes that can be characterized according to the space group using group theoretical methods. Such a treatment leads to only five Raman-active modes  Raman = A 1g + 4 E g . A vibrational pattern of all A 1g and E g phonons in the R3c structure has been proposed by Abrashev et al. 25 . According to the Raman tensors of the R3c space group, a configuration of crossed polarizer's (xy-configuration) will lead to the observation of E g modes only, while parallel polarizer's (xx-or yy-configuration) allow the observation of both A 1g and E g modes.  LaMnO 3 25 , and at 172 cm -1 in LaCoO 3 27 ). This mode will be sensitive to substitutions on the A-site via mass changes. The high-frequency modes at 399 and 451 cm -1 can be assigned to vibrational modes of the oxygen cage.
We now turn to the effect of temperature on the Raman-active phonons of LaNiO 3 and presents in Figure 2 Raman spectra from 80 K up to 900 K with sharp bands at low-temperature.
With increasing temperature all bands show the expected thermal broadening, but the overall spectral signature is qualitatively maintained and no drastic changes occur, which suggests that LaNiO 3 maintains its R3c symmetry and undergoes no phase transition in the investigated temperature range. For a more quantitative analysis, Figure 3 illustrates the evolution of the band position as a function of temperature, as deduced from a spectral deconvolution. Note that the A 1g mode could not be reliably determined above 550 K due to band broadening and overlap with the E g mode. Due to their different symmetry such a crossing is allowed.

Figure 2
Temperature-dependent Raman of a LaNiO 3 film from 83 K to 873 K, with a temperature increment of 50 K in-between the shown spectra.
Generally speaking, all bands shift to lower frequencies with increasing temperature due to thermal expansion which conditions interatomic bond lengthening. However, the most eyecatching spectral signature with increasing temperature is the softening of the A 1g mode from 222 cm -1 at 80 K to 192 cm -1 at 553 K. This pronounced shift provides further support for the assignment of this band to the rotational A 1g soft mode of LaNiO 3 , which is expected to soften stronger than other modes. As a consequence of this, the shift of the A 1g soft mode in LaNiO 6 can be interpreted as a continuous decrease of the NiO 6 octahedra tilt angle with increasing temperature, similarly to what is observed in LaAlO 3 when approaching the temperature-induced rhombohedral-to-cubic phase transition 26,28 .
From a phenomenological point of view, the behavior of a system in the vicinity of a symmetry-breaking structural phase transition can be described in the framework of Landau theory which was in the past successfully applied to the analysis of various tilted perovskite-type materials 35 . For the simplest second-order phase transition, the temperature dependence of the soft-mode frequency  is in the low-symmetry phase (T < T c ) given by ² =  0 ²(T c -T). In the inset of Figure 3 we Furthermore, we have presented a temperature-dependent Raman scattering investigation of LaNiO 3 from 80 K up to 900K. Within this temperature LaNiO 3 undergoes no structural phase transition since no drastic changes like band splitting or band appearance/disappearance are observed. However, the pronounced softening of the rotational A 1g mode points at a decreasing rhombohedral distortion towards the ideal cubic perovskite structure, which we expect from our extrapolation to occur at  1800K.


extfieldsofstudy: Biology Medicine

s2fieldsofstudy: Chemistry Biology

text:
Total Synthesis and Antimicrobial Activity of a Natural Cycloheptapeptide of Marine Origin

The present study deals with the first total synthesis of the proline-rich cyclopolypeptide stylisin 2 via a solution phase technique by coupling of the Boc-l-Pro-l-Ile-l-Pro-OH tripeptide unit with the l-Phe-l-Pro-l-Pro-l-Tyr-OMe tetrapeptide unit, followed by cyclization of the resulting linear heptapeptide fragment. The chemical structure of the finally synthesized peptide was elucidated by FTIR, 1H/13C-NMR and FAB MS spectral data, as well as elemental analyses. The newly synthesized peptide was subjected to antimicrobial screening against eight pathogenic microbes and found to exhibit potent antimicrobial activity against Pseudomonas aeruginosa, Klebsiella pneumoniae and Candida albicans, in addition to moderate antidermatophyte activity against pathogenic Trichophyton mentagrophytes and Microsporum audouinii when compared to standard drugs—gatifloxacin and griseofulvin.

Introduction
During past years, natural products derived from marine sponges have played a crucial role in the pharmaceutical research as biomedically useful agents or as lead compounds for drug development. Among them, bioactive cyclopeptides with unique structures have emerged as vital organic congeners which may prove better candidates to overcome the problem of widespread increase of microbial resistance towards conventional drugs [1]. Diverse biological activities possessed by marine OPEN ACCESS sponge-derived natural cyclic peptides include antimicrobial, cytotoxic, anti-HIV, nematocidal, anti-inflammatory, serine protease and protein tyrosine phosphatase inhibitory activity [2][3][4][5][6][7][8]. Stylisin 2, a natural cyclic heptapeptide has been isolated from the Jamaican sponge Stylissa caribica and its structure was elucidated on the basis of 1D and 2D NMR data ( 1 H, 13 C, COSY, HMQC, HMBC, ROESY), followed by determination of absolute configuration of amino acids by Marfey's analysis [9].

Results and Discussion
In present study, a disconnection strategy was employed to carry out the first total synthesis of stylisin 2. The cyclic heptapeptide molecule was split into three dipeptide units Boc-L-Pro-L-Ile-OMe (1), Boc-L-Phe-L-Pro-OMe (2), Boc-L-Pro-L-Tyr-OMe (3) and single amino acid unit L-Pro-OMe·HCl (4). The required dipeptides 1-3 were prepared by coupling of Boc-amino acids, viz. Boc-L-Pro and Boc-L-Phe with the corresponding amino acid methyl ester hydrochlorides viz. L-Ile-OMe·HCl, L-Pro-OMe·HCl and L-Tyr-OMe·HCl employing dicyclohexylcarbodiimide (DCC) as coupling agent [28]. Racemization was suitably controlled during normal peptide couplings by the use of less strenuous conditions, i.e., low temperature, controlled quantity of the base such as triethylamine (TEA) and a suitable solvent like dimethylformamide (DMF) or tetrahydrofuran (THF). The ester group of dipeptide 1 was removed by alkaline hydrolysis with LiOH and the deprotected peptide was coupled with amino acid methyl ester hydrochloride 4 using diisopropylcarbodiimide (DIPC)/DCC and triethylamine (TEA), to get the tripeptide unit Boc-L-Pro-L-Ile-L-Pro-OMe (5). Similarly, dipeptide 2 after deprotection at the carboxyl end, was coupled with dipeptide 3 deprotected at amino terminal, to get the tetrapeptide unit Boc-L-Phe-L-Pro-L-Pro-L-Tyr-OMe (6). After removal of the ester group of tripeptide 5 and the Boc group of tetrapeptide 6, these deprotected units were coupled to get the linear heptapeptide unit Boc-L-Pro-L-Ile-L-Pro-L-Phe-L-Pro-L-Pro-L-Tyr-OMe (7). The methyl ester group of the linear peptide fragment was replaced by a pentafluorophenyl (pfp)/p-nitrophenyl (pnp) ester group. The Boc-group of the resulting compound was removed using trifluoroacetic acid (TFA) and the deprotected linear fragment was now cyclized by keeping the whole contents at 0 °C for 7 days in the presence of catalytic amounts of TEA/NMM/pyridine to get cyclooligopeptide 8 (Scheme 1). The cyclization of the linear peptide was indicated by the disappearance of absorption bands at 1,750, 1,271 cm −1 and 1,389, 1,373 cm −1 (CO stretching of ester and CH deformation of tert-butyl group) in the IR spectrum of compound 8. Formation of cyclooligopeptide was further confirmed by the disappearance of the singlet at δ 1.49 corresponding to the nine protons of the Boc tert-butyl group and the singlet at d 3.54 corresponding to the three protons of the methyl ester in the 1 H-NMR spectrum of compound 8. Seven signals between δ 4.39-3.85 in the proton spectrum of compound 8 agree with the expected peptidic structure for the compound, with these signals being attributable to the alpha protons of seven amino acids. The NMR data of the synthetic stylisin 2 are identical with those of the natural product within the error range of d 0.07 ( 1 H) and 1 ( 13 C), respectively. Furthermore, the 1 H-NMR and 13 C-NMR spectra of the synthesized cycloheptapeptide showed characteristic peaks confirming the presence of all the 57 protons and 44 carbon atoms. The presence of a [M + H] + ion peak at m/z 812.9 corresponding to the molecular formula C 44 H 57 N 7 O 8 in the ESI-MS/MS spectrum of compound 8, along with other fragment ion peaks resulting from cleavage at the 'Ile-Pro' and 'Phe-Pro' amide bond levels, showed the exact sequence of attachment of all the seven amino acid moieties in a chain. In addition, elemental analysis of compound 8 afforded values (±0.03) in accordance with the molecular composition. Synthesis of stylisin 2 (8) was thus carried out successfully with good yield and pyridine was proven to be an effective base for cyclization of the linear heptapeptide fragment. Comparison of antimicrobial screening data (Table 1) suggested that cyclooligopeptide 8 exhibited higher antibacterial activity against the Gram negative bacteria Klebsiella pneumoniae and Pseudomonas aeruginosa, and potent antifungal activity against pathogenic Candida albicans with MIC values of 12.5−6 μg/mL, in comparison to a standard drug-gatifloxacin. Moreover, compound 8 displayed a moderate level of biological activity against the dermatophytes Microsporum audouinii and Trichophyton mentagrophytes with MIC values of 6 μg/mL. However, compound 8 displayed no significant activity neither against Gram positive bacteria nor against the fungus A. niger.  (6) 28(6) 27(6) 15(6) - (6)  Analysis of the antimicrobial activity data revealed that the linear heptapeptide 7 displayed less bioactivity against pathogenic bacteria and fungi compared to its cyclic form 8. This is because cyclization of peptides reduces the degree of freedom for each constituent within the ring and thus leads to substantially reduced flexibility, increased potency and selectivity for cyclic peptides. Further, the inherent flexibility of linear peptides leads to different conformations which can bind to more than one receptor molecules, resulting in undesirable adverse effects.

Experimental
Melting points were determined by open capillary method and are uncorrected. L-Amino acids, di-tert-butylpyrocarbonate (Boc 2 O), DCC, DIPC, TFA, TEA, pyridine and NMM were procured from SpectroChem Limited (Mumbai, India). IR spectra were recorded on a Shimadzu 8700 FTIR spectrophotometer using a thin film supported on KBr pellets or utilizing chloroform solutions. 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker AC NMR spectrometer (300 MHz) using CDCl 3 as solvent and TMS as internal standard. The mass spectra were recorded on a JMS-DX 303 Mass spectrometer (Jeol, Tokyo, Japan) operating at 70 eV in ESI-MS/MS mode. Optical rotation of the synthesized peptide derivatives was measured in methanol on an automatic polarimeter in a 2 dm tube at 25 °C using a sodium lamp. Elemental analyses of all compounds were performed on a Vario EL III elemental analyzer. Purity of all synthesized compounds was checked by TLC on precoated silica gel G plates utilizing chloroform/methanol in different ratios (8:2/7:3, v/v) as developing solvent system and spots were detected by exposure to iodine vapours in a tightly closed chamber.

General procedure for the preparation of linear tri/tetrapeptide segments
To a solution of amino acid methyl ester hydrochloride or dipeptide methyl ester (0.01 mol) in DMF (25 mL), TEA (0.021 mol) was added in portions at 0 °C and the reaction mixture was stirred for 15 min, maintaining the temperature between 0−5 °C. Boc-dipeptide (0.01 mol) was dissolved in DMF (25 mL) and DIPC or DCC (0.01 mol) was added in portions with stirring. Stirring was first done for 1 h at 0−5 °C and then further for 24 h at room temperature (RT). Precaution was taken not to allow the reaction temperature to exceed RT to avoid the risk of racemization. After the completion of the reaction, the reaction mixture was diluted with an equal amount of water. The precipitated solid was filtered, washed with water and recrystallized from a mixture of chloroform and petroleum ether (bp 40−60 °C) followed by cooling at 0 °C to get the title compounds.

Deprotection of tripeptide unit at the carboxyl terminal
To a solution of the tripeptide 5 (4.4 g, 0.01 mol) in THF-H 2 O (1:1, 36 mL), 0.36 g (0.015 mol) of LiOH was added at 0 °C. The mixture was stirred at room temperature for 1 h and then acidified to pH 3.5 with 1 N H 2 SO 4 . The aqueous layer was extracted with Et 2 O (3 × 25 mL). The combined organic extracts were dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure. The crude product was finally crystallized from methanol and ether to afford pure deprotected compound.

Deprotection of tetrapeptide unit at amino terminal
Tetrapeptide 6 (6.36 g, 0.01 mol) was dissolved in CHCl 3 (15 mL) and treated with CF 3 COOH (2.28 g, 0.02 mol). The resulting solution was stirred at room temperature for 1 h, washed with saturated NaHCO 3 solution (25 mL). The organic layer was dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure. The crude product was purified by crystallization from CHCl 3 and petroleum ether (bp 40-60 °C) to give the pure deprotected tetrapeptide unit.

Procedure for the synthesis of the linear heptapeptide unit
Deprotected tetrapeptide (5.36 g, 0.01 mol) was dissolved in tetrahydrofuran (THF, 35 mL). To this solution, TEA (2.8 mL, 0.021 mol) was added in portions at 0 °C and the resulting mixture was stirred for 15 min. Deprotected tripeptide (4.25 g, 0.01 mol) was dissolved in THF (35 mL) and DIPC (1.26 g, 0.01 mol) was added in portions to the above mixture with stirring, maintaining the temperature between 0−10 °C. Stirring was continued at room temperature for 36 h, after which the reaction mixture was filtered and the filtrate was washed with 5% NaHCO 3 and saturated NaCl solutions (30 mL each). The organic layer was dried over anhydrous Na 2 SO 4 , filtered and evaporated in vacuum. The crude product was recrystallized from a mixture of chloroform and petroleum ether (bp 40−60 °C) followed by cooling at 0 °C.

Biological activity studies
3.6.1. Antibacterial screening Newly synthesized linear and cyclic heptapeptides 7 and 8 were evaluated for their antibacterial potential against two Gram-positive bacteria-Bacillus subtilis and Staphylococcus aureus-and two Gram-negative bacteria-Pseudomonas aeruginosa and Klebsiella pneumonia-at concentrations of 50−6.25 μg/mL by using a modified Kirby-Bauer disc diffusion method [29]. MIC values of the test compounds were determined by the Tube Dilution Technique. Both linear and cyclic heptapeptides were dissolved separately using DMF to prepare a stock solution of 1 mg/mL. Stock solution was aseptically transferred and suitably diluted with sterile broth medium to contain seven different concentrations of each test compound ranging from 200−3.1 μg/mL in different test tubes. All the tubes were inoculated with one loopful of one of the test bacteria. The process was repeated with different test bacteria and different samples. Tubes inoculated with bacterial cultures were incubated at 37 °C for 18 h and the presence/absence of growth of the bacteria was observed. From these results, MIC of each test compound was determined against each test bacterium. A possible spore suspension was prepared in sterile distilled water from 5 days old culture of the test bacteria growing on nutrient broth media. About 20 mL of the growth medium was transferred into sterilized Petri plates and inoculated with 1.5 mL of spore suspension (spore concentration ~6 × 10 4 spores/mL). Filter paper disks of 6 mm diameter and 1 mm thickness were sterilized by autoclaving at 121 °C (15 psi) for 15 min. Each Petri plate was divided into five equal portions along the diameter to place one disc. Three discs of test sample were placed on three portions together with one disc with reference drug-gatifloxacin and a disk impregnated with the solvent (DMF) as negative control. The Petri plates inoculated with bacterial cultures were incubated at 37 °C for 18 h. Diameters of the inhibition zones (in mm) were measured and the average diameters for test sample were calculated in triplicate. The diameters obtained for the test sample were compared with that produced by the standard drug. The results of antibacterial studies are presented in Table 1. 3.6.2. Antifungal screening Serial plate dilution method was used for the evaluation of antifungal activity against the diamorphic fungal strain C. albicans and three other fungal strains, including A. niger and two cutaneous fungal strains, M. audouinii and T. mentagrophytes, at concentrations of 50−6.25 μg/mL [30]. MIC values of the test compounds were determined by employing the same technique as used for antibacterial studies using DMSO instead of DMF and tubes inoculated with fungal cultures were incubated at 37 °C for 48 h. After incubation, the presence/absence of fungal growth was observed and MIC of test compounds was determined against each test fungus. A spore suspension in normal saline (0.91% w/v of NaCl) was prepared from culture of the test fungi on Sabouraud's broth media. After transferring growth medium, Petri plates were inoculated with spore suspension. After drying, wells were made using an agar punch and test samples, reference drug-griseofulvin and negative control (DMSO) were placed in labeled wells in each Petri plate. The Petri plates inoculated with fungal cultures were incubated at 37 °C for 48 h. Antifungal activity was determined by measuring the diameter of the inhibition zone for triplicate sets. Activity of each compound was compared with reference standard. The results of antifungal studies are given in the Table 1.

Conclusions
The first total synthesis of the natural peptide stylisin 2 (8) was accomplished in good yield via coupling reactions utilizing different carbodiimides. Diisopropylcarbodiimide (DIPC)/TEA coupling method proved to be yield-effective, in comparison to methods utilizing DCC/TEA or NMM, providing 10-12% better yield. A pentafluorophenyl ester proved to be better for the activation of the acid functionality of the linear heptapeptide unit when compared to p-nitrophenol. Pyridine was found to be a good base for intramolecular cyclization of the linear peptide fragment in comparison to TEA and NMM. The synthesized cycloheptapeptide displayed high antibacterial activity against K. pneumoniae and P. aeruginosa, along with potent antifungal activity against C. albicans. In comparison, Gram negative bacteria were found to be more sensitive than Gram positive bacteria towards the newly synthesized peptide. On passing toxicity tests, synthesized cyclooligopeptide 8 may prove good candidate for clinical studies and can be new antimicrobial drug of future.