An amino acid. A nonessential amino acid, you might be relieved to know. Glycine is the simplest possible amino acid, and if you chip a hydrogen ion off either end of one (a sharp stone adze or a kitchen knife will do) you can stick two of them together, and it's still an amino acid, called either diglycine or glycylglycine. The chemical structure of the glycine molecule admits indefinite chaining of this kind*.

An amino acid has the general formula CHR(NH2)COOH. That is, a pivotal carbon atom has an amino radical NH2, a carboxyl radical COOH, and some other distinguishing radical R. Glycine, the simplest case, has plain H for its R. The standard symbol for glycine is Gly.

There are two different glycyl ions: I don't know what the proper chemical names for them are, but you can either knock a hydrogen off the amino to give CHR(NH-)COOH or you can knock a hydroxy off the carboxyl to give CHR(NH2)CO+. The combination of these two gives common or garden glycylglycine. These are symbolized Gly- and -Gly.

A third possibility is to knock off both the H+ and the OH- (these combine to give water so put some towels down first), and this gives you another ion, -Gly-. This one can go in the middle of the other two. Repeat to taste.

Glycylglycylglycylglycylglycine has a Gly- at one end, three lots of -Gly- in the middle, and a -Gly at the far end.

Reaction rates for glycylglycylglycylglycylglycine may be found at -- in particular EAQ1263.HTM and EAQ1264.HTM, but the whole long list is interesting if you like that sort of thing.

* I can't swear to the indefinite: there might well be some stereochemical restriction that prevents it polymerizing to arbitrary length.