 By the mid-1930s, analytical ultracentrifugation studies by Theodor Svedberg had shown that proteins had a well-defined chemical structure, and were not aggregations of small molecules.The same studies appeared to show that the molecular weight of proteins fell into a few well-defined classes related by integers, such as "M""w" = 2"p"3"q" Da, where "p" and "q" are nonnegative integers.However, it was difficult to determine the exact molecular weight and number of amino acids in a protein.