Sequences of amino acid residues in silk fibroin.

An outstanding problem in protein chemistry is the determination of the arrangement or sequence of amino acid residues in the peptide chains. One approach to the partial solution of this problem is through a study of the smaller degradation products present in partial hydrolysates of proteins, preferably those produced on treatment with acid (1). This method is especially attractive when the protein contains a limited number of varieties of amino acid residues. Silk fibroin of Bombyx mori is such a protein, Fischer and Abderhalden (2-10) reported the presence of glycylalanine, alanylglycine, and glycyltyrosine among the products of acid hydrolysis of Glk. Stein, Moore, and Bergmann (11) analyzed such hydrolysates for the two alanine peptides and found considerable amounts of each. No finding of alanylalanine, tyrosyltyrosine, or glycylglycine has been, reported for hydrolysates of B. mori fibroin. Nevertheless, the direct linkage of these pairs of identical amino acids in the protein is not excluded, because no method assuring the isolation of any particular peptide, if present, has been used. The quantitative significance of those sequences which have been reported (alanylglycine, glycylalanine, glycyltyrosine) has not been established, For example, a purely random sequence of the amino acid residues in fibroin would lead to the prediction, on a statistical basis, of rather frequent juxtaposition of alanine and glycine residues, and an accumulation, during hydrolysis, of appreciable quantities of the corresponding dipeptides