Basic Amino-acids of Silk Fibroin

Abstract

IN studies of the constitution of proteins, the basic amino-acids are of particular interest. With silk fibroin, for example, the basic amino-acids are the least frequent of the constituent amino-acid residuee and are therefore indicative of the approximats minimum molecular weight. Coleman and Howitt¹, deduced from the available analytical data that, assuming the molecule to contain one histidine residue, there was present a total of some 390 residues which, accepting the experimental mean residue weight of 78, implied a molecular weight of approximately 30,000; a molecular weight of this order was found by osmotic pressure measurements made on the renatured protein. In the latter connexion, however, Holmes and Smith², by ultracentrifuge and diffusion methods, found a mean molecular weight of 60,000–150,000 (average 84,000) for water-soluble fibroin. This higher value of the mean molecular weight suggests that, assuming no great error to exist in the value of about 0.4 per cent for the histidine content of fibroin, there are at least two histidine residues in the fibroin molecule. In order to strengthen the available experimental evidence on this point, a careful determination has been made of the basic amino-acids in silk fibroin.