Polypeptides

POLYPEPTIDES (peptides), complex amino acids formed, according to Em. Fischer (E. Fischer), by the combination of amino acids with the elimination of water at the expense of the amino and carboxyl groups; from the residues of the amino and carboxyl groups, the peptide group CO.NH is formed; the bond of amino acids in peptides by means of this group is called a peptide bond. According to the number of constituent amino acids, di-, tri-, tetra-, and polypeptides are distinguished. The names of peptides are constructed, according to Fischer, from the names of the constituent amino acids, with the name of the last amino acid remaining standard, while the names of the others, entering the molecule in the form of acid radicals, take the ending -yl; e.g., a dipeptide of two molecules of glycocoll (abbreviated as glycine) is called glycyl-glycine, NH₂.CH₂.COOH + H.NH.CH₂.COOH = H₂O + NH₂.CH₂CO.NH.CH₂COOH; of three glycocolls - diglycyl-glycine, NH₂.CH₂CO.NH.CH₂.CO.NH.CH₂COOH; of leucine, glycine, and phenylalanine - leucyl-glycyl-phenylalanine, NH₂.CH.CH₂.CH(CH₃)₂.CO.NH.CH₂.CO.NH.CH(CH₂.C₆H₅).COOH. Abderhalden synthesized a nonadecapeptide consisting of residues of 19 amino acids (4 leucines and 15 glycines), with a molecular weight of 1,246. During the formation of polypeptides from amino acids, a large number of isomers can be predicted; for 20 amino acids, their quantity is expressed by a 19-digit number, taking into account only the different arrangement of amino acids in the polypeptide molecule; if all types of isomerism are taken into account, then the number of isomers, according to Fischer, reaches 1.28x10²⁷. Some polypeptides are easily soluble in water, others are difficultly soluble; many polypeptides give a biuret reaction similar to peptones. Some high-molecular-weight polypeptides are precipitated by (NH₄)₂SO₄ like albumoses. Polypeptides form methylene compounds with formaldehyde and are titrated in 50% alcohol as acids. Polypeptides are not cleaved by pepsin and react differently to the action of various proteases, with various factors influencing the cleavability of polypeptides by proteases, e.g., the sequence of constituent amino acids, structure, configuration (see Walden inversion), the number of amino acid groups, and their nature; substitution of the hydrogen of the free NH₂ group of a polypeptide makes it incapable of being cleaved by erepsin. Polypeptides containing glycocoll, phenylalanine, and proline in their composition are difficult to cleave by proteases. Em. Fischer and others succeeded in isolating polypeptides during the partial cleavage of proteins by acids. The cleavage of dipeptides by acids, alkalis, and erepsin proceeds according to the type of monomolecular reactions. Many properties of proteins can be explained by the presence of a polypeptide bond in them. Fischer considers this bond to be the main one in the protein molecule (polypeptide theory of protein molecule structure).