Circular Dichroism of the Tyrocidines and Gramicidin S-A

The cyclic decapeptides gramicidin S-A and tyrocidines A, B, and C provide excellent models for the testing of current notions on the circular dichroism of complex polypeptides. Experimental data utilizing both the naturally occurring peptides and, for the first time, hydrogenated derivatives, are presented. Significant contributions of the aromatic residues to the spectra are noted in the 200 to 230 mµ region. Aggregation of the tyrocidines is shown markedly to affect their spectra. Finally, the results are discussed in terms of the mode of aggregation of the tyrocidines.