The tyrocidine antibiotic polypeptides are known to exhibit self-association. They were therefore used as test substances to document the behavior to be expected by the method of thin-film dialysis. Self-association with these peptides is indicated by concentration dependence, a reverse curvature of the escape plot, and a retardation of the rate of diffusion by addition of salt but an acceleration by the addition of ethanol or other hydrophobic bond breaking solvent additives. A survey of dipeptides by thin-film dialysis has revealed that L-lysyl-L-lysine, L-lysyl-L-arginine, and L-histidyl-L-histidine exhibit a behavior characteristic of self-association in a pH region where all the acidic and basic T A he so-called quaternary structures of proteins and nucleotides are thought to result from the interplay of various binding forces such as hydrophobic bonds, coulombic forces, 7r bonds, and hydrogen bonds. The contribution of each individual interaction or small section of the molecule is considered to be relatively weak as compared with most covalent bonds but their sum in concerted effect can be strong. Such a theory agrees well with experience but leaves much to be understood about the detailed nature of each type of interaction and its contribution to the whole. A truly clear understanding is made uncertain by the many variables
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