Circular dichroism of polypeptides

Abstract

The polypeptides, together with the polynucleotides, are polymers which possess the unique property, under the right conditions, of forming ordered structures in dilute isotropic solution. This general attribute is responsible for the precise specificity of structure, and consequently of biochemical function, of the naturally occurring proteins and nucleic acids in their most usual native environment, aqueous solution. It gives rise also to the possibility of extraordinarily delicate biological control mechanisms, based on the induction of conformational changes in the macromolecule by small alterations in the environment. The most versatile methods available for the study of polypeptide conformation in solution are without doubt the spectroscopic ones, and of these the measurement of optical activity is preeminent. The ordered structure of these molecules—and we shall consider here primarily the synthetic homopolyamino acids—makes it possible to regard them essentially as one dimensional unimolecular crystals, and indeed they possess a number of important physical attributes associated with the crystalline state. Thus the collapse of the ordered structure when the environment is changed commonly occurs in a sharp cooperative manner, recalling a crystalline phase transition (melting). More particularly the ordered system of chromophores (peptide groups) gives rise to certain of the spectroscopic characteristics of molecular crystals, and it is these phenomena which provide us with the principal means of studying synthetic polypeptides and proteins in solution. It is evident then that a better understanding of these effects will be of the greatest relevance to the study of conformations and conformational changes in these molecules.