Identification of β,β-turns and unordered conformations in polypeptide chains by vacuum ultraviolet circular dichroism

Abstract

Different conformations of polypeptides were characterized by measurements of the circular dichroism (CD) extended into the vacuum ultraviolet region. (i) The linear ,-pleated sheet structure was characterized in a broad ultraviolet region down to 165 nm by examination of copolypeptides composed of alternating hydrophobic and hydrophilic aminoacid residues, e.g., poly(Lys-Leu-Lys-Leu). A short-wavelength intense band was foud at about 169 nm, which is characteristic of a-pleated sheet conformation. (ii) The #-turns were experimentally measured using poly(Ala2-Gly2) in a broad spectral region down to 165 nm with accuracy. The observed CD spectrum is in excellent qualitative agreement with the theoretical curve calculated by Woody for the a-turns of type II and/or I of Venkatachalam. The similarity in shape between the theoretical curve and the observed CD spectra suggests a dominance of a-turn segments in the poly(Ala Glyz) structure. The presence of ,-turns in poly(Ala-Gly2) is also in agreement with the characterization of this polypeptide by solid state methods (electron microscopy and x-ray diffraction). The (ii) The CD spectrum of the unordered chain of these alternating copolypeptides in salt-free solution is observed in the vacuum ultraviolet region.The knowledge of optical parameters, particularly from circular dichroism (CD), of different conformations encountered in polypeptides is of primary importance since it may represent a basis for structural investigation of proteins in solution. In contrast to a relatively good agreement between theoretical and experimental results concerning the a-helical conformation in polypeptides and proteins (1-4), the understanding of the optical properties of other structural forms, such as the 3-forms and the unordered conformation, is far from satisfactory.The fl-pleated sheet structure has been investigated in synthetic polypeptides such as poly(L-lysine) under various conditions (5-7), e.g., poly(L-serine) (8), poly(L-threonine), poly(L-valine), and poly(L-isoleucine) (9), and in proteins such as silk fibroin (10). As pointed out by Kubota and Fasman (9), large differences exist in the CD spectra of these compounds, although each was considered to be that of a f-structure. Most probably the causes of these discrepancies reside in the rich variety of fl-structures which may consist not only of antiparallel and parallel fl-chains, but also of various fl-bends (or reverse ,8-turns), not yet described in the characterization of CD spectra, and of twisted f-pleated sheets found in globular proteins (11).The costs of publication of this article were defrayed in part by the payment of page charges from funds made available to support the research which is the subject of the article. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.Another difficulty is related to the more or less expressed turbidity of fl-pleated sheet structures insolution. The form of the CD spectrum associated with ...

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