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 β-pleated sheet structure was characterized in a broad ultraviolet region down to 165 nm by examination of copolypeptides composed of alternating hydrophobic and hydrophilic amino-acid residues, e.g., poly(Lys-Leu-Lys-Leu). A short-wavelength intense band was found at about 169 nm, which is characteristic of β-pleated sheet conformation. ( ii ) The β-turns were experimentally measured using poly(Ala 2 -Gly 2 ) 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 β-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 β-turn segments in the poly(Ala 2 -Gly 2 ) structure. The presence of β-turns in poly(Ala 2 -Gly 2 ) is also in agreement with the characterization of this polypeptide by solid state methods (electron microscopy and x-ray diffraction). The CD spectrum of β-turns is characterized by a very intense band at 207.5 nm and strong negative bands at 191 and 169 nm. Copolypeptides such as poly(Ala 2 -Gly 3 ) and poly(Ala 3 -Gly 3 ) yielded a similar type of CD spectrum, analysis of which indicates that a large fraction of their residues is contained in β-turn regions. ( iii ) The CD spectrum of the unordered chain of these alternating copolypeptides in salt-free solution is observed in the vacuum ultraviolet region.