Helix to coil transition in poly-L-amino acids. II. N.M.R. study of model compounds and Poly-γ-benzyl-L-glutamate

Abstract

The proton magnetic resonance spectroscopy of simple amides and poly-L- amino acids has been investigated with particular reference to the question of charging of the amide group in strong acids. It is found that the downfield chemical shift of the α-CH resonance of poly-γ- benzyl-L-glutamate (PBG) which accompanies the helix to coil transition can be conveniently divided into two parts on the basis of experiments with model systems. The first is due to the collapse of the helix to an uncharged random coil and the second represents the charging of the amide groups of the random coil. For PBG samples of low molecular weight, two α-CH resonances are observed due to residues in helical (uncharged) and random-coil (charged) forms. The rate of exchange between them is of the order of 400 sec-1. The proton resonances of the NH proton, COOH proton of the acid, and α-CH proton of dichloroacetic acid (DCA) are shown to be less useful than the α-CH proton. A critical evaluation has been made of all the evidence relevant to the charging of the amide group of polypeptides in mixtures of non-interacting solvents and organic acids e.g. trifluoroacetic acid (TFA) and DCA. It is concluded that viscosity, infrared, dielectric constant, electric birefringence, and conductivity studies give overwhelming support to the concept of charging. Nuclear magnetic resonance, circular dichroism, and optical rotatory dispersion are less sensitive indicators of small amounts of charging, and hence studies should be made with samples of low molecular weight (where there is a greater fractional degree of charging). Such studies made here by n.m.r. methods give strong support for charging. The features of the flexible helix model first proposed in Part I are delineated. It consists of helical segments separated by short, charged random-coil breaks at both ends and in the interior of the molecule. Although elaborated in some detail for the case of PBG, it is likely to occur quite generally for those polypeptides which are soluble in organic solvents and undergo the helix to coil transition.