13C n.m.r. relaxation study of poly(aspartic acid)

Abstract

13C n.m.r. relaxation parameters ( T 1, line widths and NOE factors) and chemical shifts were measured in dependence on pH for several samples of poly(α- l-Asp) differing in molecular mass and polydispersity, and for samples of poly(α,β- d,l-Asp) differing in molecular mass. Poly(α- l-Asp) dissolved at pH 9 and acidified to pH 6-4 can appear in two different forms, A and B. In n.m.r. spectra these forms differ mainly by the width of the bands of side chain carbons which is larger in form A. Conditions for a reproducible generation of either of the two forms were not found although the effect of molecular mass (3–6 × 10 4), polydispersity, sample concentration (2.0-0.2 mol l −1), temperature of dissolution (4°C–37°C), concentration of NaCl (0–4 mol l −1) or rate of acidification (1 pH unit s −1 week −1) and rate of mixing were investigated. The dynamics of poly(α,β- d,l-Asp) is almost unaffected by change of pH. The relaxation parameters of poly(α- l-Asp) at pH 9 and 4 differ more in form A than in form B. Analysis of relaxation data for the methine carbon of poly(α- l-Asp) in form A by means of the isotropic model with a log x 2 distribution of correlation times yields a correlation time of 1 ns at pH 4. This indicates that the dynamics of the backbone is dominated by rapid segmental motions even at pH 4. The dependence of chemical shifts on pH indicates that the chemical shift values are determined mainly by the ionization of the carboxyl group in the side chain rather than by a conformational transition. The evaluated relaxation parameters suggest, when compared with those of polypeptides with known conformational behaviour, that the two forms of poly(α- l-Asp) differ in conformation (α-helix, β-structure).