C-13 NUCLEAR MAGNETIC-RELAXATION OF POLY (DELTA-GLUTAMIC ACID) IN AQUEOUS-SOLUTION

The molecular conformation and dynamics of poly(D-glutamic acid) in aqueous solution were studied by 13C nuclear magnetic resonance spectroscopy. Chemical shift, spin—lattice relaxation time (T1), spin—spin relaxation time (T2), and the nuclear Overhauser enhancement (NOE) were measured as functions of pH at 300 K. Increasing pH resulted in upfield shifts of C and peptide C′ carbons, reflecting the helix-to-coil transition and downfield shifts of C, C, and C carbons, reflecting the ionization of the side-chain carboxyl group. T1 T2, and NOE increased with increasing pH. The effective reorientational correlation time (τ) of C′ obtained from the combination of T and NOE was 2.8 nsec in the helix region. This indicates that τ of C′ is determined by not only the overall motion of the molecule but also by appreciable local segmental motions of the backbone. When going to the coil, τ of C decreases by a factor of 4.3 as a result of the onset of rapid segmental motion. There is a progressive increase in T values of the side-chain carbons as going away from the backbone, suggesting that the end of the side-chain undergoes more a rapid internal reorientation than the others even in the helix state. The peptide carbonyl carbons relaxes more slowly in D2O than in H2O, suggesting that the relaxation is contributed appreciably from the amide proton which is exchanged with deuterium in D2O solution. © 1979, The Society of Polymer Science, Japan. All rights reserved.