We have determined the 13C spin-lattice relaxation times (T1) of the various types of carbons in helical and random-coil poly(γ-benzyl L-glutamate) ((BzlGlu)n) samples of molecular weight 7,000, 17,000, and 46,000 in CDCl3-CF3-COOH mixtures, by means of proton-decoupled natural-abundance 13C partially relaxed Fourier transform nuclear magnetic resonance (nmr) spectra at 14.2 kG. We have also measured the nuclear Overhauser enhancement (NOE) of the various carbons by means of integrated intensities in decoupled and undecoupled 13C spectra. The T1 values of Cα change by less than a factor of 2 when going from helical to random-coil (BzlGlu)n. However, the NOE changes from about 1.1 (10% intensity enhancement) to more than 2 (100% intensity enhancement). The very low NOE for the α-carbons of helical (BzlGlu)n is evidence for a rotational correlation time too slow to satisfy the extreme narrowing condition. The combination of T1 and NOE values yields effective rotational correlation times (τeff) for Cα of 24-32 nsec for the helix and only about 0.8 nsec for the random coil. These results suggest that τeff of Cα of the helical species is dominated by overall rotation with little or no contribution from segmental motion. When going to the random-coil species, τeff of Cα decreases by a factor of about 30 as a result of the onset of rapid segmental motion. The α-carbon T1 values of helix and coil differ only slightly because of their particular positions on different sides of the minimum in the functional relation between the spin-lattice relaxation time and the correlation time. In both the helical and random-coil molecules, there is a progressive increase in T1, of the protonated carbons of the side chains when going away from the polymer backbone, as a result of fast internal rotations. In the helical molecules, there is a large increase in NOE when going from Cα to Cβ, because τeff acquires contributions from internal motions with rates that satisfy the extreme narrowing condition. In contrast, in random-coil (BzlGlu)n τeff of Cα is already in the extreme narrowing limit, and changes in NOE along the side chains are small.
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