Determination of Rotational Mobilities of Backbone and Side-Chain Carbons of Poly(γ-benzyl L-glutamate) in the Helical and Random-Coil States from Measurements of Carbon-13 Relaxation Times and Nuclear Overhauser Enhancements

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 (τ_eff) for Cα of 24-32 nsec for the helix and only about 0.8 nsec for the random coil. These results suggest that τ_eff 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, τ_eff 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 τ_eff acquires contributions from internal motions with rates that satisfy the extreme narrowing condition. In contrast, in random-coil (BzlGlu)_n τ_eff of Cα is already in the extreme narrowing limit, and changes in NOE along the side chains are small.