Effect of internal librational motions on the 13C nuclear magnetic resonance relaxation times of proteins and peptides

Abstract

Many existing experimental anomalies concerning the main chain and the side chain protonated carbon relaxation times in large peptides and proteins are explained by a simple model of internal librational motions in which, for all proteins, τ(libration)= 10^–11 s. The mean librational angle is ± 20° for all α-carbons, and this increases by ≈± 6° for each step down a non-cyclic side chain. A general conclusion is that many published rotational correlation times obtained from T₁ values are too large by a factor of 1.5–2.0; the corrected times agree better with τ_R values obtained from light-scattering experiments. It is also shown that, in the absence of exchange-broadening, the linewidths of protonated carbon resonances will be generally narrower than those expected for a rigid protein. Further detailed measurements at 45 and 68 MHz, supporting these conclusions, are described for ribonuclease-A and bovine serum albumin.