Determination of the antisymmetric part of the chemical shift anisotropy tensor via spin relaxation in nuclear magnetic resonance

Abstract

Relaxation processes induced by the antisymmetric part of the chemical shift anisotropy tensor (henceforth called anti-CSA) are usually neglected in NMR relaxation studies. It is shown here that anti-CSA components contribute to longitudinal relaxation rates of the indole (15)N nucleus in tryptophan in solution at different magnetic fields and temperatures. To determine the parameters of several models for rotational diffusion and internal dynamics, we measured the longitudinal relaxation rates R(1)=1/T(1) of (15)N, the (15)N-(1)H dipole-dipole (DD) cross-relaxation rates (Overhauser effects), and the cross-correlated CSA/DD relaxation rates involving the second-rank symmetric part of the CSA tensor of (15)N at four magnetic fields B(0)=9.4, 14.1, 18.8, and 22.3 T (400, 600, 800, and 950 MHz for protons) over a temperature range of 270<T<310 K. A good agreement between experimental and theoretical rates can only be obtained if the CSA tensor is assumed to comprise first-rank antisymmetric (anti-CSA) components. The magnitude of the hitherto neglected antisymmetric components is of the order of 10% of the CSA.