Estimating quadrupole couplings of amide deuterons in proteins from direct measurements of 2H spin relaxation rates.

The measurements of longitudinal and transverse 2H spin relaxation rates of backbone amide deuterons (D(N)) in the [U-13C,15N]-labeled protein ubiquitin show that the utility of amide deuterons as probes of backbone order in proteins is compromised by substantial variability of D(N) quadrupolar coupling constants (QCC) from one amide site to another. However, using the dynamics parameters of 15N-2H bond vectors evaluated from 15N relaxation data, site-specific QCC values can be estimated directly from D(N)R1 and R2 rates providing useful information on hydrogen bonding in proteins. In agreement with previous indirect scalar relaxation-based measurements, the D(N) QCC values estimated directly from R1 and R2 2H relaxation rates correlate with the inverse cube of the X-ray structure-derived hydrogen bond distances in ubiquitin: QCC=(232+/-2.3)+(118+/-17) summation operator(i)(cosalpha)r(i)(-3) where r is the inter-nuclear hydrogen bond distance in ångströms, and alpha is the N-D....O(i) angle.