Measurement of Methyl 2H Quadrupolar Couplings in Oriented Proteins. How Uniform Is the Quadrupolar Coupling Constant?

Abstract

2H quadrupolar and one-bond 13C−13C dipolar couplings have been measured at methyl sites in a uniformly 13C and fractionally 2H labeled sample of the N-terminal drk SH3 domain, weakly aligned in a dilute solution of Pf1 phage. An average ratio between the 2H quadrupolar and 13C−13C dipolar couplings of 19.2 ± 0.1 is measured. Assuming rapid rotation about the one-bond 13Cmethyl−13C axis and an angle of 109.5° between the unique principal axis of the electric field gradient tensor and the methyl averaging axis (13Cmethyl−13C bond), an average value of 167 ± 1 kHz is obtained for the quadrupolar coupling constant, e2qQ/h. The profile of 2H quadrupolar vs 13C−13C dipolar couplings suggests that the use of a uniform value for the quadrupolar coupling constant is a good approximation in the analysis of 2H relaxation data measured at methyl sites in proteins.