DETERMINATION OF C-13-ALPHA RELAXATION-TIMES IN UNIFORMLY C-13 N-15-ENRICHED PROTEINS/

Relaxation times of C-13(alpha) carbons of uniformly C-13/N-15-enriche d probes have been investigated. The relaxation behaviour was analyzed in terms of a multispin system. Pulse sequences for the determination of T-1, T-2 and the heteronuclear NOE of C-13(alpha) in uniformly C-1 3/N-15-enriched ribonuclease Tl are presented. The experiments perform ed in order to obtain T-1 and the heteronuclear NOE were similar to th ose of the corresponding N-15 experiments published previously. The de termination of T-2 for the C-alpha-carbon in a completely labeled prot ein is more complicated, since the magnetization transfer during the T -2 evolution period owing to the scalar coupling of C-alpha-C-beta mus t be suppressed. Various different pulse sequences for the T-2 evoluti on period were simulated in order to optimize the bandwidth for which reliable T-2 relaxation times can be obtained. A proof for the quality of these pulse sequences is given by fitting the intensity decay of i ndividual H-1-C-13(alpha) cross peaks, in a series of (H-1,C-13)-ct-HS QC spectra with a modified CPMG sequence as well as a T-1 rho sequence for the transverse relaxation time, to a single exponential using a s implex algorithm.