EFFECT OF MAGNETIC-FIELD STRENGTH ON THE LINEWIDTH AND SPIN-LATTICE RELAXATION-TIME OF THE THIOCYANATE CARBON OF CYANYLATED BETA-LACTOGLOBULIN-B - OPTIMIZATION OF THE EXPERIMENTAL PARAMETERS FOR OBSERVING THIOCYANATE CARBONS IN PROTEINS

The linewidths and spin-lattice relaxation times of the C-13-n.m.r. si gnal at 109.7 p.p.m. due to the thiocyanate carbon of intact [cyanato- C-13]cyanylated-beta-lactoglobulin-B have been determined at magnetic field strengths of 1.88, 6.34 and 11.74 T as well as the spin-lattice relaxation times of its backbone alpha-carbon atoms. The linewidths we re directly proportional to the square of the magnetic field strength and we conclude that, at magnetic field strengths of 6.34 T or above, more than 70% of the linewidth will be determined by chemical-shift an istropy. We estimate that the spin-lattice relaxation time resulting f rom the chemical-shift anistropy of the thiocyanate carbon is 1.52 +/- 0.1 s and we conclude that for magnetic field strengths of 6.34 T and above the observed spin-lattice relaxation time of the thiocyanate ca rbon will be essentially independent of magnetic field strength. Using the rigid-rotor model we obtain estimates of the rotational correlati on time of [cyanato-C-13]cyanylated-beta-lactoglobulin-B and of the ch emical-shift anisotropy shielding tenser of its thiocyanate carbon. We have calculated the linewidths and spin-lattice relaxation times of t hiocyanate carbons at magnetic field strengths of 1.88-14.1 T in prote ins with M(r) values in the range 10000-400000. The effects of magneti c field strength on the resolution and signal-to-noise ratios of the s ignals due to thiocyanate carbons attached to proteins of M(r) greater than 10000 are discussed.