A sulfur-33 nuclear quadrupole resonance study of S -labeled L-cystine.

The sulfur electric-field-gradient tensor for a disulfide bond in S -labeled L-cystine has been investigated by S nuclear quadrupole resonance (NQR). S -labeled L-cystine is synthesized by introduction of disulfide ions prepared from elemental S-sulfur into an amino acid derivative, the side chain of which is iodinated. In its NQR spectrum, sharp single peaks are observed at between 24.63 and 24.90 MHz in the temperature range from 80 to 298 K. The two-dimensional nutation echo S NQR experiment is carried out at 160 K, and the quadrupole coupling constant, C , and the asymmetric parameter, η , are obtained to be 46.9(9) MHz and 0.6(1), respectively. The calculated S electric-field-gradient tensor components with respect to the molecular frame is briefly discussed.