A sulfur-33 nuclear quadrupole resonance study of ³³ S₂ -labeled L-cystine

The sulfur electric-field-gradient tensor for a disulfide bond in ³³ S₂ -labeled L-cystine has been investigated by ³³ S nuclear quadrupole resonance (NQR). ³³ S₂ -labeled L-cystine is synthesized by introduction of disulfide ions prepared from elemental ³³ 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 ³³ S NQR experiment is carried out at 160 K, and the quadrupole coupling constant, C_Q , and the asymmetric parameter, η_Q , are obtained to be 46.9(9) MHz and 0.6(1), respectively. The calculated ³³ S electric-field-gradient tensor components with respect to the molecular frame is briefly discussed.