SPIN-LATTICE RELAXATION-TIME AND TEMPERATURE-DEPENDENCE OF F-19 NUCLEAR-MAGNETIC-RESONANCE SPECTRA OF CYSTEINE-CONTAINING PEPTIDE IRON(II) COMPLEXES

The F-19 NMR signals of cysteine-containing peptide ligands, such as Z -Cys(l)-Pro-Leu-Cys(2)-Gly-X [Z = PhCH2OCO; X = NH(C6H4F-m), NH(C6H4F- p), NH(CH2C6H4F-p) or NH(CH2CH2C6H4-Fp)] were isotropically shifted bo th down- and up-field by co-ordination to Fe(II) in [NEt4]2[Fe(Z-cys-P ro-Leu-cys-Gly-X)2]. The shifted signals show very short spin-lattice relaxation times (T1) in the range 5-55 ms, while the corresponding fr ee peptide ligands give much longer T1 values (1500-3500 ms). The temp erature dependence of the F-19 NMR spectra indicates that the isotropi c shifts are caused by the dipolar mechanism when the groups X are NH( CH2C6H4F-p) and NH(CH2CH2C6H4F-p), while when X = NH(C6H4F-p) and NH(C 6H4F-m) both contact and dipolar mechanisms are involved. The observed behaviours are explained by the formation of X NH ... S Cys(2) hydrog en bonds and the presence of pi-pi interactions between the aromatic g roup and the sulfur atom of the cysteine residue.