Spin–lattice relaxation time and temperature dependence of fluorine-19 nuclear magnetic resonance spectra of cysteine-containing peptide iron(II) complexes

Abstract

The ¹⁹F NMR signals of cysteine-containing peptide ligands, such as Z-Cys(1)-Pro-Leu-Cys(2)-Gly-X [Z = PhCH₂OCO; X = NH(C₆H₄F-p), NH(CH₂C₆H₄F-p) or NH(CH₂CH₂C₆H₄-Fp)] were isotropically shifted both down- and up-field by co-ordination to Fe^II in [NEt₄]₂[Fe(Z-cys-Pro-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 free peptide ligands give much longer T₁ values (1500–3500 ms). The temperature dependence of the ¹⁹F NMR spectra indicates that the isotropic 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(C₆H₄F-m) both contact and dipolar mechanisms are involved. The observed behaviours are explained by the formation of X NH ⋯ S Cys(2) hydrogen bonds and the presence of π–π interactions between the aromatic group and the sulfur atom of the cysteine residue.