The use of ¹¹³Cd NMR chemical shifts as a structural probe in tetrathiolate metalloproteins

A wide range of ¹¹³Cd chemical shifts has been observed for ¹¹³Cd-substituted metalloproteins ranging from - 100 ppm, for Cd with octahedral oxygen ligands, to + 760 ppm tor tetrahedral sulfur ligands. In particular the ¹¹³Cd chemical shifts of tetrahedral sulfur bound sites, for proteins such as rubredoxin and desulforedoxin, appear around 720-745 ppm. New ¹¹³Cd chemical shift data for ¹¹³Cd-substituted, overexpressed and mutated homologous desulforedoxin-like Fe(S-Cys)₄ proteins, have been obtained and a correlation between the ¹¹³Cd chemical shift and structure at the metal site has been observed. This subtle effect of geometry at the metal centre on ¹¹³Cd chemical shifts can be explained in terms of an increase in the paramagnetic term for the chemical shift of the ¹¹³Cd nucleus as distortion of the tetrathiolate centre is increased.