Contact-shifted NMR of spinach ferredoxin: Additional resonances and partial assignments

Abstract

We have extended the study of the contact-shifted resonances in the nmr spectra of oxidized and reduced spinach ferredoxin to lower magnetic field than reported by others. We have found two resonances not observed previously: one at 37 ppm, corresponding to as many as eight protons, in the oxidized protein; and the other at 43 ppm, corresponding to two protons, in the reduced protein. Assignments are proposed for these resonance and for those previously reported. We conclude that amino acid residues other than cysteine may participate in coordinating the iron atoms. By comparing the line widths in our 60 MHz spectra with those in the published 220 MHz spectra, by evaluating hyperfine coupling constants, and by an independent measurement of the electron paramagnetic resonance line width at 230 [deg] K, we estimate the electronic spin-relaxation time of the reduced protein to be ca. 1 x 10-11 sec. With this value for [tau] we determine the dipolar contribution to the line width and hence estimate the distances between the contact-shifted protons and the paramagnetic center.