The iron electron-nuclear double resonance (ENDOR) of two-iron ferredoxins from spinach, parsley, pig adrenal cortex and Pseudomonas putida

Abstract

The iron electron-nuclear double resonance (ENDOR) spectra of reduced iron-sulfur proteins (two-iron ferredoxins) from spinach, parsley, pig adrenal cortex and Pseudomonas putida unequivocally show two inequivalent iron atoms at the active sites of each of these proteins. The frequencies of the ENDOR lines establish the total electronic spin in the ground state to be . The principal values of the hyperfine tensor have been determined for each of the iron atoms and these values are consistent with and lend considerable support to the model of a high-spin Fe(III) atom and a high-spin Fe(II) atom antiferromagnetically coupled to form an system. The measured principal axis components of the effective hyperfine tensors for are as follows (1 and 2 refer to the inequivalent iron sites): These data are consistent with Site 1 being ferric and Site 2, ferrous iron. The primes indicate that the A-tensor principal axes for Site 1 (Fe(III)) are apparently rotated about the x-axis with respect to the g-tensor axes by an angle [theta] (20[deg] [les] [theta] [les] 40[deg]). The orientations of the A-tensors for Site 2 (Fe(II)) have not been determined and hence the values presented are the observed values of the A-tensors along the x, y, and z-axes of the g-tensor for this complex.A brief introduction to the theory of ENDOR is given.