Analysis of the temperature dependence of the 1H and 13C isotropic shifts of horse heart ferricytochrome c:: Explanation of Curie and anti-Curie temperature dependence and nonlinear pseudocontact shifts in a common two-level framework

The H-1 and C-13 hyperfine shifts of the heme methyls of horse heart ferricytochrome c have been measured over the temperature range 278-328 K in order to interpret the "anomalous" temperature-dependence of the hyperfine shifts in terms of their pseudocontact and contact shifts. By taking advantage of the available pseudocontact shifts for protein nuclei measured at 303 and 323 K (Santos, H.; Turner, D. L. fur. J. Biochem. 1992, 206, 721-728), the metal-centered pseudocontact shifts have been analyzed in terms of a thermally accessible excited state lying 355-590 cm(-1) to higher energy which has a magnetic susceptibility tensor with the rhombic anisotropy, Delta chi(rh), which is rotated by 90 degrees to that of the ground state. The metal-centered pseudocontact shifts have been evaluated at all temperatures at which the chemical shifts were measured, and these calculated values were used to evaluate the contact shifts of each heme methyl for the two nuclei. The temperature dependence of the heme methyl contact shifts for both H-1 and C-13, assuming a thermally accessible excited state, was then used to evaluate the spin density for the four beta-pyrrole heme carbons to which the methyls are attached. The ligand-centered pseudocontact shifts have been estimated and found to give a modest contribution to the experimental behavior. The H-1 and C-13 data are highly self-consistent. The present analysis provides deep insight into the electron distribution on the porphyrin ring in low-spin Fe(III) hemes.