Heme-apoprotein interaction in the modified oxyhemoglobin-bis(N-maleimidomethyl)ether and in oxyhemoglobin at high Cl-concentration detected by resonance Raman scattering.

The dispersion of the depolarization ratio of oxidation and spin-marker lines of oxyhemoglobin-bis(N-maleimidomethyl)ether and oxyhemoglobin at high Cl- concentration (1 M) have been examined for different pH values in the neutral and alkaline regions. The oxidation marker line at 1375 cm-1 shows no pH-dependence in the physiological region for oxyHb-bis(N-maleimidomethyl)ether and a comparatively small variation for oxyHb at a Cl- concentration higher than 0.4 M. The spin-marker line at 1638 cm-1 exhibits a strong pH-dependence of depolarization ratio for high Cl- concentration, but a minor pH-induced variation for oxyHb-bis(N-maleimidomethyl)ether. Interpretation of these data yield the following conclusions: (1) The oxidation marker line monitors symmetry-lowering distortions of the heme group introduced by central coupling to the protein via the Fe-N bond, whereas the spin-marker line monitors peripheral coupling due to heme-protein interaction in the heme pocket. (2) At low Cl- concentrations (below 0.3 M) both types of coupling are present. These are induced by the salt bridge between His 146 beta and Asp94 beta and flexibility of the FG corner. (3) At high Cl- concentrations the salt bridge is missing, eliminating central coupling. (4) In oxyhemoglobin-bis(N-maleimidomethyl)ether, due to constraint of the bis(N-maleimidomethyl)ether bridging the FG corner and eliminating its flexibility and the missing salt bridge, both central and peripheral coupling are drastically reduced.