Resonance Raman investigation of dioxygen bonding in oxycobaltmyoglobin and oxycobalthemoglobin: structural implication of splittings of the bound O--O stretching vibration.

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Abstract
Splittings related to the stretching vibration of bound dioxygen in hemoproteins were detected by resonance Raman spectroscopy. With excitation at 406.7 nm 3 isotope-sensitive lines are observed in [sperm whale] oxycobaltmyoglobin (oxyCoMb) [or in [human] oxycobalthemoglobin A (oxyCoHbA)] at 1103 (1107), 1137 (1137) and 1153 (1152) cm-1, of which the most intense one appears at 1137 cm-1. The 1st 2 frequencies arise from resonance interaction between a v(O.sbd.O) mode at .apprxeq. 1122 cm-1 and an accidentally degenerate porphyrin ring mode at 1123 (1121) cm-1, whereas the 3rd one represents an unperturbed v(O.sbd.O) vibration from a different species. These 2 v(O.sbd.O) modes at .apprxeq. 1122 and .apprxeq. 1153 cm-1 shift to .apprxeq. 1066 and .apprxeq. 1096 cm-1, respectively, upon 16O2 .fwdarw. 18O2 substitution. The same resonance interaction may also occur in oxyFeMb (probably also in oxyFeHbA), because it exhibits an intensity increase at 1125 cm-1 upon 16O2 .fwdarw. 18O2 substitution, although the v(O.sbd.O) vibrations have not been observed directly. Concomitant enhancement is observed in the v(Co.sbd.O) vibration at 539 (537) cm-1, which is considerably lower than the v(Fe.sbd.O) frequency at .apprxeq. 570 cm-1 in oxyFeMb and oxyFeHbA. The Co.sbd.O bond is longer and weaker than the Fe.sbd.O bond. Enhancement of both v(O.sbd.O) and v(Co.sbd.O) indicates the existence of a charge-transfer transition underlying the Soret band, which may be assigned as .pi.*(.pi.*O2/dxz) O2/dxz) .fwdarw. .sigma.*(dz2Co/.pi.g*). The presence of 2 v(O.sbd.O) vibrations (at .apprxeq. 1122 and .apprxeq. 1152 cm-1) but only 1 v(Co.sbd.O) mode (at .apprxeq. 538 cm-1) means that the 2 spp. in oxyCoMb or oxyCoHbA have the same Co.sbd.O bond lengths but different O.sbd.O bond lengths. The bound dioxygen in a bent end-on configuration may have 2 allowed orientations, which differ in the extent of sp2(N.epsilon.) .fwdarw. .pi.*(O2) donation from distal histidine.