Absence of heme-localized strain in T state hemoglobin: insensitivity of heme-imidazole resonance Raman frequencies to quaternary structure.

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Abstract
Substitution of pentadeuterated 2-methylimidazole in (2-methylimidazole)-Fe(II)-protoporphyrin IX, a model complex for deoxyHb, shifted 3 bands in the low-frequency resonance Raman spectrum 380 .fwdarw. 373 cm-1, 348 .fwdarw. 345 cm-1 and 220 .fwdarw. 218 cm-1. The first of these was assigned primarily to Fe-imidazole stretching and the other 2 were assigned to porphyrin deformation modes with substantial Fe-pyrrole stretching contributions. The 3 bands were observed in deoxyHb and Mb [myoglobin]. The Fe-pyrrole modes were at essentially the same frequencies in the 2 proteins, but the Fe-imidazole mode was 6 cm-1 lower in deoxyHb than Mb, implying a slight alteration in the heme-imidazole linkage. No change greater than 2 cm-1 was observed when Hb Kempsey was switched from the R to the T state. This observation placed an upper limit on the energy stored in the Fe-imidazole bond of T state deoxyHb which was estimated to be < 0.2 kcal/mol (< 836.8 J/mol).