Resonance Raman spectra of manganese myoglobin and its azide complex. Assignment of a new charge-transfer band to azide (pi) to porphyrin (pi) transition.
The enhancement of bound azide vibrations at 650 [depolarized (dp), bending] and 2039 cm-1 (dp, antisymmetric stretch) upon excitation at approximately 400-460 nm indicates the existence of a new charge-transfer transition in manganese(III) myoglobin-azide complex. The assignments of these two vibrational modes are based on the agreement of their 15N3 isotope shifts (22 and 70 cm-1) with the calculated values (22 and 69 cm-1), the depolarized nature, and their close proximity to the corresponding vibrations in ionized azide. The Mn-(III)-N3 stretch has not been observed in the present study although the Fe(III)-N3 stretch at 413 cm-1 (polarized) was reported [Asher, S. A., Vickery, L. E., Schuster, T. M., & Sauer, K. (1977) Biochemistry 16, 5849]. The RR spectra of MnIIIMb-azide between 150 and 300 cm-1 differ dramatically from those of FeIIIMb-azide exicted in the 640-nm charge-transfer band or near the Soret band. There are lines at 170 and 282 cm-1 (both polarized) in the MnIIIMb-azide spectra which exhibit extremely large resonance enhancements and are unshifted by 15N3 isotope substitution. These two lines, having no analogue in other heme protein spectra, may be tentatively assigned to the out-of-plane porphyrin ring vibrations, with the latter involving significant Mn(III)-N(pyrrole) stretch. The enhancement of non totally symmetric azide modes suggests that the charge-transfer state may be mixed with other excited electronic states (possibly band Va or band VI) via Herzberg-Teller vibronic couplings. The lack of enhancement of the Mn(III)-N3 stretch leads to our present assignment of azide (pi) to porphyrin (pi) charge-transfer transition rather than azide (pi) to metal (dz2) or azide (n) to metal (dz2).