Resonance Raman Evidence for Cleavage of the Fe-N (His-F8) Bond in the Subunit of the T-Structure Nitrosylhemoglobin
Nagai, K., Welborn, C., David Dolphin, and Kitagawa, T.
Resonance Raman spectra are observed for hybrid nitrosylHbs (NO-Hb) reconstituted from normal and meso-deuterated hemes and also for penta- and hexacoordinated NO-heme complexes. Upon meso deuteration of both hemes of the α and β subunits [α (D)2 NOβ (D)2 NO], the Raman lines of stripped NO-Hb at 1636 (dp), 1584 (ap), and 1502 (p) cm-1 are shifted by -10, -20, and -7 cm-1, respectively, and those at 1306 (ap) and 1228 (dp) cm-1 disappear. The spectral changes caused by meso deuteration are in good agreement with those observed previously for octaethylporphyrinatonickel(II), for which the vibrational assignments of resonance Raman lines have been established. Accordingly, the 1636-, 1584-, 1502-, 1306-, and 1228- cm-1 lines of NO-Hb are assigned to ν10, ν19, ν3, ν21, and ν13, respectively Upon conversion from the R to the T structure, both ν19 and ν3 are shifted to higher frequencies by 4 cm-1 and ν10 is split into 2 lines at 1645 and 1637 cm-1. The 1645- cm-1 line remains unshifted after meso deuteration of the β heme [α (H)2 NOβ (D)2 NO], and, conversely, the 1637- cm-1 line remains unshifted after meso deuteration of the α heme [α (D)2 NOβ (H)2 NO]. On the basis of the Raman spectra of the model NO-heme complexes, the 1645- and 1637- cm-1 lines are assigned to the penta- and hexacoordinated NO-heme complexes. Thus the Fe-Nε (His-F8) bond of the α subunit within NO-Hb is disrupted in the T structure, whereas the NO-heme of the β subunit adopts the hexacoordinated structures. The Raman spectra of the isolated α NO and β NO chains are appreciably different. The Raman lines of the isolated α NO chain at 1606 (p), 1587 (ap), and 1504 (p) cm-1 are shifted by -6, +5, and +4 cm-1 when it is incorporated into the T-structure tetramer. In contrast, none the Raman lines of the isolated β NO chain is shifted in the T-structure tetramer.