Experimental Determination of an Isolated trans-Dinitrosyl Manganese(II) Heme Analogue

The lack of direct proof in either natural or synthetic systems for trans-dinitrosyl hemes, a key intermediate in the reactions of heme proteins (e.g. soluble guanylate cyclase (sGC), cytochrome c ' and So H-NOX) with nitric oxide (NO), has hampered understanding of the exact reaction mechanisms, such as the formation of the five-coordinate heme complex with NO at the proximal side (5c NOP). Herein, we report the first isolation of a dinitrosyl metalloporphyrin complex, the six-coordinate, low-spin {Mn(NO)(2)}(7) species [Mn(TPP)(NO)(2)] (TPP2-=meso-tetraphenylporphyrin dianion). The complex shows distinct features, such as an elongated axial bond (1.877(9) vs. 1.641(5) angstrom), a higher NO stretching bond position (1760 vs. 1735 cm(-1)) and an isotropic resonance at g = 2.0, in sharp contrast to those of five-coordinate mononitrosyl analogues. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and EPR studies provided deep insight into the reaction processes, demonstrating different responses of porphyrinates to NO.