The quantum yield for NO photodissociation from iron protoporphyrin 1-methylimidazole nitrosyl, FePP(1-MeIm)(NO), in the presence of excess 1-Melm is wavelength independent, Φ, = 0.08-0.1, and the NO binding rate to the five-coordinate heme, Fe(PP)(l-MeIm), is [k5 NO = 1.7 ± 0.7×108M-1s-1; for Fe(PP)(NO), Φ1=0.05-0.08. This quantum yield is much higher than believed earlier but nevertheless appears to be significantly less than unity; the result is important to an understanding of heme-ligand photodissociation. In contrast for myoglobin and T- and R-state hemoglobin, k5 = 1.8 × 107 M-1 S-1 and Φ1 = 10-3. The observations for model systems and proteins (and comparable results for CO) can be understood self-consistently within a scheme for ligand binding and photorelease that incorporates as an intermediate a (heme, ligand) encounter pair, in the one case surrounded by a solvent cage and in the other embedded in the heme pocket of a protein. At ambient temperature, dissociation of a (heme model, NO) encounter pair in solution is several times more likely than bond formation. In contrast, because diffusion into and out of the protein heme pocket is restricted, a NO molecule in the pocket is over 100 times more likely to bind than to escape.
ASJC Scopus subject areas
- Colloid and Surface Chemistry