Mechanistic Studies of Nitric Oxide Reactions with Water Soluble Iron(II), Cobalt(II), and Iron(III) Porphyrin Complexes in Aqueous Solutions:  Implications for Biological Activity

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298−318 K and the hydrostatic pressure range 0.1−250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive ΔS ‡ and ΔV ‡ values were observed for NO binding to and release from iron(III) complexes FeIII(TPPS) and FeIII(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Δ and Δ for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k on = 1.5 × 109 and 1.9 × 109 M-1 s-1 for FeII(TPPS) and CoII(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the FeII complexes (k on = 3.6 × 107 M-1 s-1 for FeII(TPPS)) displays negative Δ and Δ values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from FeII(TPPS)(NO) and CoII(TPPS)(NO) by trapping free NO gave k off values of 6.3 × 10-4 s-1 and 1.5 × 10-4 s-1. The respective MII(TPPS)(NO) formation constants calculated from k on/k off ratios were 2.4 × 1012 and 1.3 × 1013 M-1, many orders of magnitude larger than that (1.1 × 103 M-1) for the reaction of FeIII(TPPS) with NO.