Intrinsic properties of nitric oxide binding to ferrous and ferric hemes

Gas phase studies offer an ideal medium whereby structural and reactivity properties of charged spe-cies may be unveiled in the absence of solvent, matrix or counterion effects. In this environment NO binds to iron(II)- and iron(III)-hemes with comparable kinetics and equilibrium parameters, conclu-sively elucidating the factors determining the widely different affinity in protic solvents or in heme proteins. IRMPD spectroscopy of the isolated species provides unambiguous characterization of the gaseous nitrosyl heme complexes.