Nitrous-Acid-Mediated Synthesis of Iron-Nitrosyl-Porphyrin: pH-Dependent Release of Nitric Oxide

Two ironnitrosylporphyrins, nitrosyl[meso-tetrakis(3,4,5-trimethoxyphenylporphyrin]iron(II) acetic acid solvate (3) and nitrosyl[meso-tetrakis(4-methoxyphenylporphyrin]iron(II) CH2Cl2 solvate (4), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen-atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}7 class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (48) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric-oxide-free iron(III)porphyrin can be re-nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these FeII complexes at high pH values seems to be similar to that in nitrophorin, a nitric-oxide-transport protein, which formally possesses FeIII. However, because the release of NO occurs from ferrousnitrosylporphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein.