DOI: 10.1002/asia.201200518 Nitrous-Acid-Mediated Synthesis of Iron–Nitrosyl–Porphyrin: pH- Dependent Release of Nitric Oxide Jagannath Bhuyan [a] and Sabyasachi Sarkar* [b] Introduction Nitric oxide, a bioregulatory signaling molecule, can react with Fe II - and Fe III –heme to form {(heme)FeNO} moieties of {FeNO} 7 - and {FeNO} 6 type. [1] Nitrophorin, a nitric-oxide- transport protein that is found in the saliva of blood-feeding insects, is an Fe III –heme protein that reversibly binds nitric oxide. [2] This protein binds with nitric oxide at low pH values (in the saliva of blood-feeding insects) and releases it at high pH values (about pH 7.5) in the blood stream of the host to improve the flow of blood by inflating the blood- flowing channels. The binding of the released histamine to the nitric-oxide-free iron atom of nitrophorins prevents its role in immune response that help to prolong the process of blood-feeding, thereby leading to the transmission of T. cruzi, which is the cause of Chagas’ disease. The release of NO into the blood stream at high pH value and the binding of free histamine at the site of released NO on the heme moiety of nitrophorin results in an immune response to pre- vent the detection of the insect bite. [3] There is a lot of ambi- guity regarding the Fe-N-O angle in nitrophorin, even though the iron center is believed to be in its ferric state. [4] Most interestingly, nitrobindin, which is a recently discov- ered protein from the plant Arabidopsis thaliana that is structurally similar to nitrophorins, stores NO in its ferrous state in an anaerobic environment and then releases it in an aerobic environment. [5] In addition, in cd1 nitrite reductase, which reduces nitrite into nitric oxide, it was believed that NO was released from ferric–d1-heme. However, the en- hanced rate of NO-dissociation from the ferrous–heme of cd1 nitrite reductase has been recently reported. [6] A study of the spatial- and electron-distribution in the ferrous–d1- heme–NO complex from Pseudomonas aeruginosa cd1 NIR has also been performed to understand the unique proper- ties that are responsible for the relatively fast release from ferrous–d1-heme–NO. [7] There are also reports of a ferrous– heme protein that reversibly binds nitric oxide. [8] In addi- tion, the physiological actions of the nitroxy anion (NO À ), which is the one-electron-reduction product of nitric oxide (NO), is not properly understood. Little is known about the release of the nitroxy anion from heme protein. [9] To under- stand more about the interactions between NO and the heme group, we wanted to investigate the reaction between nitrous acid and iron–porphyrin. Herein, we report the syn- thesis of iron–nitrosyl–porphyrin by using nitrous acid with in situ oxygen-atom abstraction by triphenylphosphine. We have used iron–porphyrins, such as meso-tetrakis(3,4,5- trimethoxyphenyl)porphyrinironACHTUNGTRENNUNG(III) chloride (1) and meso- tetrakis(4-methoxyphenyl)porphyrinironACHTUNGTRENNUNG(III) chloride (2), for such nitrosylation reactions with nitrous acid. These iron–porphyrins have been easily oxidized to generate their corresponding isoporphyrins or higher-valent iron–porphyr- ins. [10] These iron–nitrosyl–porphyrins release NO at a rela- Abstract: Two iron–nitrosyl–porphyr- ins, nitrosyl[meso-tetrakis(3,4,5-trime- thoxyphenylporphyrin]iron(II) acetic acid solvate (3) and nitrosyl[meso-tet- rakis(4-methoxyphenylporphyrin]iro- n(II) CH 2 Cl 2 solvate (4), were synthe- sized 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 (4–8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric- oxide-free ironACHTUNGTRENNUNG(III)–porphyrin can be re-nitrosylated by using nitrous acid and triphenylphosphine. The rapid re- lease of NO from these Fe II complexes at high pH values seems to be similar to that in nitrophorin, a nitric-oxide- transport protein, which formally pos- sesses Fe III . However, because the re- lease of NO occurs from ferrous–nitro- syl–porphyrin under aerobic conditions, these compounds are more closely re- lated to nitrobindin, a recently discov- ered heme protein. Keywords: EPR spectroscopy · iron · nitric oxide · porphyrinoids · X-ray diffraction [a] J. Bhuyan Department of Chemistry Indian Institute of Technology Kanpur Kanpur-208016, U.P. (India) [b] Prof. S. Sarkar Department of Chemistry Bengal Engineering and Science University Shibpur, Howrah-711103 (West Bengal) E-mail : abya@iitk.ac.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201200518. Chem. Asian J. 2012, 00,0–0  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1& && These are not the final page numbers! ÞÞ