Esterification of hemins with trimethyloxonium tetrafluoroborate.

Esterification of naturally occurring hemins and porphyrins is often a prerequisite for structural studies of these compounds. Porphyrins and hemins are in general more easily purified as their esters than as the free carboxylic acids. In addition, esterification is the most feasible procedure for preparing derivatives of tetrapyrroles for studies by mass spectrometry. The most convenient and frequently employed method for hemin and porphyrin esterification has been treatment with an alcohol and mineral acid. Although this method is highly successful in derivatizing the carboxyl group, bond cleavage or rearrangement of various acid-labile tetrapyrroles may result from the strongly acidic conditions, and derivatization of other functional groups may occur. For example, treatment of heme a with methanol-sulfuric acid results in partial methylation of both the formyl and hydroxyl groups (1). Although porphyrins can be esterified with diazomethane, this reaction may very well modify side chains and is completely unsuitable for hemin carboxylic acids (2). Conversion of an acid-labile hemin to its porphyrin methyl ester by way of the free porphyrin is not feasible since removal of iron from the hemin requires acid conditions. Recent studies in this laboratory have been directed toward determining the structure of the unique formylhemin prosthetic group of a green hemeprotein isolated from human and bovine erythrocytes (3.4). The very labile character of the isolated hemin has led us to suspect the presence of a moiety which can be cleaved or derivatized in acid. An acid-labile group has likewise been postulated to be a component of the hemin prosthetic group of cytochrome oxidase (5). Thus, a method of esterification which would selectively derivatize carboxyl groups of