3 - The Structure and Chemistry of Functional Groups

Abstract

This chapter reviews the structure and chemistry of different functional groups of those chlorophylls the structures of which have been known for some time: chlorophylls a, b, protochlorophyll, and bacteriochlorophyll. The determination of the structures of the chlorobium chlorophylls and chlorophyll d, and what is known of the structures of chlorophylls c and e, and bacteriochlorophyll b, are treated. The chlorophylls and their derivatives, the structures of some of which are discussed in the chapter, are regarded as derivatives of the unknown compound phorbin, and the known compounds chlorin and porphin. Fischer's conventional system of numbering the pyrrole rings, their β-positions, and the methine bridges or meso positions, is consistent for chlorin and phorbin, at the sacrifice of rigid adherence to I.U.C rules. In the narrow sense, porphyrin designates substituted porphins, as distinct from chlorins. The two nitrogens of metal-free porphyrins that do not bear hydrogens are more strongly basic than the other two. The ability of products of degradation of chlorophyll to form water-soluble mono- and deprotonated species enabled Willstatter to separate them by differential extraction out of ether with HCl of increasing strength and to characterize them by their hydrochloric acid number.