A comparative study of the allomerization reaction of chlorophyll a and bacteriochlorophyll a

Abstract

A combined spectroscopic and chromatographic approach has been employed to study the products of allomerization of chlorophyll a (chl a), bacteriochlorophyll a (bchl a) and bacterioviridin (bvir) under a variety of conditions. Using high-performance liquid chromatography, mass spectrometry (MS), tandem-MS, NMR spectroscopy, UV–VIS absorption spectroscopy and protected surface-enhanced resonance Raman spectroscopy we have identified the allomers formed under all the conditions. Analysis of the different product distributions enables us to reach conclusions about the reaction mechanism. Water is identified as the source of hydroxyl in the allomerization reaction and it is firmly established that a C(13²)-CO₂Me group is required for allomerization to occur. Under identical allomerization conditions, bchl a yields a distribution of products that is different from those given by chl a and bvir. This observation has allowed us to demonstrate that it is the bonding at the C(7)–C(8) position of chlorophylls and not the presence of a conjugated carbonyl functionality that influences the reactivity in ring E.