12 - Absorption and Fluorescence Spectra of Bacterial Chlorophylls in Situ

Abstract

This chapter discusses the absorption and fluorescence spectra of bacterial chlorophylls in situ. Three types of interaction with the local environment have been postulated to explain the absorption spectra of the bacterial chlorophylls (Bchl) in situ: (1) chlorophyll–protein interaction, (2) chlorophyll–chlorophyll interaction and (3) chlorophyll–carotenoid interaction. The interaction of chlorophyll with protein and lipid, as is observed in distinct complexes, such as the Bchl–protein complexes obtained from green bacteria, is undoubtedly an important factor in determining the spectra in situ. Chlorophyll–chlorophyll interaction resulting from the formation of large chlorophyll aggregates in vivo has been suggested to explain the very large wavelength differences, 80–100 mμ, often existing between several of the red bands in vivo and the single band observed in solution. The presence in most purple bacteria of two or three Bchl absorption bands between 800 and 900 mμ is consistent with the theory that Bchl exists in two or three states of aggregation in vivo. Chlorophyll–carotenoid interaction has been proposed to explain the great variability in the fine structure of the near-infrared absorption spectrum of Bchl a in purple bacteria. It seems reasonable to consider the carotenoids along with lipids as nonpolar elements of the environment of the chlorophylls in vivo.