Organization of antenna and photo-reaction centre chlorophylls on the molecular level

Abstract

Recent investigation, mainly by nuclear magnetic (n.m.r.) and electron spin (e.s.r.) resonance spectroscopy, provide experimental support for the view that both antenna or light-harvesting chlorophyll and photo-reaction centre chlorophyll are aggregated species, but are aggregated in quite different ways. The spectral properties of antenna chlorophyll appear to be best explained in terms of chlorophyll oligomers, (Chl ₂ ) _n , in which the chlorophyll molecules are bound to each other via keto C = 0 • • • Mg coordination interactions, whereas light conversion occurs in special pairs of chlorophyll molecules probably held in the necessary configuration by a water molecule between them: [Chl H ₂ O Chl]. Photo-reactive bacteriochlorophyll a appears to have a very similar special pair structure, but antenna bacteriochlorophyll a appears to be a much more complex entity than is the antenna in green plants, and it is possible that a variety of bacteriochlorophyll a -water adducts are involved in antenna behaviour. Considerations of photosynthetic membrane structure suggest that antenna chlorophyll and the special pair photo-active chlorophyll may occupy the annular region of the thylakoid lipid bilayer.