Antenna chlorophyll in photosynthetic membranes. A study by resonance Raman spectroscopy

Raman spectra of antenna chlorophyll a and chlorophyll b were selectively obtained from chloroplasts of green plants and from monocellular algae, using resonance enhancement in the respective Soret bands of these molecules, at 35 K. It is shown that:

Antenna chlorophyll a molecules occur in at least five discrete categories, distinguished by different extramolecular bonding of their 9-keto carbonyl groups.

These vibrational categories are probably identical in nature and number among the different organisms studied, but differ in their relative populations.

Chlorophyll b molecules occur in at least two different categories differing by the strength of the interactions of their 3-formyl C = 0 groups. These vibrational categories also appear as universal.

Most chlorophyll a and b molecules have their magnesium atoms bound to a single foreign ligand, whose nature may depend on the population considered.

Resonance Raman spectra of antenna structures, including those of organisms devoid of chlorophyll b, were compared to resonance Raman spectra of chlorophyll a and b in monomeric, oligomeric and hydrated polymeric states, at room temperature and at 35 K. No sizable amount of antenna chlorophyll a or b occurs as dry or hydrated oligomers, or polymers. The antenna molecules are thus necessarily bound to foreign molecules, probably proteins, through H-bonding on their formyl and/or keto carbonyl groups and through bonding of their magnesium atoms.