An analysis of the visible absorption spectrum of chlorophyll a monomer, dimer, and oligomers in solution

Abstract

The visible absorption spectra of chlorophyll a monomer, dimer, and oligomers in solution have been analyzed with respect to peak positions, extinction coefficients, oscillator strengths, and dipole strengths. Exciton theory has been used to relate features in the dimer and oligomer spectra to features in the monomer spectrum. Conversion from penta-to hexacoordination at Mg strongly red shifts the Qx transition so that Qx(O,O) appears between Qy(O,O) and QJ(O,l). The relative bulk solvent environmental shifts of the Qy transition in carbon tetrachloride and n-octane have been computed. Because chlorophyll a oligomers have absorption spectra strikingly similar to antenna chlorophyll in green plants, oligomeric chlorophyll a is proposed as a model for antenna chlorophyll.

Effect of pigment composition on absorption properties of phytoplankton

“…Empirical evidence on the decon~position of absorption spectra of chl a in solution (Cotton et al 1974, Shipman et al 1976, Katz et al 1977, and of chlorophyll-protein complexes (French et al 1972, Larkum & Barrett 1983, also favours use of Gaussian curves for approximating the individual absorption bands of pigments.…”

Detection limitations of photomultiplier tubes in pulsed-laser emission spectroscopy: time-resolved fluorescence spectra of chlorophyll solutions

“…This would lead to the conclusion that the intrinsic fluorescence intensity of the aggregated and monomeric species are not all that different. Simple exciton theory predicts that for a symmetric dimeric chlorophyll species, one fluorescence transition should have non-vanishing intensity and the position of the transition could indeed be close to that of the ligated monomer (donor and acceptor) species (12). On the other hand, if the dimer is asymmetric we expect non-vanishing intensity for both components in the fluorescence spectrum.…”

“…The solutions used in the experiments were not free of water; in fact, the number of H 2 0 molecules exceeded the number of chlorophyll molecules by more than two orders of magnitude. Because chlorophyll a in wet pyridine may form penta-and hexa-coordinated complexes with pyridine and water (12,13), only a small amount (if any) of free monomer chlorophyll a molecules may be found in these solutions. Also of importance to this work is the knowledge that an equilibrium exists between Chl a and Chl a '.…”

Molecular emission and electronic structure of associated chlorophyll a

“…These solutions contain, besides the monohydrate, various aggregates of Chla. It is known that a chemical equilibrium exists between the species in solution (16,17) as is evident by the appearance of red-shifted absorption features in both the Soret and Q,, spectral regions. In the solvents CClj and hexane, vapour phase osmometry (35) data were directly attributed to species having an average aggregation number of approximately 2.…”

“…In T-shaped dimer (hereafter referred to as the dimer) where bondcontrast, such co-facial dimers are less stable in solution. They ing occurs (16)(17)(18)(19)(20)25,26) between the keto group on ring V of are thought to be a minority species in our solutions. and the resonance Raman excitation profiles can provide one with sufficient data to obtain the positions of energy levels lying in the Q and Soret bands of the species in solution.…”