AGGREGATION OF CHLOROPHYLL-A PROBED BY RESONANCE LIGHT-SCATTERING SPECTROSCOPY

We report the resonance light scattering (RLS) spectra of chlorophyll a aggregated in a 9:1 solution of formamide and pH 6.8 phosphate buffe r. The aggregate formed after 2 h of mixing, referred to as Chl(469), shows a strong scattering feature at 469 nm (Soret band) and a much we aker feature at 699 nm (Q(y) band). A kinetic investigation confirmed that the aggregation process is cooperative, and also detected one int ermediate (Chl(458)) with a strong RLS spectrum but only a weak CD spe ctrum. We propose that aggregation proceeds via at least three steps: 1) formation of a nucleating species, probably a dimer of chlorophylls ; 2) formation of large aggregates with little or no secondary structu re (e.g., Chl(458)); and 3) conformational change to form helical aggr egate (Chl(469)).