Self quenching of chlorosome chlorophylls in water and hexanol-saturated water

The optical properties of a methyl ester homolog of bacteriochlorophyll d (BChl d_M) and bacteriochlorophyll c (BChl c) in H₂O, hexanol-saturated H₂O and methanol were studied by absorption, fluorescence emission, and circular dichroism (CD). In H₂O, BChl d_M spontaneously forms an aggregate similar to that formed in hexane, with absorption maximum at 730 nm and fluorescence emission at 748 nm. For the pigment sample in hexanol-saturated H₂O, while the absorption peaks at 661 nm, only slightly red-shifted compared to the monomer, the fluorescence emission is highly quenched. When diluted 2-3 fold with H₂O, the absorption returns to around 720 nm, characteristic of an aggregate. The CD spectrum of the H₂O aggregate exhibits a derivative-shaped feature with positive and negative peaks, while the amplitude is lower than that of chlorosomes. The Fourier transform infrared spectra of BChl d_M aggregates in H₂O and hexane were measured. A 1644 cm^-1 band, indicative of a bonded 13¹-keto group, is detected for both samples. A marker band for 5-coordinated Mg was observed at 1611 cm^-1 for the two samples as well. To study the kinetic behavior of the samples, both single-photon counting (SPC) fluorescence and transient absorption difference spectroscopic measurements were performed. For BChl d_M in hexanol-saturated H₂O, a fast decay component with a lifetime of 10 to 14 ps was detected using the two different techniques. The fast decay could be explained by the concentration quenching phenomenon due to a high local pigment concentration. For the pigment sample in H₂O, SPC gave a 16 ps component, whereas global analysis of transient absorption data generated two fast components: 3.5 and 26 ps. The difference may arise from the different excitation intensities. With a much higher excitation in the latter measurements, other quenching processes, e.g. annihilation, might be introduced, giving the 3.5 ps component. Finally, atomic force microscopy was used to examine the ultrastructure of BChl d_M in H₂O and hexanol-saturated H₂O. Pigment clusters with diameters ranging from 15 to 45 nm were observed in both samples.