Excitation Energy Trapping and Dissipation by Ni-Substituted Bacteriochlorophyll a in Reconstituted LH1 Complexes from Rhodospirillum rubrum

Abstract

Bacteriochlorophyll a with Ni(2+) replacing the central Mg(2+) ion was used as an ultrafast excitation energy dissipation center in reconstituted bacterial LH1 complexes. B870, a carotenoid-less LH1 complex, and B880, an LH1 complex containing spheroidene, were obtained via reconstitution from the subunits isolated from chromatophores of Rhodospirillum rubrum . Ni-substituted bacteriochlorophyll a added to the reconstitution mixture partially substituted the native pigment in both forms of LH1. The excited-state dynamics of the reconstituted LH1 complexes were probed by femtosecond pump-probe transient absorption spectroscopy in the visible and near-infrared spectral region. Spheroidene-binding B880 containing no excitation dissipation centers displayed complex dynamics in the time range of 0.1-10 ps, reflecting internal conversion and intersystem crossing in the carotenoid, exciton relaxation in BChl complement, and energy transfer from carotenoid to the latter. In B870, some aggregation-induced excitation energy quenching was present. The binding of Ni-BChl a to both B870 and B880 resulted in strong quenching of the excited states with main deexcitation lifetime of ca. 2 ps. The LH1 excited-state lifetime could be modeled with an intrinsic decay time constant in Ni-substituted bacteriochlorophyll a of 160 fs. The presence of carotenoid in LH1 did not influence the kinetics of energy trapping by Ni-BChl unless the carotenoid was directly excited, in which case the kinetics was limited by a slower carotenoid S1 to bacteriochlorophyll energy transfer.

Internal Conversion and Vibrational Energy Redistribution in Chlorophyll A

“…Shi et al demonstrated that the time-constant for internal conversion increases with an increase in the solvent dielectric constant . Dong et al further estimated the vibrational relaxation time to be 1.5 ps for ChlA solvated in common solvents such as ethyl acetate and ethyl ether. , Even though the internal conversion in chlorophylls itself is surprisingly understudied, it has received significant attention in various other light-harvesting pigments such as bacteriochlorophylls, − carotenoids, − and porphyrins. Within a wider view, importance of understanding internal conversion is evident from a number of related studies in biological macromolecules such as nucleic acids , and many other organic/inorganic molecules or compounds. ,, …”

Tuning the Photophysical Features of Self-Assembling Photoactive Polypeptides for Light-Harvesting

“…The absorption maxima of Sph (400–530 nm), from the first stages of reconstitution are red-shifted by ~15 nm with respect to the maxima in acetone, and their positions stay constant during the folding of the antenna (Figure 3). The maxima of Sph absorption are located in the same positions as in the spectra of fully assembled and isolated LH1 [29,32]. The assembly of the subunits can also be induced in the absence of Crt, either by lowering the detergent concentration below its CMC value or by the use of a co-solvent, for instance acetone, and then the B870 complex is formed (not shown) [18].…”

“…Pure Sph-LH1 was eluted with 200 mM NaCl in 0.03% LDAO. To insert Ni-BPheo into LH1, a portion of Ni-BPheo dissolved in acetone was added to the B780-B820-B870 mixture before titration with Sph, following a previously published method [29].…”

Intramolecular charge-transfer enhances energy transfer efficiency in carotenoid-reconstituted light-harvesting 1 complex of purple photosynthetic bacteria

“…rubrum strain G9+ according to the reported protocols 52 , 53 . The B820 heterodimers were isolated from the native LH1 complex as described previously 54 – 56 following solubilization with 1.2% octyl-β-glucopyranoside (β-OG) in 50 mM phosphate buffer (pH 7.0) with the presence of 10 mM sodium ascorbate. The B820 heterodimers were further purified by means of sucrose density gradient (0.6–0.8 M) ultracentrifugation (165,000 × g × 16 h at 4 °C) and stored in the freezer (–30 °C) until required.…”