Picosecond Laser Spectroscopy and Optically Detected Magnetic Resonance on a Model Photosynthetic System
Year: 1986
Type: article
Abstract: Fluorescence‐detected magnetic resonance of triplets in zero magnetic field (FDMR), fluorescence fading (FF) due to triplet‐formation, both at 4.2 K, and prompt fluorescence decay kinetics (FDK) at room temperature have been measured for free pheophorbide‐a (f‐Pheo) and bound (b‐Pheo) to a synthetic polypeptide (L‐Lys‐L‐Ala‐L‐Ala)n, dissolved in dimethylformamide (DMF). Fluorescence decay kinetics measurements of f‐Pheo in DMF yielded 1‐5 ns lifetimes, for b‐Pheo in DMF a ~ 50 ps decay‐component was found emitting at 730–750 nm. Zero‐field splitting parameters |D| and |E| of the lowest triplet state T1 were determined from FDMR spectra as (337 and 24) 10‐4 cm‐1 for f‐Pheo and (359 and 25) 10‐4 cm‐1 for b‐Pheo, both in DMF. Decay rate constants of the three spin levels of T1 of b‐Pheo (Kx= 1200 50 s‐1, ky= 440 25 s‐1, kz= 80 5 s‐1) and relative steady‐state populations (Nx= 28 2%, Ny= 47 2%, Nz= 26 2%) determined from FF curves predict a fluorescence decrease at the D–E and D + E FDMR transitions, whereas experimentally a fluorescence increase is observed. The FDMR sign‐inversion results from singlet‐singlet energy transfer from b‐Pheo monomers to their aggregates, followed by fast intersystem crossing to T1. These results indicate that aggregates are formed by two or more b‐Pheo molecules at different positions on the folded polypeptide chain. This situation resembles that in chlorophyll‐proteins, containing low‐lying traps, resulting from interaction of chromophores with other chromophores and with the protein environment.
Source: Photochemistry and Photobiology
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2014728075