Selective Raman scattering from the core chlorophylls in photosystem I viapreresonant near-infrared excitation

The Raman scattering characteristics of photosystem I (PSI) over the 200-17 00-cm(-1) frequency range have been examined using near-infrared (NIR) exci tation (lambda(ex), = 800 nm). The salient features observed in the spectra are as follows: (1) The Raman spectra are characteristic of neutral,pentac oordinate chlorophyll a (Chl a) molecules regardless of the oxidation state of the primary electron donor, P700. No Raman bands are observed for the o xidized primary donor, P700(+), despite the fact that the 800-nm excitation wavelength is coincident with a NIR absorption feature of the m-cation rad ical species. (2) The redox state of P700 has a strong influence on the tem perature dependence of the preresonance Raman (PRR) scattering intensities of the neutral Chls in PSI (Q(y) absorption maxima 660-720 nm). When P700 i s neutral, the PRR intensities of the neutral Chls decrease approximately 4 -fold on going from 77 to 200 K. When P700 is chemically or photochemically oxidized, the PRR Scattering intensities of the neutral Chls are essential ly temperature independent. The PRR intensities of the chemically oxidized sample at 77 K are approximately 50% larger than those of the neutral sampl e, whereas those at 200 K are about 4-fold larger and are comparable to tho se Ir observed at 77 K. The observation that the redox state of WOO dramati cally alters the PRR scattering intensities of the neutral Chls suggests th at the dominant contribution to the PRR spectrum obtained with 800 nm excit ation is from either neutral Chls that are electronically coupled to P700 o r these molecules plus the neutral pair of Chls in P700. The structure of P SI indicates that the P700 dimer is in close proximity to only a handful of core monomeric Chls. Thus, these findings suggest that NIR-excitation PRR spectroscopy provides a selective probe of the core Chls in PSI and may aid in the characterization of the -electronic coupling among these Chls.