Electron transfer pathways and protein response to charge separation in photosynthetic reaction centers: time-resolved high-field ENDOR of the spin-correlated radical pair P865(+)QA(-).

Recently we reported the first observation of time-resolved (TR) high-frequency (HF) electron nuclear double resonance (ENDOR) of the transient charge separated state P865(+)Q(-)A in purple photosynthetic bacterial reaction centers (RC) (Poluektov, O. G., et al. J. Am. Chem. Soc. 2004, 126, 1644-1645). The high resolution and orientational selectivity of HF ENDOR allows us to directly probe protein environments by spectrally selecting specific nuclei in isotopically labeled samples. A new phenomenon associated with the spin correlated radical pair (SCRP) nature of P865(+)Q(-)A was observed. The TR-HF ENDOR spectra of protein nuclei (protons) surrounding deuterated QA(-) exhibit a derivative-like, complicated line shape, which differs considerably from the HF ENDOR spectrum of the protein nuclei surrounding thermally equilibrated QA(-). Here, a theoretical analysis of these observations is presented that shows that the positions and amplitudes of ENDOR lines contain information on hyperfine interactions (HFI) of a particular nucleus (a proton of the protein) with both correlated electron spins. Thus, spin density delocalization in the protein environment between the SCRP donor and acceptor molecules can be revealed via HF ENDOR. Novel approaches for acquiring and analyzing SCRP ENDOR that simplify interpretation of the spectra are discussed. Furthermore, we report here that the positions of the ENDOR lines of the SCRP shift with an increase in the time after laser flash, which initiates electron transfer. These shifts provide direct spectroscopic evidence of reorganization of the protein environment to accommodate the donor-acceptor charge-separated state P865(+)QA(-).

ENDOR of spin-correlated radical pairs in photosynthesis at high magnetic field: a tool for mapping electron transfer pathways.

A new phenomenon has been detected in the time-resolved electron-nuclear double resonance (ENDOR) spectra of the spin-correlated radical pairs in photosynthetic reaction center proteins. The observed effects result from both increased resolution and orientational selectivity provided by high magnetic field EPR and are manifest as specific, derivative-type lines in the ENDOR spectrum. Importantly, the positions and amplitudes of these lines contain information on the interaction of a particular nucleus with both correlated electron spins. Thus, spin density delocalization in the protein environment between the donor and acceptor in the SCRP can be revealed via SCRP ENDOR, providing a unique opportunity to probe the electron-transfer pathways in natural and artificial photosynthetic assemblies.