Electron nuclear double resonance evidence supporting a monomeric nature for P700 in spinach chloroplasts

Proton electron nuclear double resonance (ENDOR) spectra of P700(+) in spinach chloroplasts and in photosystem I particles have been obtained and compared with the corresponding ENDOR spectrum of monomeric chlorophyl a(+) (Chla(+)) cation radical. The hyperfine couplings for P700(+) can be interpreted in terms of those expected for a monomer Chla(+) radical. The reduction in alpha-carbon spin densities observed for the in vivo species when compared to the in vitro radical is attributed to differences in the composition of the ground-state orbital for the two systems. For P700(+), a mixture of 75% D(0)/25% D(1), in which D(0) and D(1) represent the ground-and first excited-state orbitals calculated by Petke et al. for Chla(+) [Petke, J. D., Maggiora, G. M., Shipman, L. L. & Christoffersen, R. E. (1980) Photochem. Photobiol. 31, 243-257], gives good agreement between calculated and experimental spin-density reduction factors. Interaction of the pigment ion with its protein environment such as through ligation of the central Mg atom, hydrogen bonding to the 9-keto-carbonyl group, and electrostatic interactions with charged amino acid residues are proposed as factors responsible for the lowering in energy of the D(1) level in vivo. Combined with similar previous proposals for P680(+) of photosystem II, the data suggest that both primary donor cation radicals of green plant photosynthesis can be viewed as monomeric Chla(+) species in which the D(1) orbital makes a significant contribution to the spin-density distribution.