An EPR/ENDOR study of the asymmetric hydrogen bond between the quinone electron acceptor and the protein backbone in Photosystem I

Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A1 z2 in photosystem (PS) I has been studied using pulsed Q-band ENDOR spectroscopy. With deuterated quinone in protonated PS I particles it is demonstrated that the observed radical anion has only one hydrogen-bond hyperfine coupling (hfc) tensor with tensor components above the 2 MHz range. Below 2 MHz the protein matrix protons dominate and a second weak H-bond could not be detected. The spectral resolution of pulsed Q-band ENDOR is critically required to separate the signals of the H-bond proton from those of the primary chlorophyll acceptor, A0 z2, which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A1 z2 is found to be close to axial symmetry with a small isotropic component, as expected from a predominantly dipolar electron–proton spin interaction in a hydrogen-bond. The principal tensor components are Ak ¼ ðþÞ7:7; MHz A’ ¼ ð2Þ4:9 MHz, Aiso ¼ ð2Þ0:7 MHz. The magnitude of the dipolar tensor corresponds to an unusually short H-bond which can be estimated from the point-dipole approximation (,1.5 ^ 0.1 A ° ). Based on previous studies with A- and B-branch specific site-directed mutants of the A1 site of PS I and the chosen photoaccumulation protocol, the observed A1 z2 radical anion can be assigned to the QK–A site of the A-branch. The observed H-bond hfc tensor is compared to those determined for related quinone radical anions observed in frozen protic solution as well as in the QA site of type II bacterial reaction centers. q 2004 Elsevier B.V. All rights reserved