Hydrogen bonding to the photoaccumulated secondary acceptor radical anion A 1 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, A 0 z2 , which cannot be avoided to be formed to some extent in the photoaccumulation procedure. The determined H-bond hfc tensor of A 1 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 A k ΒΌ ðþΓ7:7; MHz A ' ΒΌ Γ°2Γ4:9 MHz, A iso ΒΌ Γ°2Γ0:7 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 Λ). Based on previous studies with A-and B-branch specific site-directed mutants of the A 1 site of PS I and the chosen photoaccumulation protocol, the observed A 1 z2 radical anion can be assigned to the Q K -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 Q A site of type II bacterial reaction centers.