In photosystem I (PS I) the phylloquinone secondary acceptor functions as a one electron gate similar to the Q(A) quinone secondary acceptor in type II reaction centers. The quinone radical anion formed during charge separation as part of the P-700(+.) A(1)(-.) radical pair state is known to have a highly asymmetric electron spin density distribution, which is attributed to asymmetric hydrogen bonding with the protein environment. Here, the native phylloquinone is replaced by specifically C-13-labeled 2-methyl-1,4-naphthoquinones (2-methyl-4-C-13-1,4-naphthoquinone and 2-C-13-methyl-1,4-naphthoquinone) to probe the spin density distribution in two relevant quinone ring positions. The menB phylloquinone biosynthetic pathway mutant was used because it allows for efficient in vitro quinone replacement in isolated PS I trimers. X-and Q-band time-resolved EPR spectroscopy was used to determine the C-13 hyperfine tensors. in the functional P-700(+.) A(1)(-.) state. For 2-methyl-4- C-13-1,4-naphthoquinone anion radical the largest hyperfine tensor component A(zz) = 44 MHz was found to be considerably larger than those determined in bacterial reaction centers. The PS I structure at 2.5 A resolution shows a single H-bond from the backbone NH group of a specific leucine residue. Such a highly asymmetric H-bonding is confirmed here for the functional P-700(+.) A(1)(-.)state. Experimental evidence for an unusual backbone H-bond strength is analyzed by comparing the quinone binding site with that in bacterial reaction centers and possible mechanisms of increased H-bond strength are considered.