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In photosystem II (PSII), redox-active tyrosine Z (TyrZ) forms a low-barrier H-bond with Nε of D1-His190. The PSII crystal structures show that Nδ of D1-His190 donates an H-bond to the carbonyl O of D1-Asn298. However, at a level of ∼2 Å resolution, a clear discrimination between the -NH 2 and -C═O groups of the asparagine side chain may not be possible based on the electron density map. Using quantum mechanical/molecular mechanical calculations, we investigated the energetics of the D1-Asn298 conformations. In the D1-Asn298-rotated conformation, where the amide N group donates an H-bond to deprotonated Nδ of D1-His190, oxidation of S 2 resulted in formation of a neutral radical, either TyrZ • or D1-His190 • . This suggests that in the D1-Asn298-rotated conformation, the redox potential ( E m ) values of TyrZ/D1-His190 are lower than the E m of the Mn 4 CaO 5 cluster due to deprotonated D1-His190. The large disorder of a water molecule (water 1117A) at D1-Asn298 in the crystal structure as well as the absence of water 1117A in the Sr 2+ -substituted crystal structure may be associated with coexistence of the two D1-Asn298 conformations in the crystals.

Mechanism of Radical Formation in the H-Bond Network of D1-Asn298 in Photosystem II