Density functional calculated spin densities and hyperfine couplings for hydrogen bonded 1,4-naphthosemiquinone and phyllosemiquinone anion radicals:: a model for the A1 free radical formed in photosystem I

The B3LYP hybrid density functional method is used to calculate spin densities and hyperfine couplings for the 1,4-naphthosemiquinone anion radical and a model of the phyllosemiquinone anion radical. The effect of hydrogen bonding on the spin density distribution is shown to lead to a redistribution of pi spin density from the semiquinone carbonyl oxygens to the carbonyl carbon atoms. The effect of in plane and out of plane hydrogen bonding is examined. Out of plane hydrogen bonding is shown to give rise to a significant delocalisation of spin density on to the hydrogen bond donor heavy atom. Excellent agreement is observed between calculated and experimental hyperfine couplings. Comparison of calculated hyperfine couplings with experimental determinations for the Al phyllosemiquinone anion radical present in Photosystem I (PS I) of higher plant photosynthesis indicates that the in vivo radical may have a hydrogen bond to the O4 atom only as opposed to hydrogen bonds to each oxygen atom in alcohol solvents. The hydrogen bonding situation appears to be the reverse of that observed for Q(A) in the bacterial type II reaction centres where the strong hydrogen bond occurs to the quinone O1 oxygen atom. For different types of reaction centre the presence or absence of the non-heme Fe(II) atom may well determine which type of hydrogen bonding situation prevails at the primary quinone site which in turn may influence the direction of subsequent electron transfer. (C) 1999 Elsevier Science B.V. All rights reserved.