| Resumo: |
Photosystem I (PSI) and II (PSII) possess chlorophyll pairs P~A~/P~B~ and P~D1~/P~D2~, respectively. These chlorophylls are the primary electron donors in the light-induced electron transfer. After the electron transfer, the radical cation remains on these chlorophyll pairs, forming [P~A~/P~B~]^·+^ and [P~D1~/P~D2~]^·+^. The positive charge distributions over the two chlorophylls were reported to be 10/90-50/50 for P~A~^·+^/P~B~^·+^ [1,2] and 70/30-80/20 for P~D1~^·+^/P~D2~^·+^ [3,4]. To clarify the origin of the distributions, we calculated ratios of P~A~^·+^/P~B~^·+^ and P~D1~^·+^/P~D2~^·+^ with a quantum mechanical/molecular mechanical (QM/MM) approach and the redox potentials (_E_~m~) of monomeric chlorophylls P~A~, P~B~, P~D1~, and P~D2~ with an electrostatic continuum-model approach, using the crystal structures of PSI [5] and PSII [6]. 
1) Our QM/MM calculation reproduced the experimentally measured ratios of P~A~^·+^/P~B~^·+^ [1,2] and P~D1~^·+^/P~D2~^·+^ [3,4]. The calculated ratios were strongly correlated with the calculated _E_~m~ values. 2) We analyzed residues on puseudo-symmetrical subunit pairs PsaA/PasB and D1/D2 that shifted _E_~m~ of P~A~, P~B~, P~D1~, and P~D2~ and identified the residue pairs responsible for the P~A~^·+^/P~B~^·+^ and the P~D1~^·+^/P~D2~^·+^ ratios. In PSII, the difference in the electrostatic protein environments between D1 and D2 was significant in determining the P~D1~^·+^/P~D2~^·+^ ratio, whereas geometric differences between P~A~ and P~B~ (P~A~ as the C13^2^ epimer of chlorophyll a and the H-bond pattern) played a role in determining the P~A~^·+^/P~B~^·+^ ratio in PSI.

References:
[1] A. N. Webber, W. Lupitz, _Biochim. Biophys. Acta. 1507_ (2001) 61.
[2] M. Pantelidou, P. R. Chitnis et al., _Biochemistry 43_ (2004) 8380.
[3] I. H. Davis, P. Heathcote et al., _Biochim. Biophys. Acta. 1143_ (1993) 183.
[4] T. Okubo, T. Tomo et al., _Biochemistry 46_ (2007) 4390.
[5] P. Jordan, P. Fromme et al., _Nature 411_ (2001) 909.
[6] Y. Umena, K. Kawakami et al., _Nature 473_ (2011) 55. 

|
