PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

Probing the electronic structure of axial histidines in photosynthetic reaction centres using MAS/NMR. A Alia^*,1, Ido de Boer¹, Peter Gast², Huub de Groot¹, Jorg Matysik¹, ¹ SSNMR, Leiden Institute of Chemistry, Leiden, The Netherlands² Department of Biophysics, Huygens Laboratorium, Leiden, The Netherlands

ABSTRACT- Histidines serve as the axial ligand to Chl/BChl in all known photosynthetic reaction centers (RC). In bacterial reaction centers, mutation of the axial histidines of the primary electron donor has been demonstrated to affect kinetics of electron transfer, symmetry of the electron spin distribution. In the present study, different MAS NMR techniques are applied in order to explore the electronic and protonic structure of the histidines co-ordinating to Mg2+ in bacterial reaction centres. The 2D homonuclear dipolar correlation MAS NMR spectrum of [¹³C₆,¹⁵N₃]-histidine labeled bacterial RC shows four separate correlation networks corresponding to at least four different types of histidines. Due to significant ring current effects, the 2D heteronuclear (¹H-¹³C) dipolar correlation spectrum of histidine labeled bacterial reaction centers shows a clear separation between histidines in close proximity to the BChl cofactors and the other histidines in the reaction centres. A full 1H and 13C resonance assignment of axial histidines is presented. The DFT calculations corroborate that coordinating histidines carry 0.2 electron equivalent of positive charge in ground state. ¹³C-photo-CIDNP MAS NMR studies provide insight into the crucial charge separation process since the resonances detected by photo-CIDNP are from the ground state electronic structure after charge separation and recombination. ¹³C photo-CIDNP spectra show significant nuclear-spin polarisation of the ring carbons of axial histidines with a clear upfield shift of 13 ppm and 7 ppm, respectively. These resonances correspond to negatively charged histidines as also supported by DFT calculations. These initial photo-CIDNP results indicate that during charge separation process histidines might be switching their electronic structure toward negatively charged character in order to stabilise radical cation states of primary chlorophyll.

KEY WORDS: axial histidines, bacterial reaction center, MAS/NMR