13C−1H heteronuclear dipolar correlation studies of the hydrogen bonding of the quinones inRhodobacter sphaeroides R26 reaction centers

Abstract

The photosynthetic reaction center (RC) of the photosynthetic bacteriumRhodobacter sphaeroides R26 contains two quinones, QA and QB. Solid-state heteronuclear (1H−13C) dipolar correlation spectroscopy has been used to study the binding of the quinones in the ground state for RCs reconstituted with l-13C ubiquinone-10. Lee-Goldburg cross-polarization buildup curves are recorded to determine distancesrCH between the l-13C carbon labels and the protons involved in the polarization transfer. The l-13C of both QA and QB have intermolecular correlations with protons that resonate downfield, in the region of hydrogen-bonding protons. The distances between the carbon labels and the correlated protons are short, 0.21±0.01 nm. Hence the nuclear magnetic resonance provides evidence for strong hydrogen-bonding interactions at the l-C=O of both QA and QB for RCs in the ground state. The environment of the l-13C of the QB is structurally heterogeneous compared to that of the QA. The data can be reconciled with a strong H-bonding interaction of the l-C=O of QA with Ala M260 NH, and with complex hydrogen bonding involving NH of Ile-L224 and of Gly-L225, and possibly the Ser-L223 hydroxyl group of the l-C=O of the QB, in the proximal site.