Magic Angle Spinning Nuclear Magnetic Resonance of Photosynthetic Components

Abstract

This chapter is the first extensive review of magic angle spinning (MAS) NMR studies of photosynthetic components. We describe how the chemical environment of the (M)Y210 in [4′-13C]tyrosine Rhodobacter sphaeroides R26 reaction centers was investigated and how labeled Rb. sphaeroides 2.4.1 (M)Y210W mutant reaction centers were used to assign the response of (M)Y210 in R26 to a narrow signal at δfi ppm. According to the MAS NMR, the Y(M)210 is in a homogeneously ordered region of the complex, and probably contributes to the fine-tuning of the energy levels of prosthetic groups involved in electron transfer. In another MAS NMR investigation, analysis of the chemical shifts of labels in the carotenoid supports a 15–15′-cis configuration for the spheroidene in the Rb. sphaeroides RC. This is a detail of the structure that can not yet be resolved unambiguously with modern X-ray techniques. A temperature-dependent asymmetry was reported for QA, since the CP/MAS signal of carbonyl 4 is not observable at temperatures T⩾255 K, contrary to the signals from the opposite carbonyl 1. Together, these studies represent the first systematic MAS investigation of a membrane protein complex. Finally, some new technical developments are discussed. A novel example of MAS photo-CIDNP was discovered recently, and yields strong emissive signals for QA depleted, or pre-reduced, 15N labeled RC. Using MAS dipolar correlation Spectroscopy on uniformlyenriched chlorophyll a/water aggretates, a complete assignment for 13C resonances was obtained and intermolecular transfer of coherence was observed. This development is an important step forward towardsstructuredetermination in multispin labeled proteins and complexes that are not accessible to diffraction methods.