Photo-CIDNP 13C Magic Angle Spinning NMR on Bacterial Reaction Centres: Exploring the Electronic Structure of the Special Pair and Its Surroundings

Abstract

Photochemically induced dynamic nuclear polarisation (photoCIDNP) in intact bacterial reaction centres has been observed by 13Csolid state NMR under continuous illumination with white light. Strong intensity enhancement of 13C NMR signals of the aromatic rings allows probing the electronic ground state of the two BChl cofactors of the special pair at the molecular scale with atomic selectivity. Differences between the two BChl cofactors are discussed. Several aliphatic 13C atoms of cofactors, as well as 13C atoms of the imidazole ring of histidine residue(s), show nuclearspin polarisation to the same extent as the aromatic nuclei of the cofactors. Mechanisms and applications of polarisation transfer are discussed.

Photo-CIDNP MAS NMR beyond the T1 limit by fast cycles of polarization extinction and polarization generation

“…Already at a d 4 delay of 2 ms (spectrum shown in red), strong signals appear exclusively in the aromatic region of the 13 C NMR spectrum. This is in contrast to previous experiment under continuous illumination, in which also light-induced signals appear in the aliphatic region [23,25]. Data obtained at 60, 133 and 500 ms (spectra shown in green, light blue and dark blue, respectively) show the exact kinetics of polarization transfer to the aliphatic carbons on the ms timescale which can be understood in terms of spin diffusion towards the steadystate envelope (Spectrum 8B).…”

“…A strong solid-state photo-CIDNP effect has been observed with 4-ALA labeled WT RCs under continuous illumination under white light [23,25] leading to entirely negative signals [9]. In Fig.…”

“…Upon isotope labeling, natural spin diffusion spreads the nuclear polarization into the vicinity of the active cofactors. The effect is used in continuous illumination experiments to map the electronic ground-state of the entire active cofactors as well as their surrounding amino acids of the protein [23][24][25][26]. On the other hand, spin diffusion leads to a redistribution of the primary polarization and obscures the primary photo-CIDNP MAS NMR intensities which provide information on electron spin densities.…”

Observation of the solid-state photo-CIDNP effect in entire cells of cyanobacteria Synechocystis

“…Signals of C-17 are indeed expected in this region. Since for continuous illumination experiments of selectively labelled RCs, labelled aliphatic carbons may gain intensity indirectly by spin diffusion from the labelled aromatic carbons nearby (Matysik et al 2001), the origin of the enhancement is not obvious. A possible explanation may be that these positive light-induced signals indeed originate from PS2, while the light-induced signals in the aromatic region originate from PS1.…”

Nanosecond-Flash 15N Photo-CIDNP MAS NMR on Reaction Centers of Rhodobacter sphaeroides R26

“…The samples were loaded into optically transparent 4-mm sapphire rotors. For continuous illumination experiments, a setup specially designed for the Bruker MAS probe has been applied [4,30]. Light and dark spectra were obtained by simple Bloch decay followed by a spin echo pulse sequence.…”