The field-dependence of the solid-state photo-CIDNP effect in two states of heliobacterial reaction centers

Solid-State NMR Analysis of the Dynamics of Cofactors: Comparison of Heliobacterial and Purple Bacterial Reaction Centers

Photosynthetic reaction centers (RCs) serve as natural engines converting solar energy to chemical energy. Understanding the principles of efficient charge separation and light-induced electron transfer (ET) between the chlorophyll-type pigments might guide the synthesis for artificial photosynthetic systems. We present detailed insight into the dynamics at the atomic level using solid-state NMR techniques applied to the RCs of Heliobacillus (Hb.) mobilis (HbRCs) and the purple bacterium Rhodobacter (R.) sphaeroides (PbRCs). It is assumed that heliobacteria were among the first phototrophic organisms; therefore, their RC can be regarded as ancient. They are constructed homodimerically with perfect C2 symmetry, enabling ET over both branches of cofactors. Modern RCs of R. sphaeroides wild-type (WT) have higher redox power and are functionally highly asymmetric. The dynamics of the cofactors in both RCs has been explored using nuclear hyperpolarization, induced by the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. Based on the individual incorporation of 13C positions of the cofactors (through supplementation by 13C-δ-aminolevulinic acid), photo-CIDNP magic-angle spinning (MAS) NMR experiments provide access to the local dynamics of the cofactors along the ET path over a broad range of time scales. Theoretical analysis of the dynamic deformation of these macrocycles is also discussed in terms of function. The dynamics observed in HbRCs appears to be correlated to ET. The cofactors in PbRC are significantly less dynamic than those in the HbRC. Relevance for efficiency and redox properties are discussed.

Light-Induced 1H NMR Hyperpolarization in Solids at 9.4 and 21.1 T

The inherently low sensitivity of nuclear magnetic resonance (NMR) spectroscopy is the major limiting factor for its application to elucidate structure and dynamics in solids. In the solid state, nuclear spin hyperpolarization methods based on microwave-induced dynamic nuclear polarization (DNP) provide a versatile platform to enhance the bulk NMR signal of many different sample formulations, leading to significant sensitivity improvements. Here we show that 1H NMR hyperpolarization can also be generated in solids at high magnetic fields by optical irradiation of the sample. We achieved this by exploiting a donor-chromophore-acceptor molecule with an excited state electron-electron interaction similar to the nuclear Larmor frequency, enabling solid-state 1H photochemically induced DNP (photo-CIDNP) at high magnetic fields. Through hyperpolarization relay, we obtained bulk NMR signal enhancements εH by factors of ∼100 at both 9.4 and 21.1 T for the 1H signal of o-terphenyl in magic angle spinning (MAS) NMR experiments at 100 K. These findings open a pathway toward a general light-induced hyperpolarization approach for dye-sensitized high-field NMR in solids.

Magic Angle Spinning Solid-State 13C Photochemically Induced Dynamic Nuclear Polarization by a Synthetic Donor-Chromophore-Acceptor System at 9.4 T

Solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) is a nuclear magnetic resonance spectroscopy technique in which nuclear spin hyperpolarization is generated upon optical irradiation of an appropriate donor-acceptor system. Until now, solid-state photo-CIDNP at high magnetic fields has been observed only in photosynthetic reaction centers and flavoproteins. In the present work, we show that the effect is not limited to such biomolecular samples, and solid-state 13C photo-CIDNP can be observed at 9.4 T under magic angle spinning using a frozen solution of a synthetic molecular system dissolved in an organic solvent. Signal enhancements for the source molecule larger than a factor of 2300 are obtained. In addition, we show that bulk 13C hyperpolarization of the solvent can be generated via spontaneous 13C-13C spin diffusion at natural abundance.

Electronic Structures of Radical-Pair-Forming Cofactors in a Heliobacterial Reaction Center

Photosynthetic reaction centers (RCs) are membrane proteins converting photonic excitations into electric gradients. The heliobacterial RCs (HbRCs) are assumed to be the precursors of all known RCs, making them a compelling subject for investigating structural and functional relationships. A comprehensive picture of the electronic structure of the HbRCs is still missing. In this work, the combination of selective isotope labelling of 13C and 15N nuclei and the utilization of photo-CIDNP MAS NMR (photochemically induced dynamic nuclear polarization magic-angle spinning nuclear magnetic resonance) allows for highly enhanced signals from the radical-pair-forming cofactors. The remarkable magnetic-field dependence of the solid-state photo-CIDNP effect allows for observation of positive signals of the electron donor cofactor at 4.7 T, which is interpreted in terms of a dominant contribution of the differential relaxation (DR) mechanism. Conversely, at 9.4 T, the emissive signals mainly originate from the electron acceptor, due to the strong activation of the three-spin mixing (TSM) mechanism. Consequently, we have utilized two-dimensional homonuclear photo-CIDNP MAS NMR at both 4.7 T and 9.4 T. These findings from experimental investigations are corroborated by calculations based on density functional theory (DFT). This allows us to present a comprehensive investigation of the electronic structure of the cofactors involved in electron transfer (ET).

Electronic structure and energetics of a heterodimeric BChl g'/Chl a' special pair generated by exposure of Heliomicrobium modesticaldum to dioxygen

Heliobacteria are anoxygenic phototrophs that have a Type I homodimeric reaction center containing bacteriochlorophyll g (BChl g). Previous experimental studies have shown that in the presence of light and dioxygen, BChl g is converted into 81-OH-chlorophyll aF (hereafter Chl aF), with an accompanying loss of light-driven charge separation. These studies suggest that the reaction center only loses the ability to transfer electrons once both BChl g' molecules of the P800 special pair have been converted to Chl aF'. The present work confirms that the partially converted BChl g'/Chl aF' special pair remains functional in samples exposed to dioxygen by demonstrating its presence using hyperfine couplings obtained from Q-band 1H ENDOR, 2D 14N HYSCORE and DFT methods. The DFT calculations of the BChl g'/BChl g' homodimeric primary donor, which are based on the recently published X-ray crystal structure, predict that the unpaired electron spin is equally delocalized over both BChl g' molecules and provide an excellent match to the experimental hyperfine couplings of the anaerobic samples. Exposure to dioxygen leads to substantial changes in the hyperfine interactions, indicative of greater localization of the unpaired electron spin. The measured hyperfine couplings are reproduced in the DFT calculations by replacing one of the BChl g' molecules of the primary donor with a Chl aF' molecule. The calculations reveal that the spin density becomes localized on BChl g' in the heterodimeric primary donor. Time-dependent DFT calculations demonstrate that conversion of either or both of the accessory BChl g molecules and/or one of the BChl g' molecules of P800 to Chl aF' results in minor effects on the energy of the charge-separated states. In contrast, if both of the BChl g' molecules of P800 are converted a large increase in the energy of the charge-separated state occurs. This suggests that the reaction center remains functional when only one half of the dimer is converted, however, conversion of both halves of the P800 dimer leads to loss of function.

Optically Enhanced Solid-State 1H NMR Spectroscopy

Low sensitivity is the primary limitation to extending nuclear magnetic resonance (NMR) techniques to more advanced chemical and structural studies. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is an NMR hyperpolarization technique where light is used to excite a suitable donor-acceptor system, creating a spin-correlated radical pair whose evolution drives nuclear hyperpolarization. Systems that exhibit photo-CIDNP in solids are not common, and this effect has, up to now, only been observed for ¹³C and ¹⁵N nuclei. However, the low gyromagnetic ratio and natural abundance of these nuclei trap the local hyperpolarization in the vicinity of the chromophore and limit the utility for bulk hyperpolarization. Here, we report the first example of optically enhanced solid-state ¹H NMR spectroscopy in the high-field regime. This is achieved via photo-CIDNP of a donor-chromophore-acceptor molecule in a frozen solution at 0.3 T and 85 K, where spontaneous spin diffusion among the abundant strongly coupled ¹H nuclei relays polarization through the whole sample, yielding a 16-fold bulk ¹H signal enhancement under continuous laser irradiation at 450 nm. These findings enable a new strategy for hyperpolarized NMR beyond the current limits of conventional microwave-driven DNP.

Photo-CIDNP in Solid State

Photo-CIDNP (photo-chemically induced dynamic nuclear polarization) refers to nuclear polarization created by the spin-chemical evolution of spin-correlated radical pairs (SCRPs). This phenomenon occurs in gases, liquids and solids. Based on the solid-state photo-CIDNP effect observed under magic-angle spinning (MAS), photo-CIDNP MAS NMR has been developed as analytical method. Here we report the origin, the theory and the state of the art of this method.

Nuclear spin-hyperpolarization generated in a flavoprotein under illumination: experimental field-dependence and theoretical level crossing analysis

The solid-state photo-chemically induced dynamic nuclear polarization (photo-CIDNP) effect generates non-equilibrium nuclear spin polarization in frozen electron-transfer proteins upon illumination and radical-pair formation. The effect can be observed in various natural photosynthetic reaction center proteins using magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and in a flavin-binding light-oxygen-voltage (LOV) domain of the blue-light receptor phototropin. In the latter system, a functionally instrumental cysteine has been mutated to interrupt the natural cysteine-involving photochemistry allowing for an electron transfer from a more distant tryptophan to the excited flavin mononucleotide chromophore. We explored the solid-state photo-CIDNP effect and its mechanisms in phototropin-LOV1-C57S from the green alga Chlamydomonas reinhardtii by using field-cycling solution NMR. We observed the 13C and, to our knowledge, for the first time, 15N photo-CIDNP signals from phototropin-LOV1-C57S. Additionally, the 1H photo-CIDNP signals of residual water in the deuterated buffer of the protein were detected. The relative strengths of the photo-CIDNP effect from the three types of nuclei, 1H, 13C and 15N were measured in dependence of the magnetic field, showing their maximum polarizations at different magnetic fields. Theoretical level crossing analysis demonstrates that anisotropic mechanisms play the dominant role at high magnetic fields.

Analysis of the Electronic Structure of the Special Pair of a Bacterial Photosynthetic Reaction Center by 13 C Photochemically Induced Dynamic Nuclear Polarization Magic-Angle Spinning NMR Using a Double-Quantum Axis

The origin of the functional symmetry break in bacterial photosynthesis challenges since several decades. Although structurally very similar, the two branches of cofactors in the reaction center (RC) protein complex act very differently. Upon photochemical excitation, an electron is transported along one branch, while the other remains inactive. Photochemically induced dynamic nuclear polarization (photo-CIDNP) magic-angle spinning (MAS) ¹³ C NMR revealed that the two bacteriochlorophyll cofactors forming the "Special Pair" donor dimer are already well distinguished in the electronic ground state. These previous studies are relying solely on ¹³ C-¹³ C correlation experiments as radio-frequency-driven recoupling (RFDR) and dipolar-assisted rotational resonance (DARR). Obviously, the chemical-shift assignment is difficult in a dimer of tetrapyrrole macrocycles, having eight pyrrole rings of similar chemical shifts. To overcome this problem, an INADEQUATE type of experiment using a POST C7 symmetry-based approach is applied to selectively isotope-labeled bacterial RC of Rhodobacter (R.) sphaeroides wild type (WT). We, therefore, were able to distinguish unresolved sites of the macromolecular dimer. The obtained chemical-shift pattern is in-line with a concentric assembly of negative charge within the common center of the Special Pair supermolecule in the electronic ground state.

Analysis of electron donors in photosystems in oxygenic photosynthesis by photo-CIDNP MAS NMR

Both photosystem I and photosystem II are considerably similar in molecular architecture but they operate at very different electrochemical potentials. The origin of the different redox properties of these RCs is not yet clear. In recent years, insight was gained into the electronic structure of photosynthetic cofactors through the application of photochemically induced dynamic nuclear polarization (photo-CIDNP) with magic-angle spinning NMR (MAS NMR). Non-Boltzmann populated nuclear spin states of the radical pair lead to strongly enhanced signal intensities that allow one to observe the solid-state photo-CIDNP effect from both photosystem I and II from isolated reaction center of spinach (Spinacia oleracea) and duckweed (Spirodela oligorrhiza) and from the intact cells of the cyanobacterium Synechocystis by (13)C and (15)N MAS NMR. This review provides an overview on the photo-CIDNP MAS NMR studies performed on PSI and PSII that provide important ingredients toward reconstruction of the electronic structures of the donors in PSI and PSII.