Symmetry Break of Special Pair: Photochemically Induced Dynamic Nuclear Polarization NMR Confirms Control by Nonaromatic Substituents

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).

Stabilization of a flavoprotein for solid-state photo-CIDNP MAS NMR at room temperature by embedding in a glassy sugar matrix

Hyperpolarization via the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect can be detected in frozen solutions of electron transfer proteins generating a radical-pair upon illumination. The effect has been observed in various natural photosynthetic reaction centers and in light-oxygen-voltage (LOV) sensing domains incorporating a flavin mononucleotide (FMN) as chromophore. In LOV domains, where a highly conserved cysteine is mutated to a flavin to interrupt its natural photochemistry, a radical-pair is generated by electron transfer from a nearby tryptophan to the photoexcited triplet state of FMN. During the photocycle, both the LOV domain and the chromophore are photochemically degraded, e.g., by the formation of singlet oxygen. This limits the time for collection of hyperpolarized nuclear magnetic resonance (NMR) data. We show that embedding of the protein into a trehalose sugar glass matrix stabilizes the protein for 13C solid-state photo-CIDNP NMR experiments which can be conducted at room temperature in a powder sample. Additionally, this preparation allows for incorporation of high amounts of protein further boosting the intensity of the detected signals from FMN and tryptophan at natural abundance. Signal assignment is aided by quantum chemical calculations of absolute shieldings. The underlying mechanism for the surprising absorption-only signal pattern is not yet understood. Comparison to calculated isotropic hyperfine couplings imply that the enhancement is not due to the classical radical-pair mechanism (RPM). Analysis of the anisotropic hyperfine couplings associated with solid-state photo-CIDNP mechanisms also show no simple correlation, suggesting a more complex underlying mechanism.

Photoinduced charge transfer in Zn(II) and Au(III)-ligated symmetric and asymmetric bacteriochlorin dyads: A computational study

The excited-state properties and photoinduced charge-transfer (CT) kinetics in a series of symmetrical and asymmetrical Zn- and Au-ligated meso-meso-connected bacteriochlorin (BChl) complexes are studied computationally. BChl derivatives, which are excellent near-IR absorbing chromophores, are found to play a central role in bacterial photosynthetic reaction centers but are rarely used in artificial solar energy harvesting systems. The optical properties of chemically linked BChl complexes can be tuned by varying the linking group and involving different ligated metal ions. We investigate charge transfer in BChl dyads that are either directly linked or through a phenylene ring (1,4-phenylene) and which are ligating Zn or Au ions. The directly linked dyads with a nearly perpendicular arrangement of the BChl units bear markedly different properties than phenylene linked dyads. In addition, we find that the dielectric dependence of the intramolecular CT rate is very strong in neutral Zn-ligated dyads, whereas cationic Au-ligated dyads show negligible dielectric dependence of the CT rate. Rate constants of the photo induced CT process are calculated at the semiclassical Marcus level and are compared to fully quantum mechanical Fermi's golden rule based values. The rates are calculated using a screened range separated hybrid functional that offers a consistent framework for addressing environment polarization. We study solvated systems in two solvents of a low and a high scalar dielectric constant.

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.

On the Role of the Special Pair in Photosystems as a Charge Transfer Rectifier

The special pair, a bacteriochlorophyll a (BChl) dimer found at the core of bacterial reaction centers, is known to play a key role in the functionality of photosystems as a precursor to the photosynthesis process. In this paper, we analyze the inherent affinity of the special pair to rectify the intrapair photo-induced charge transfer (CT). In particular, we show that the molecular environment affects the nuclear geometry, resulting in symmetry breaking between the two possible intrapair CT processes. To this end, we study the relationships of the intrapair CT and the molecular geometry with respect to the effective dielectric constant provided by the molecular environment. We identify the special pair structural feature that breaks the symmetry between the two molecules, leading to CT rectification. Excited state energies, oscillator strengths, and electronic coupling values are obtained via time-dependent density functional theory, employing a recently developed framework based on a screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM). We analyze the rectification capability of the special pair by calculating the CT rates using a first-principles-based Fermi's golden rule approach.

Studying hydrogen bonding and dynamics of the acetylate groups of the Special Pair of Rhodobacter sphaeroides WT

Although the cofactors in the bacterial reaction centre of Rhodobacter sphaeroides wild type (WT) are arranged almost symmetrically in two branches, the light-induced electron transfer occurs selectively in one branch. As origin of this functional symmetry break, a hydrogen bond between the acetyl group of P_L in the primary donor and His-L168 has been discussed. In this study, we investigate the existence and rigidity of this hydrogen bond with solid-state photo-CIDNP MAS NMR methods offering information on the local electronic structure due to highly sensitive and selective NMR experiments. On the time scale of the experiment, the hydrogen bond between P_L and His-L168 appears to be stable and not to be affected by illumination confirming a structural asymmetry within the Special Pair.

Assignment of NMR resonances of protons covalently bound to photochemically active cofactors in photosynthetic reaction centers by 13C-1H photo-CIDNP MAS-J-HMQC experiment

Modified versions of through-bond heteronuclear correlation (HETCOR) experiments are presented to take advantage of the light-induced hyperpolarization that occurs on ¹³C nuclei due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect. Such ¹³C-¹H photo-CIDNP MAS-J-HMQC and photo-CIDNP MAS-J-HSQC experiments are applied to acquire the 2D ¹³C-¹H correlation spectra of selectively ¹³C-labeled photochemically active cofactors in the frozen quinone-blocked photosynthetic reaction center (RC) of the purple bacterium Rhodobacter (R.) sphaeroides wild-type (WT). Resulting spectra contain no correlation peaks arising from the protein backbone, which greatly simplifies the assignment of aliphatic region. Based on the photo-CIDNP MAS-J-HMQC NMR experiment, we obtained assignment of selective ¹H NMR resonances of the cofactors involved in the electron transfer process in the RC and compared them with values theoretically predicted by density functional theory (DFT) calculation as well as with the chemical shifts obtained from monomeric cofactors in the solution. We also compared proton chemical shifts obtained by photo-CIDNP MAS-J-HMQC experiment under continuous illumination with the ones obtained in dark by classical cross-polarization (CP) HETCOR. We expect that the proposed approach will become a method of choice for obtaining ¹H chemical shift maps of the active cofactors in photosynthetic RCs and will aid the interpretation of heteronuclear spin-torch experiments.

13C → 1H transfer of light-induced hyperpolarization allows for selective detection of protons in frozen photosynthetic reaction center

In the present study, we exploit the light-induced hyperpolarization occurring on ¹³C nuclei due to the solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect to boost the NMR signal intensity of selected protons via inverse cross-polarization. Such hyperpolarization transfer is implemented into ¹H-detected two-dimensional ¹³C-¹H correlation magic-angle-spinning (MAS) NMR experiment to study protons in frozen photosynthetic reaction centers (RCs). As a first trial, the performance of such an experiment is tested on selectively ¹³C labeled RCs from the purple bacteria of Rhodobacter sphaeroides. We observed response from the protons belonging to the photochemically active cofactors in their native protein environment. Such an approach is a potential heteronuclear spin-torch experiment which could be complementary to the classical heteronuclear correlation (HETCOR) experiments for mapping proton chemical shifts of photosynthetic cofactors and to understand the role of the proton pool around the electron donors in the electron transfer process occurring during photosynthesis.

Field-cycling NMR with high-resolution detection under magic-angle spinning: determination of field-window for nuclear hyperpolarization in a photosynthetic reaction center

Several parameters in NMR depend on the magnetic field strength. Field-cycling NMR is an elegant way to explore the field dependence of these properties. The technique is well developed for solution state and in relaxometry. Here, a shuttle system with magic-angle spinning (MAS) detection is presented to allow for field-dependent studies on solids. The function of this system is demonstrated by exploring the magnetic field dependence of the solid-state photochemically induced nuclear polarization (photo-CIDNP) effect. The effect allows for strong nuclear spin-hyperpolarization in light-induced spin-correlated radical pairs (SCRPs) under solid-state conditions. To this end, ¹³C MAS NMR is applied to a photosynthetic reaction center (RC) of the purple bacterium Rhodobacter (R.) sphaeroides wildtype (WT). For induction of the effect in the stray field of the magnet and its subsequent observation at 9.4 T under MAS NMR conditions, the sample is shuttled by the use of an aerodynamically driven sample transfer technique. In the RC, we observe the effect down to 0.25 T allowing to determine the window for the occurrence of the effect to be between about 0.2 and 20 T.

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.

Contribution of bacteriochlorophyll conformation to the distribution of site-energies in the FMO protein

The structural data for the Fenna-Matthews-Olson (FMO) protein indicate that the bacteriochlorophylls (BChls) display a significant degree of conformational heterogeneity of their peripheral substituents and the protein-induced nonplanar skeletal deformations of the tetrapyrrole macrocycle. As electronic properties of chromophores are altered by such differences, a conformational effect may influence the site-energies of specific pigments and thus play a role in mediating the excitation energy transfer dynamics, but this has not yet been established. The difficulty of assessing this question is shown to be partly the result of the inability of the sequential truncation approach usually employed to account for interactions between the conformations of the macrocycle and its substituents and an alternative approach is suggested. By assigning the BChl atoms to meaningful atom groups and performing all possible permutations of partial optimizations in a full-factorial design, where each group is either frozen in the crystal geometry or optimized in vacuo, followed by excited state calculations on each resulting structure (PM6//ZIndo/S), the specific effects of the conformations of each BChl component as well as mutual interactions between the molecular fragments on the site-energy can be delineated. This factorial relaxation procedure gives different estimates of the macrocycle conformational perturbation than the approach of sequentially truncating the BChl periphery. The results were evaluated in the context of published site-energies for the FMO pigments from three species to identify how conformational effects contribute to their distribution and instances of cross-species conservation and functional divergence of the BChl nonplanarity conformational contribution are described.

Elastic Vibrations in the Photosynthetic Bacterial Reaction Center Coupled to the Primary Charge Separation: Implications from Molecular Dynamics Simulations and Stochastic Langevin Approach

Primary electron transfer reactions in the bacterial reaction center are difficult for theoretical explication: the reaction kinetics, almost unalterable over a wide range of temperature and free energy changes, revealed oscillatory features observed initially by Shuvalov and coauthors (1997, 2002). Here the reaction mechanism was studied by molecular dynamics and analyzed within a phenomenological Langevin approach. The spectral function of polarization around the bacteriochlorophyll special pair PLPM and the dielectric response upon the formation of PL(+)PM(-) dipole within the special pair were calculated. The system response was approximated by Langevin oscillators; the respective frequencies, friction, and energy coupling coefficients were determined. The protein dynamics around PL and PM were distinctly asymmetric. The polarization around PL included slow modes with the frequency 30-80 cm(-1) and the total amplitude of 130 mV. Two main low-frequency modes of protein response around PM had frequencies of 95 and 155 cm(-1) and the total amplitude of 30 mV. In addition, a slowly damping mode with the frequency of 118 cm(-1) and the damping time >1.1 ps was coupled to the formation of PL(+)PM(-) dipole. It was attributed to elastic vibrations of α-helices in the vicinity of PLPM. The proposed trapping of P excitation energy in the form of the elastic vibrations can rationalize the observed properties of the primary electron transfer reactions, namely, the unusual temperature and ΔG dependences, the oscillating phenomena in kinetics, and the asymmetry of the charge separation reactions.