Physical Chemistry Chemical Physics

Understanding photosynthetic light harvesting requires knowledge of the molecular mechanisms that dissipate excess energy in thylakoids. However, it remains unclear how the physical environment of light-harvesting complex II (LHCII) influences the process of chlorophyll de-excitation. Here, we demonstrate that protein-protein interactions between LHCIIs affect the optical properties of LHCII and thus influence the total energy budget. Aggregation of LHCII in the dark altered its absorption properties, independent of the amount of prior light exposure. We also revisited the triplet excited state involved in light-induced fluorescence quenching and found another relaxation pathway involving emission in the green region, which might be related to triplet excited energy transfer to neighboring carotenoids and annihilation processes that result in photoluminescence. LHCII- containing liposomes with different protein densities exhibited altered fluorescence and scattering properties. Our results suggest that macromolecular reorganization affects overall optical properties, which need to be addressed to compare the level of energy dissipation.

Understanding the diversity in the enzymatic mechanism have utmost importance as it can temporally control the catalytic efficiency. Recent literature suggests that by influencing mechanical properties of hybrid materials, the catalytic efficiency of the enzymatic reactions can be altered significantly. Here, taking a computational and experimental approach, we have dig out the fate of the kinetics of enzyme reaction systems exhibiting relatively complex mechanism than usual Michaelis Menten kinetics involving multiple substrate/enzyme/enzyme-substrate complex. We have developed a numerical recipe improvising stochastic reaction-diffusion approach to explore the role of mechanical stress like compression-decompression cycle (C-D) on modulating the output of enzymatic reaction. The proposed methodology can be used as a theoretical tool to understand how to enhance the catalytic activity and setup appropriate reaction conditions by efficiently using the catalytic cycles. Hence, our methodology will be crucial to identify the most effective strategy to efficiently convert substrate into product in this type of mechanoresponsive materials, which will enable future development of cost-effective biomaterials. In future, the insights gained from this work may find enormous application in drug delivery, tissue engineering, bio-sensing and bio-catalysis.

Plant hormones are small molecules that regulate plant growth, development, and responses to biotic and abiotic stresses. Plant hormones are specifically recognized by the binding site of their receptors. In this work, we investigated the role of water displacement and reorganization at the binding site of plant receptors on the binding of eight classes of phytohormones (auxin, jasmonate, gibberellin, strigolactone, brassinosteroid, cytokinin, salicylic acid, and abscisic acid) using extensive molecular dynamics simulations and inhomogeneous solvation theory. Our findings demonstrated that displacement of water molecules by phytohormones contributes to free energy of binding via entropy gain and is associated with free energy barriers. Also, our results have shown that displacement of unfavorable water molecules in the binding site can be exploited in rational agrochemical design. Overall, this study uncovers the role of water molecules in plant hormone perception, which creates new avenues for agrochemical design to target plant growth and development.

Ions perform crucial functions in biological systems. However, the understanding of the relation between polyelectrolyte gel and ion is confined to the effects of swelling by salt solutions on macro- and micro-properties of gel. Here we explore the microstructure evolution of gel with changing crosslinking density and concentration of types of salts measured by dynamic light scattering (DLS). The dissociation-association of the ion bonds will induce a stretched exponential mode in the correlation function. The stretched exponent {beta} reflects the subchain dynamics of gel strands and undergoes a transition from 0.33 to 0.5 when the multivalent salt concentration increases to the vicinity of phase separation boundary. It demonstrates that the Gaussian chain-like gel strands with Rouse dynamics transform into molten-globule-like strands with Zimm dynamics at high multivalent salt concentration.

In this study, the effect of the chemical nature of the confinement on the folding and thermal stability of the telomere G-quadruplex (G4) has been investigated by studying the folding pattern of different telomere DNA sequences with varying numbers and arrangements of thymine loop nucleobases in the presence of anionic and cationic nanosized water pools. The findings suggest that both anionic and cationic water pools fold the telomere sequences into G4 of the same topology. However, the thermal stability of the folded G4 in the cationic water pool is significantly lower than that of the anionic case. The overall data indicate that the topology of the folded G4 is insensitive to the nature of the confinement, however, the thermal stability of the folded telomeric G4 depends significantly on the chemical nature of the confinement. It is plausible that the interfacial water inside the cationic water pools has a different orientation and hydrogen bonding than the case of anionic water pools, which may cause the different thermal stability of the G4 on these two water pools. These findings may be important in understanding the folding and stability of telomere G4 inside the confined cellular system.

For the cofactor-free 1-H-3-hydroxy-4-oxoquinaldine-2,4-dioxygenase (HOD), the dioxygen (O2) dependent steps are rate-limiting along with a spin state crossover to the singlet spin state. Here, the primary triplet O2 molecule activation on the 2-methyl-3-hydroxy-4(1H)-quinolone (MHQ) is investigated, and the catalytic role of the intersystem crossing effects is highlighted by directly comparing results from the Born-Oppenheimer dynamics and non-adiabatic surface hopping dynamics. This work confirms non-adiabatic dynamical effects are essential to modulate the O2 activation on the substrate MHQ. The time scale of the equilibration and conversion from triplet to singlet state should be in the range of a few hundreds of femtoseconds. We hope this work provides us a fresh look at the underlying physics of dioxygen activation reactions involving more than one spin state.

Photosynthetic organisms have evolved to work under low and high lights in photoprotection, acting as a scavenger of reactive oxygen species. The light dependent xanthophyll cycle involved in this process is performed by a key enzyme (present in the thylakoid lumen) Violaxanthin De-Epoxidase (VDE) in the presence of violaxanthin and ascorbic acid substrates. Phylogenetically, VDE is found to be connected with an ancestral enzyme Chlorophycean Violaxanthin De-Epoxidase (CVDE) present in the green algae on the stromal side of the thylakoid membrane. However, the structure and functions of CVDE were not known. In search of functional similarities involving this cycle, the structure, binding conformation, stability, and interaction mechanism of CVDE are explored with the two substrates in comparison to VDE. The structure of CVDE was determined by homology modeling and validated. In-silico docking (of first-principles-optimized substrates) revealed it has a larger catalytic domain than VDE. A thorough analysis of the binding affinity and stability of four enzyme-substrate complexes are performed by computing free energies and its decomposition, the root-mean-square deviation (RMSD) and fluctuation (RMSF), the radius of gyration, salt-bridge and hydrogen bonding interactions in molecular dynamics. Based on these, violaxanthin interacts with CVDE to the similar extent as that of VDE, hence its role is expected to be the same for both the enzymes. On the contrary, ascorbic acid has a weaker interaction with CVDE than VDE. As these interactions drive epoxidation or de-epoxidation process in the xanthophyll cycle, it immediately discerns that either ascorbic acid does not take part in de-epoxidation or this process requires a different cofactor because of the weaker interaction of ascorbic acid with CVDE in comparison to VDE.

Upon binding to cytosolic DNA, the cyclic GMP-AMP synthase (cGAS) is activated to catalyze the synthesis of cGAMP, which then activates downstream effectors and induces innate immune responses. The activation of cGAS relies on the formation of cGAS-DNA oligomers and liquid phase condensation, which are sensitive to the length and concentration of DNA. For a thorough understanding of such a length-and concentration-dependent activation, the details of the cGAS-DNA oligomerization are required. Here, with molecular dynamics (MD) simulations, we report the structure of the cGAS-DNA monomer (the cGAS1-DNA1 complex), in which the DNA binds simultaneously to the major parts of two DNA-binding sites as observed in the cGAS-DNA dimer (the cGAS2-DNA2 complex) and the active site is largely immature. Energetic analysis reveals that two cGAS1-DNA1 complexes are just slightly less stable than the cGAS2-DNA2 complex and the energy barrier for the formation of cGAS2-DNA2 complex from two cGAS1-DNA1 complexes is high, suggesting that cGAS-DNA oligomerization is unfavored thermodynamically and kinetically in low concentration of cGAS and DNA. However, the formation of cGAS4-DNA2 complex from one molecule of cGAS2-DNA2 complex between cGAS and long DNA and two molecules of cGAS are energetically favored without energy barrier.

Polyampholyte gel is a perfect physics model to mimic condensed state of proteins. We have studied the hierarchical dynamics of polyampholyte gels by dynamic light scattering. In addition to the normal gel mode, which indicates the gel elasticity, we also discovered a new mode with a stretched exponential decay with the stretched exponent {beta} = 1/3, and a diffusive exponential decay, which indicates the coupled motion between counterion and the polyampholyte backbone. After dialysis to low salt concentration, the coupled motion of the counterion will go away, so that there are only two modes. Combined with a newly developed theory, we attribute this stretched exponential mode to hierarchical dynamics of the segments between two crosslinking junctions, whose segmental distribution obeys Poisson distribution. As salt concentration inside the gel increases, {beta} decreases from 0.38 to 0.33, which is consistent with theoretical results. The gel with the molar charge ratio R=1, which is at the charge balance point, has the highest value {beta} = 0.38. As long as R deviates further away from the charge balance point from either side, the {beta} values decrease. When the gel is 100% positive charged, their dynamic light scattering results will go back to that of the normal polyelectrolyte gels.

HMGB1, a nuclear DNA-binding protein, can be secreted by activated immune cells or passively released from damaged cells. In such cases, HMGB1 functions as an alarmin that activates the immune system. Excessive inflammation may lead to pathogenesis, whereas this response can be dampened by polyanion binding, which impedes further receptor recognition. Moreover, HMGB1 is known to form liquid droplets in the cellular environment--a phase separation directly linked to its proper function. While the A-Box domain is believed to be primarily responsible for heparin binding due to its conserved binding site, the association and phase separation behavior of HMGB1 may be mediated by the B-box domain, owing to its extended hydrophobic regions. In this study, we first demonstrated that the B-box protein forms 30-nm large self-associates while maintaining its structure. Next, using molecularly sensitive EPR spectroscopy, we showed that the presence of these protein associations significantly enhances interactions with heparin. Notably, the local conformational changes induced by heparin are similar in both individual protein chains and their self-associated forms. To explain this effect, AlphaFold modeling was employed, revealing that the formation of protein multimers induces charge redistribution, resulting in an extended positively charged region that enhances electrostatic attraction to negatively charged polyanions, such as heparin.

Exoelectrogenesis is the ability of living cells to export electrons. In photosynthetic organisms, exoelectrogenesis is of particular interest because it can be used for transduction of solar energy into electric current in biophotovoltaics or reducing power for biocatalysis. Previously, we identified and characterized the green microalga Parachlorella kessleri MACC-38 producing unprecedented current largely dependent on the photosynthetic electron transport chain (PETC). In this study, our comparative photosynthetic characterization demonstrates that chlororespiration is a crucial factor maintaining the PETC redox balance in MACC-38. Our data points that the oxidative pentose phosphate pathway (OPPP) is activated during exoelectrogenesis to meet the increased demand of reducing power. We hypothesize that chlororespitration prevents oversaturation of PETC during OPPP activation. These findings provide valuable insights into the fundamental mechanisms of exoelectrogenesis in green algae. By elucidating the complex interconnection between PETC, OPPP and exoelectrogenesis, our results pave the way towards improved bioelectrochemical and biocatalysis technologies.

Zeaxanthin (Zea) is a key component in the energy-dependent, rapidly reversible, non-photochemical quenching process (qE) that regulates photosynthetic light harvesting. Previous transient absorption (TA) studies suggested that Zea can participate in direct quenching via Chlorophyll (Chl) to Zea energy transfer. However, the contamination of intrinsic exciton-exciton annihilation (EEA) makes the assignment of TA signal ambiguous. In this study, we present EEA-free TA data using Nicotiana benthamiana thylakoid membranes, including wild type and three NPQ mutants (npq1, npq4, and lut2) generated by CRISPR/Cas9 mutagenesis. Results show a strong correlation between excitation energy transfer from excited Chl Qy to Zea S1 and the xanthophyll cycle during qE activation. Notably, a Lut S1 signal is absent in the npq1 thylakoids which lack zeaxanthin. Additionally, the fifth-order response analysis shows a reduction in the exciton diffusion length (LD) from 55 {+/-} 5 nm to 38 {+/-} 3 nm under high light illumination, consistent with the reduced range of exciton motion being a key aspect of plants response to excess light.

Photosystem II uses water as the ultimate electron source of the photosynthetic electron transfer chain. Water is oxidized to dioxygen at the Oxygen Evolving Complex (OEC), a Mn4CaO5 inorganic core embedded in the lumenal side of PSII. Water-filled channels are thought to bring in substrate water molecules to the OEC, remove the substrate protons to the lumen, and may transport the product oxygen. Three water-filled channels, denoted large, narrow, and broad, that extend from the OEC towards the aqueous surface more than 15 [A] away are seen. However, the actual mechanisms of water supply to the OEC, the removal of protons to the lumen and diffusion of oxygen away from the OEC have yet to be established. Here, we combine Molecular Dynamics (MD), Multi Conformation Continuum Electrostatics (MCCE) and Network Analysis to compare and contrast the three potential proton transfer paths during the S1 to S2 transition of the OEC. Hydrogen bond network analysis shows that the three channels are highly interconnected with similar energetics for hydronium as calculated for all paths near the OEC. The channels diverge as they approach the lumen, with the water chain in the broad channel better interconnected that in the narrow and large channels, where disruptions in the network are observed at about 10 [A] from the OEC. In addition, the barrier for hydronium translocation is lower in the broad channel, suggesting that a proton from the OEC could access the paths near the OEC, and likely exit to the lumen via the broad channel, passing through PsbO.

Liquid-liquid phase separation of biomolecules is crucial for maintaining the functional organization in biological systems. Intrinsically disordered proteins are particularly prone to form phase-separated condensates in response to various physicochemical triggers. While the effect of ionic strength and temperature on phase separation dynamics have been studied extensively, the influence of pH is less explored. Here, we study a model glycine-rich protein present in the tick bioadhesive, given its capability to undergo phase separation. After confirming its disordered nature through spectroscopy, we investigated its pH dependence and underlying molecular mechanisms. Our findings reveal that pH significantly influences the protein hydrophobicity via ionic residues, driving notable variations in the coacervation behavior (propensity, progression) and in shaping the material properties (viscosity, interfacial activity) of the formed condensates. Given the ubiquitous presence of disordered proteins in biology, this study provides valuable insights about the broad implications of the pH-dependent behavior of intrinsically disordered proteins.

The dynamics of the aggregated light-harvesting complex (LHCII) associated with its antennae pigments can be crucial for a transition between light harvesting and dissipative states pivotal for non-photochemical quenching (NPQ). To this end, aggregation of chlorophyll-a (CLA) without the LHCII and pigment binding LHCII monomers in the plant thylakoid membranes have been investigated using coarse-grained molecular dynamics simulations at 293 K. Both CLA without the LHCII and pigment-binding LHCII monomers dynamically form and break dimers and higher-order aggregates in thylakoids within the simulation time. The contact lifetime and waiting time distributions of CLA dimers exhibit multiple time scales including most populated fast time scales and less populated slow time scales. The survival probability of CLA dimer in the absence of the LHCII follows a non-exponential decay with multiple residence time scales, leading to a time-dependent rate, unlike conventional rate theory. Such non-exponential decay of survival manifests the emergence of dynamic disorder in CLA without the LHCII resulting from the coupling between time scales of dimer formation and higher-order aggregates. The conformational fluctuations of the LHCII known for inter-CLA coupling variation occur on multiple time scales comparable to the LHCII dimer residence time scales leading to less probable but comparable and more probable slower inter-CLA fluctuations. This indicates the dynamic coupling in the LHCII conformations and their aggregates with the antennae pigments can result in dynamic disorder which will be highly relevant for the light-harvesting efficiency and regulation of NPQ. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=111 SRC="FIGDIR/small/638782v1_ufig1.gif" ALT="Figure 1"> View larger version (57K): org.highwire.dtl.DTLVardef@5fd895org.highwire.dtl.DTLVardef@8456cforg.highwire.dtl.DTLVardef@5f5c69org.highwire.dtl.DTLVardef@abf6d0_HPS_FORMAT_FIGEXP M_FIG C_FIG

Thermodynamics and structural transitions on protein surfaces remain relatively understudied and poorly understood. Wrapping of DNA on proteins provides a paradigm for studying protein surfaces. We used magnetic tweezers to investigate a prototypical DNA-interacting protein, i.e., the single-stranded DNA binding protein (SSB). SSB binds DNA with distinct binding modes the mechanism of which is still elusive. The measured thermodynamic parameters relevant to the SSB-DNA complex are salt-dependent and discontinuous at the bind-mode transitions. Our data indicate that free SSB undergoes salt-induced first-order structural transitions. The conclusion was supported by the infrared spectroscopy of SSB in salt solutions. Ultrafast infrared spectroscopy further suggests that the transitions are correlated with surface salt bridges. Our work not only unravels a long-standing mystery of the different binding site sizes of SSB, but also would inspire interests in thermodynamics of protein surfaces.

Protein-DNA complexes are stabilized by various interactions forming an interaction network between the protein and DNA molecules. Any change in the system - whether through mutations in the protein or DNA, external factors, or protein conformational transitions -- can alter this interaction network, thereby affecting structural and functional aspects. Employing all-atom classical molecular dynamics, we investigated how the interaction network in the ZTA TF-DNA is rewired when key arginine residues in ZTA are mutated to oppositely charged glutamic acids. Using the MMPBSA technique, we calculated per-residue binding energies for all systems and correlated binding affinity with structural features. Our detailed mechanistic study shows that when key arginine residues are mutated, new interactions are formed either around the mutation site and/or in other ZTA monomer. Through load-sharing, the system attempts to counter-balance the interaction load, leading to reorganization of the interaction network. As the number of mutations increases from single-to-double site, the system is able to partially maintain its structural stability. However, with multi-site mutations, even after reorganization of the interaction network, system cannot sustain its structural stability and therefore becomes destabilized. Despite the structural symmetry of the ZTA TF, we observed asymmetric monomer contributions upon mutation. Overall, our rigorous mechanistic studies provide deeper insights into the mechanism of interaction network reorganization in ZTA-DNA system. These comprehensive insights may be useful for tuning binding affinity and structural adaptability under adverse conditions. Since ZTA is a key factor in the Epstein-Barr virus (EBV), this study will be central to understanding DNA recognition and developing drug therapeutics targeting viral transcription factors in EBV.

There has been a surge in antibiotic resistance in recent years, making traditional antibiotics less effective against key pathogens. RNA has recently emerged as a potential target for antibiotics due to its involvement in crucial microbial functions. It is possible to expand the range of therapeutic targets by using RNA-based therapies, but it remains necessary to improve the molecular-level understanding of interactions between RNA and known and potential binders. The SAM-I riboswitch, which controls the transcriptional termination of gene expression involved in sulfur metabolism in most bacteria, is an excellent ligand target. Thus, understanding its behavior with and without ligand complexes would be very helpful for drug design applications. In this manuscript, we studied the interactions between the SAM-I riboswitch and its natural ligand, SAM, which controls riboswitch function, and compared those interactions to its interactions with the very similar small molecular SAH, which does not control riboswitch function, and to its interactions with a potential binder JS4, identified via virtual screening. From our simulations, we gain a deeper understanding of small molecule interactions with the SAM-I riboswitch. The results reveal how differently the small molecules (SAM, SAH and JS4) bind to and potentially induce conformational changes in the riboswitch. Our findings offer valuable insight into the molecular mechanisms underlying riboswitch RNA-ligand interactions for the design of more effective RNA-targeting therapeutics.

Coupling between the fluctuations of DNA and its surroundings consisting of water and ions in solution remains poorly understood and relatively less investigated as compared to proteins. Here, with the help of molecular dynamics simulations and statistical mechanical analyses, we explore the dynamical coupling between DNA, water, and counterions through correlations among respective energy fluctuations in both double (ds-) and single-stranded (ss-) DNA solutions. Fluctuations in the collective DNA-Water and DNA-Ion interaction energies are found to be strongly anti-correlated across all the systems. The fluctuations of DNA self-energy, however, are weakly coupled to DNA-water and DNA-ion interactions in ds-DNA. An enhancement of the DNA-water coupling is observed in ss-DNA, where the system is less rigid. All the interaction energies exhibit 1/f noise in their energy power spectra with surprisingly prominent bimodality in the DNA-water and DNA-ion fluctuations. The nature of the energy spectra appears to be indifferent to the relative rigidity of the DNA. We discuss the role of the observed correlations in ion-water motions on a DNA duplex in the experimentally observed anomalous slow dielectric relaxation and solvation dynamics, and in furthering our understanding of the DNA energy landscape. TOC GRAPHIC O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=144 SRC="FIGDIR/small/520228v1_ufig1.gif" ALT="Figure 1"> View larger version (42K): org.highwire.dtl.DTLVardef@806797org.highwire.dtl.DTLVardef@110c9corg.highwire.dtl.DTLVardef@1531566org.highwire.dtl.DTLVardef@1a1fe12_HPS_FORMAT_FIGEXP M_FIG C_FIG

Intermolecular spin relaxation by translational motion of spin pairs have been widely used to study properties of the biomolecules in liquids. Notably, solvent paramagnetic relaxation enhancement (sPRE) arising from paramagnetic cosolutes has gained attentions for various applications, including the structural refinement of intrinsically disordered proteins, cosolute-induced protein denaturation, and the characterization of residue-specific effective near-surface electrostatic potentials (ENS). Among these applications, the transverse sPRE rate known as {Gamma} 2 has been predominantly been interpreted empirically as being proportional to <r-6>norm. In this study, we present a rigorous theoretical interpretation of {Gamma} 2 that it is instead proportional to <r-4>norm and provide explicit formula for calculating <r-4>norm without any adjustable parameters. This interpretation is independent of the type or strength of interactions and can be broadly applied, including to the precise interpretation of ENS.