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Photosystem II is a unique enzyme which catalyzes light induced water oxidation. This process is driven by highly oxidizing ensemble of four Chl molecules, PD1, PD2, ChlD1 and ChlD2 called, P680. Excitation of one of the Chls in P680 leads to the primary charge separation, P680+Pheo-. Pheo- transfers electrons sequentially to the primary quinone acceptor QA and the secondary quinone acceptor QB. P680+ in turn extracts electrons from Mn4CaO5 cluster, a site for the water oxidation. There are two redox active tyrosines, TyrZ and TyrD, found in PSII. They are symmetrically located on the D1 and D2 central proteins. Only TyrZ acts as intermediate electron carrier between P680 and Mn4CaO5 cluster, while TyrD does not participate in the linear electron flow and stays oxidized under light conditions. Both tyrosines are involved in PCET. The reduced TyrD undergoes biphasic oxidation with the fast (msec-sec time range) and the slow (tens of seconds time range) kinetic phases. We assign these phases to two populations of PSII centers with proximal or distal water positions. We also suggest that the TyrD oxidation and stability is regulated by the new small lumenal protein subunit, PsbTn. The possible involvement of PsbTn protein in the proton translocation mechanism from TyrD is suggested. To assess the possible localization of primary cation in P680 the formation of the triplet state of P680 and the oxidation of TyrZ and TyrD were followed under visible and far-red light. We proposed that far-red light induces the cation formation on ChlD1. Transmembrane interaction between QB and TyrZ has been studied. The different oxidation yield of TyrZ, measured as a S1 split EPR signal was correlated to the conformational change of protein induced by the QB presence at the QB-site. The change is transferred via H-bonds to the corresponding His-residues via helix D of the D1 protein.

Indigo N,N’-diarylimine (Nindigo) and indigo N-arylimine (Mindigo) are redox-active ligands which exhibit near-infrared absorption and can accommodate up to five ligand charge states. This dissertation explores the coordination chemistry of these ligands to further understand the role that metal-ligand combinations play on ligand-centered properties, which include electrochemical potentials, UV-Vis-NIR absorption, pKa values, hydricities, and NH bond strengths at different ligand charge states. A series of cis-Nindigo palladium complexes containing acetylacetonate (acac) and hexafluoroacetylacetonate (hfac) ligands were synthesized. The acac complexes were easier to oxidize by 0.11 to 0.16 V and absorbed at lower wavelengths compared to their hfac analogues. Complexes using indigo bis(4-methylphenylimine) were more easily reduced than complexes of indigo bis(2,6-dimethylphenylimine). Cis- and trans-Mindigo complexes of palladium acac and hfac were synthesized as the first coordination complexes of Mindigo. Trans-Mindigo complexes were more difficult to reduce by 0.33 to 0.37 V and absorbed at lower wavelengths than their cis-Mindigo counterparts. Cis-Mindigo complexes were easier to reduce and harder to oxidize than the corresponding cis-Nindigo complexes. The NH bond strengths of cis-Nindigo complexes containing Pd(acac) and Ru(bipy)2 (bipy = 2,2’-bipyridyl) fragments were determined through a potential-pKa diagram in tetrahydrofuran and acetonitrile, respectively. The NH bond strength and hydricity values of the Pd(acac) complex were comparable to the values of diaryl amines. The NH bond strength and hydricity of the Ru(bipy)2 complex were substantially smaller due to the lower oxidation potentials of this complex. In both cases, the ligand’s NH bond strengths were not affected greatly by the ligand’s charge state. Ru(acac)2 complexes of neutral, aprotic cis-Nindigo and cis-Mindigo ligands were synthesized. The Nindigo/Mindigo ligand could be protonated, and the resulting complexes demonstrated substantial temperature dependence of some of their 1H NMR chemical shifts. The NH bond strengths and hydricities of the Ru(acac)2 complexes were determined using cyclic voltammetry and pKa measurements. The NH bond strengths and hydricities of these complexes are substantially smaller than the Pd(acac) and Ru(bipy)2 complexes. Collectively, these results show that Nindigo and Mindigo can act as both a proton and electron reservoirs, and the thermodynamics of proton and electron transfer can be tuned through the choice of metal and ligand combinations. / Graduate / 2020-07-17

Photosystem II is unique! It remains the only enzyme that can oxidize water using light as energy input. Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P680+ via the tyrosine residue D1-Tyr161 (YZ). Favorable oxidation of YZ is coupled to a proton transfer along a hydrogen bond to the nearby D1-His190 residue, resulting in the neutral radical YZ•. By illuminating photosystem II at cryogenic temperatures, YZ• can be trapped in a stable state. Magnetic interaction between this radical and the CaMn4 cluster gives rise to a split electron paramagnetic resonance (EPR) signal with characteristics that depend on the oxidation state (S state) of the cluster. The mechanism by which the split EPR signals are formed is different depending on the S state. In the S0 and S1 states, split signal induction proceeds via a P680+-centered mechanism, whereas in the S2 and S3 states, our results show that split induction stems from a Mn-centered mechanism. This S state-dependent pattern of split EPR signal induction can be correlated to the charge of the CaMn4 cluster in the S state in question and has prompted us to propose a general model for the induction mechanism across the different S states. At the heart of this model is the stability or otherwise of the YZ•–(D1-His190)+ pair during cryogenic illumination. The model is closely related to the sequence of electron and proton transfers from the cluster during the S cycle. Furthermore, the important hydrogen bond between YZ and D1-His190 has been investigated by following the split EPR signal formation in the different S states as a function of pH. All split EPR signals investigated decrease in intensity with a pKa of ~4-5. This pKa can be correlated to a titration event that disrupts the essential hydrogen bond, possibly by a direct protonation of D1-His190.  This has important consequences for the function of the CaMn4 cluster as this critical YZ–D1-His190 hydrogen bond steers a multitude of reactions at the cluster.

Mechanistic studies of electrochemical proton coupled electron transfer (PCET) have attracted attention for many decades due to their importance in many fields ranging from electrocatalysis to biology. However, mechanistic research is confined to only a few groups, and challenges in this field can be found in both theory and experiment. The contributions to mechanistic studies of electrochemical PCET reaction in this thesis can be categorized under the following two headings: 1) mechanistic studies of an aminobenzoquinone modified monolayer system with multiple electron/proton transfer reaction; 2) studies that attempt to develop the relationship between thermochemical data and electrochemical PCET mechanism. An aminobenzoquinone modified monolayer showing nearly ideal electrochemical behavior and high stability was successfully prepared and used as a model system for the mechanistic study of electrochemical multiple electron/proton transfer. This model system has been proposed to undergo a 2e3H transfer at low pH electrolyte and a 2e2H transfer at high pH electrolyte. Two non-destructive electrochemical techniques (cyclic voltammetry and chronocoulmetry) have been applied for the measurement of apparent standard rate constant as a function of pH. Both pH dependent apparent formal potential and pH dependent apparent standard rate constant have been used to determine the charge transfer mechanism of this monolayer system. Under the assumption of an operative PCET mechanism (i.e. electron transfer step is the rate determining step), a theoretical description of this system has been developed based on the refinement and extension of previous models. By combining this extended theoretical model with pH dependent apparent formal potential and apparent standard rate constant, charge transfer pathways have been determined and shown to be consistent with the observed pH dependent electrochemical response, in addition, the determined pathways in this aminobenzoquinone modified monolayer are similar to previous reported pathways for benzoquinone freely dissolved in aqueous buffered electrolyte. A series of analytical expressions built in this thesis demonstrate that the parameters that differentiate stepwise mechanisms from concerted mechanisms can be classified into two aspects: thermodynamic parameters, namely acid dissociation constants, standard formal potentials; and kinetic parameters, namely standard rate constants, standard transfer coefficients. Although attempts to understand the relation between controlling parameters and electrochemical PCET mechanism (stepwise versus concerted) has been reported previously by some groups, there are still lots of unresolved aspects requiring further investigation. In this thesis, an important conclusion has been drawn which is that for the stepwise mechanism, an apparent experimentally observable kinetic isotope effect (KIE) can be induced by solvent isotope induced variation of acid dissociation constants, which contradicts previous understanding. Additionally, for the first time, values of apparent KIE, which were measured for the aminobenzoquinone modified monolayer system with stepwise PCET mechanism, were successfully explained by variation in acid dissociation constants, not by variation in standard rate constants. Based on theoretical prediction, a nitroxyl radical modified bilayer showing one electron one proton transfer reaction has been prepared in an effort to afford experimental verification. After applying similar analytical procedures as those for the aminobenzoquinone modified monolayer system, this bilayer system has been shown to follow the concerted 1e1H transfer pathway in high pH electrolytes. These latter contributions provide evidence that further development in this field will eventually lead to a comprehensive theory that can use known thermochemical variables to fully predict PCET mechanism.

Biomimetic peptides are autonomously folding secondary structural units designed to serve as models for examining processes that occur in proteins. Although de novo biomimetic peptides are not simply abbreviated versions of proteins already found in nature, designing biomimetic peptides does require an understanding of how native proteins are formed and stabilized. The discovery of autonomously folding fragments of ribonuclease A and tendamistat pioneered the use of biomimetic peptides for determining how the polypeptide sequence stabilizes formation of alpha helices and beta hairpins in aqueous and organic solutions. A set of rules for constructing stable alpha helices have now been established. There is no exact set of rules for designing beta hairpins; however, some factors that must be considered are the identity of the residues in the turn and non-covalent interactions between amino acid side chains. For example, glycine, proline, aspargine, and aspartic acid are favored in turns. Non-covalent interactions that stabilize hairpin formation include salt bridges, pi-stacked aromatic interactions, cation-pi interactions, and hydrophobic interactions. The optimal strand length for beta hairpins depends on the numbers of stabilizing non-covalent interactions and high hairpin propensity amino acids in the specific peptide being designed. Until now, de novo hairpins have not previously been used to examine biological processes aside from protein folding. This thesis uses de novo designed biomimetic peptides as tractable models to examine how non-covalent interactions control the redox properties of tyrosine in enzymes. The data in this study demonstrate that proton transfer to histidine, a hydrogen bond to arginine, and a pi-cation interaction create a peptide environment that lowers the midpoint potential of tyrosine in beta hairpins. Moreover, these interactions contribute equally to control the midpoint potential. The data also show that hydrogen bonding is not the sole determinant of the midpoint potential of tyrosine. Finally, the data suggest that the Tyr 160D2-Arg 272CP47 pi-cation interaction contributes to the differences in redox properties between Tyr 160 and Tyr 161 of photosystem II.

abstract: Sunlight, the most abundant source of energy available, is diffuse and intermittent; therefore it needs to be stored in chemicals bonds in order to be used any time. Photosynthesis converts sunlight into useful chemical energy that organisms can use for their functions. Artificial photosynthesis aims to use the essential chemistry of natural photosynthesis to harvest solar energy and convert it into fuels such as hydrogen gas. By splitting water, tandem photoelectrochemical solar cells (PESC) can produce hydrogen gas, which can be stored and used as fuel. Understanding the mechanisms of photosynthesis, such as photoinduced electron transfer, proton-coupled electron transfer (PCET) and energy transfer (singlet-singlet and triplet-triplet) can provide a detailed knowledge of those processes which can later be applied to the design of artificial photosynthetic systems. This dissertation has three main research projects. The first part focuses on design, synthesis and characterization of suitable photosensitizers for tandem cells. Different factors that can influence the performance of the photosensitizers in PESC and the attachment and use of a biomimetic electron relay to a water oxidation catalyst are explored. The second part studies PCET, using Nuclear Magnetic Resonance and computational chemistry to elucidate the structure and stability of tautomers that comprise biomimetic electron relays, focusing on the formation of intramolecular hydrogen bonds. The third part of this dissertation uses computational calculations to understand triplet-triplet energy transfer and the mechanism of quenching of the excited singlet state of phthalocyanines in antenna models by covalently attached carotenoids. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2016

abstract: Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2020

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This thesis presents the results and discussion of the investigation of the effects of Brönsted bases on the kinetics and thermodynamics of two proton-coupled electron transfer processes: the mediated oxidation of glutathione and the electrochemical oxidation of hydroquinone. Proton-coupled electron transfer (PCET) is the name given to reactions that involve the transfer of electron(s) accompanied by the exchange of proton(s). PCETs are found in many chemical and biological processes, some of current technological relevance such as the oxygen reduction reaction in fuel cells, which involves the transfer of four electrons and four protons (4e-, 4H+); or the splitting of water into protons (4H+), electrons (4e-) and oxygen (O2) efficiently achieved in photosynthesis. The study of PCET mechanisms is imperative to understanding biological processes as well as to developing more efficient technological applications. However, there are still many unanswered questions regarding the kinetic and thermodynamic performance of PCETs, and especially about the effect of different proton acceptors on the rate and mechanism of PCET reactions. This study aimed to investigate the effect of Brönsted bases as proton acceptors on the kinetics and thermodynamics of two model PCET processes, the oxidation of glutathione and hydroquinone. The analysis presented in this thesis provides insight into the influence of different proton acceptors on the mechanism of PCET and it does so by studying these reactions from a different angle, that one of the acid-base catalysis theory which has been successfully applied to the investigation of numerous chemical reactions coupled to proton transfer. We hope future research of PCETs can benefit from the knowledge of acid-base catalysis to better understand these reactions at a molecular level.