Due to the increasing applicability of ionic liquids (ILs) as different components of microemulsions (as the polar liquid, the oil phase, and the surfactant), it would be advantageous to devise a strategy by which we can formulate a microemulsion of our own interest. In this paper, we have shown how we can replace water from water-in-oil microemulsions by ILs to produce IL-in-oil microemulsions. We have synthesized AOT-derived surface-active ionic liquids (SAILs) which can be used to produce a large number of IL-in-oil microemulsions. In particular, we have characterized the phase diagram of the [C(4)mim][BF(4)]/[C(4)mim][AOT]/benzene ternary system at 298 K. We have shown the formation of IL-in-oil microemulsions using the dynamic light scattering (DLS) technique and using methyl orange (MO), betaine 30, and coumarin-480 (C-480) as probe molecules.

In photosystem II (PSII), water oxidation occurs at a MnCaO cluster and results in production of molecular oxygen. The MnCaO cluster cycles among five oxidation states, called S states. As a result, protons are released at the metal cluster and transferred through a 35 Å hydrogen-bonding network to the lumen. At 283 K, an infrared band at 2830 cm is assigned to an internal solvated hydronium ion via HO solvent exchange. This result is similar to a previous report at 263 K. Computations on an oxygen evolving complex model predict that chloride can stabilize a hydronium ion on a network of nine water molecules. In this model, a HO stretching mode at 2738 cm is predicted to shift to higher frequency with bromide and to lower frequency with nitrate substitution. The calculated frequencies were compared to S-minus-S reaction-induced Fourier transform infrared spectra acquired from chloride-, bromide-, or nitrate-containing PSII samples, which were active in oxygen evolution. As predicted, the frequency of the 2830 cm band shifted to higher energy with bromide and to lower energy with nitrate substitution. These results support the conclusion that an internal hydronium ion and chloride play a direct role in an internal proton transfer event during the S-to-S transition.

In photosynthesis, the light-driven oxidation of water is a sustainable process, which converts solar to chemical energy and produces protons and oxygen. To enable biomimetic strategies, the mechanism of photosynthetic oxygen evolution must be elucidated. Here, we provide information concerning a critical step in the oxygen-evolving, or S-state, cycle. During this S-to-S transition, oxygen is produced, and substrate water binds to the manganese-calcium catalytic site. Our spectroscopic and HO labeling experiments show that this S-to-S step is associated with the protonation of an internal water cluster in a hydrogen-bonding network, which contains calcium. When compared to the protonated water cluster, formed during a preceding step, the S-to-S transition, the S-to-S hydronium ion is likely to be coordinated by additional water molecules. This evidence shows that internal water and the hydrogen bonding network act as a transient proton acceptor at multiple points in the oxygen-evolving cycle.

In oxygenic photosynthesis, photosystem II (PSII) converts water to molecular oxygen through four photodriven oxidation events at a MnCaO cluster. A tyrosine, YZ (Y161 in the D1 polypeptide), transfers oxidizing equivalents from an oxidized, primary chlorophyll donor to the metal center. Calcium or its analogue, strontium, is required for activity. The MnCaO cluster and YZ are predicted to be hydrogen bonded in a water-containing network, which involves amide carbonyl groups, amino acid side chains, and water. This hydrogen-bonded network includes amino acid residues in intrinsic and extrinsic subunits. One of the extrinsic subunits, PsbO, is intrinsically disordered. This extensive (35 Å) network may be essential in facilitating proton release from substrate water. While it is known that some proteins employ internal water molecules to catalyze reactions, there are relatively few methods that can be used to study the role of water. In this Account, we review spectroscopic evidence from our group supporting the conclusion that the PSII hydrogen-bonding network is dynamic and that water in the network plays a direct role in catalysis. Two approaches, transient electron paramagnetic resonance (EPR) and reaction-induced FT-IR (RIFT-IR) spectroscopies, were used. The EPR experiments focused on the decay kinetics of YZ• via recombination at 190 K and the solvent isotope, pH, and calcium dependence of these kinetics. The RIFT-IR experiments focused on shifts in amide carbonyl frequencies, induced by photo-oxidation of the metal cluster, and on the isotope-based assignment of bands to internal, small protonated water clusters at 190, 263, and 283 K. To conduct these experiments, PSII was prepared in selected steps along the catalytic pathway, the S state cycle (n = 0-4). This cycle ultimately generates oxygen. In the EPR studies, S-state dependent changes were observed in the YZ• lifetime and in its solvent isotope effect. The YZ• lifetime depended on the presence of calcium at pH 7.5, but not at pH 6.0, suggesting a two-donor model for PCET. At pH 6.0 or 7.5, barium and ammonia both slowed the rate of YZ• recombination, consistent with disruption of the hydrogen-bonding network. In the RIFT-IR studies of the S state transitions, infrared bands associated with the transient protonation and deprotonation of internal waters were identified by DO and HO labeling. The infrared bands of these protonated water clusters, W (or nHO(HO), n = 5-6), exhibited flash dependence and were produced during the S to S and S to S transitions. Calcium dependence was observed at pH 7.5, but not at pH 6.0. S-state induced shifts were observed in amide C═O frequencies during the S to S transition and attributed to alterations in hydrogen bonding, based on ammonia sensitivity. In addition, isotope editing of the extrinsic subunit, PsbO, established that amide vibrational bands of this lumenal subunit respond to the S state transitions and that PsbO is a structural template for the reaction center. Taken together, these spectroscopic results su...

Photosystem II oxidizes water at a MnCaO cluster. Oxygen evolution is accompanied by proton release through a 35 Å hydrogen-bonding network to the lumen. The mechanism of this proton-transfer reaction is not known, but the reaction is dependent on chloride. Here, vibrational spectroscopy defines the functional properties of the proton-transfer network using chloride, bromide, and nitrate as perturbative agents. As assessed by peptide C═O frequencies, bromide substitution yields a spectral Stark shift because of its increase in ionic radius. Nitrate substitution leads to more complex spectral changes, consistent with an overall increase in hydrogen-bonding interactions with the peptide backbone. The effects are similar to spectral changes previously documented in site-directed mutations in a putative lumenal pathway. Importantly, the effects of nitrate are reversed by the osmolyte, trehalose. Trehalose is known to alter hydrogen-bonding interactions in proteins. Trehalose addition also reverses a shift in an internal hydronium ion signal, consistent with an alteration in its p K value and a change in the basicity of bound nitrate. The spectra provide evidence that the proton-transfer pathway contains peptide carbonyl groups, internal water, a hydronium ion, and amino acid side chains. These experiments also show that the proton-transfer pathway functionally adapts to changes in electric field, p K, and hydrogen bonding and thereby optimizes proton transfer to the lumen.