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The gas-phase geometries, binding energies (BEs), vibrational spectra, and electron density topological features of methanol (M), water (W), and methanol-water mixed clusters (M(m)W(n), where m = 0-4 and n = 0-4; m + n < or = 4) have been calculated using Hartree-Fock, second-order Møller-Plesset perturbation, and density functional theory with Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional methods. Bader's "atoms in molecules" theory has been used to analyze the hydrogen bonding network. To understand the effect of cooperativity, we have performed natural bond orbital analysis and reduced variational space decomposition analysis. The results show that BEs of methanol and mixed clusters are higher than those of water clusters due to the electron-donating nature of the methyl group. These findings are in accordance with the role of cooperative polarization and cooperative charge transfer in the methanol and mixed clusters. As the size of the cluster increases, the contribution from the cooperative effects also increases. The cooperativity contributes approximately 14 and 24% of stabilization in trimers and tetramers, respectively. The calculated nu(OH) frequencies at MP2/6-311++G(d,p) are in close agreement with the corresponding experimental values.

CdTe quantum dots (QDs) were prepared in aqueous solution by the reaction between Cd2+ and H2Te in the presence of thioglycolic acid (TGA) as a stabilizer. In order to improve the quantum yield (QY), we systematically optimized the synthesis conditions and found that the pH value significantly affected the optical properties of CdTe QDs. After synthesizing different sizes of CdTe QDs, we have extended our observation to check the effect of pH level on different sizes of CdTe QDs. Experimental results reveal that the luminescence QY as well as the lifetime of TGA-capped CdTe QDs increased noticeably when the solution was made acidic up to a certain pH level. This observation is much more pronounced in the case of smaller size QDs. The optical spectroscopy studies imply that the pH-dependent behavior of the luminescence of CdTe QDs is caused by structural changes on the surface rather than the size of the QDs. Turbidity observed at low pH indicates the formation of aggregates of QDs. This is confirmed by steady-state spectroscopy and atomic force microscopy studies. We observed that both the luminescence intensity and absorbance changes were reversible when the pH level of the solution was brought back its original value.

The plasma membrane calcium-ATPase (PMCA) helps to control cytosolic calcium levels by pumping out excess Ca2+. PMCA is regulated by the Ca2+ signaling protein calmodulin (CaM), which stimulates PMCA activity by binding to an autoinhibitory domain of PMCA. We used single-molecule polarization methods to investigate the mechanism of regulation of the PMCA by CaM fluorescently labeled with tetramethylrhodamine. The orientational mobility of PMCA-CaM complexes was determined from the extent of modulation of single-molecule fluorescence upon excitation with a rotating polarization. At a high Ca2+ concentration, the distribution of modulation depths reveals that CaM bound to PMCA is orientationally mobile, as expected for a dissociated autoinhibitory domain of PMCA. In contrast, at a reduced Ca2+ concentration a population of PMCA-CaM complexes appears with significantly reduced orientational mobility. This population can be attributed to PMCA-CaM complexes in which the autoinhibitory domain is not dissociated, and thus the PMCA is inactive. The presence of these complexes demonstrates the inadequacy of a two-state model of Ca2+ pump activation and suggests a regulatory role for the low-mobility state of the complex. When ATP is present, only the high-mobility state is detected, revealing an altered interaction between the autoinhibitory and nucleotide-binding domains.

CdTe quantum dots (QDs) were synthesized in aqueous solution using thioglycolic acid (HS-CH2COOH, TGA) as a stabilizer. The phenomenon of "on" and "off" luminescence intermittency (blinking) of CdTe QDs in PVA and trehalose was investigated by single-molecule optical microscopy, and we identified that the intermittencies of single QDs were correlated with the interaction of water molecules absorbed on the QD surface. The "off" times, the interval between adjacent "on" states, remained essentially unaffected with an increase in excitation intensity. Every QD showed a similar power law behavior for the "off" time distribution regardless of the excitation intensity and aqueous environment of the QDs. In the case of "on" time distribution, power law behavior with an exponential cutoff tail is observed at longer time scales. The time traces indicated that the "on" time was inversely proportional to the excitation intensity; the duration of "on" time became shorter with increasing excitation intensity. An increase in the duration of "on" time was observed in trehalose with respect to that in PVA. We obtained a clear decrease in the power law exponent when PVA was replaced with trehalose. These observations indicate that the luminescence blinking statistics of water-soluble single CdTe QDs is significantly dependent on the aqueous environment, which is interpreted in terms of passivation of the surface trap states of QDs.

The important biosynthetic intermediate chorismate reacts thermally by two competitive pathways, one leading to 4-hydroxybenzoate via elimination of the enolpyruvyl side chain, and the other to prephenate by a facile Claisen rearrangement. Measurements with isotopically labeled chorismate derivatives indicate that both are concerted sigmatropic processes, controlled by the orientation of the enolpyruvyl group. In the elimination reaction of [4-2 H]chorismate, roughly 60% of the label was found in pyruvate after 3 h at 60 °C. Moreover, a 1.846±0.057 2 H isotope effect for the transferred hydrogen atom and a 1.0374±0.0005 18 O isotope effect for the ether oxygen show that the transition state for this process is highly asymmetric, with hydrogen atom transfer from C4 to C9 significantly less advanced than C-O bond cleavage. In the competing Claisen rearrangement, a very large 18 O isotope effect at the bond-breaking position (1.0482±0.0005) and a smaller 13 C isotope effect at the bond-making position (1.0118±0.0004) were determined. Isotope effects of similar magnitude characterized the transformations catalyzed by evolutionarily unrelated chorismate mutases from Escherichia coli and Bacillus subtilis. The enzymatic reactions, like their solution counterpart, are thus concerted [3,3]-sigmatropic processes in which C-C bond formation lags behind C-O bond cleavage. However, as substantially larger 18 O and smaller 13 C isotope effects were observed for a mutant enzyme in which chemistry is fully rate determining, the ionic active site may favor a somewhat more polarized transition state than that seen in solution.The unimolecular rearrangement of chorismate (1) to prephenate (2) is a key step in the biosynthesis of the aromatic amino acids tyrosine and phenylalanine (Scheme 1). Formally, this reaction can be described as a pericyclic Claisen rearrangement, a 3,3-sigmatropic process that is rarely exploited in cellular metabolism. Secondary tritium isotope effects have suggested that the uncatalyzed reaction is concerted but asynchronous, with C-O bond cleavage preceding C-C bond fomation, 1 while stereochemical studies have established a chair-like geometry for the transition state. 2 These conclusions are supported by RHF/6-31* calculations. 3Chorismate mutases accelerate the chorismate to prephenate rearrangement more than a million fold. 4 Crystallographic studies have shown that these enzymes fall into two structurally distinct classes. 5 The overwhelming majority of chorismate mutases belong to the AroQ family, whose * To whom correspondence should be addressed at cleland @enzyme.wisc.edu (W.W.C.) hilvert@org.chem.ethz.ch (D.H.) Supporting Information Available. Coordinates and computational details for calculation of the theoretical isotope effects, plus derivations of equations 2 and 3 (10 pages). This material is available free of charge via the Internet at http://pubs.acs.org. The origins of the catalytic rate enhancement achieved by chorismate mutases have been elusive. The enzymatic reactions, ...

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

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