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.
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