Proton dynamics in an extended array of hydrogen bonds: normal coordinates, proton transfer and macroscopic quantum entanglement in the ground state

After a brief introduction to neutron scattering techniques, illustrated with the scattering function for harmonic oscillators, some new aspects of proton dynamics in the KHCO₃ crystal are presented. The full scattering function for the proton modes measured on single crystals provides a graphic view of proton dynamics. Vibrational states are fully characterized with three quantum numbers. The effective oscillator mass of 1 amu confirms the decoupling of protons from the lattice. Combining infrared, Raman and inelastic neutron scattering techniques, the double minimum potential for the transfer of a single proton along hydrogen bonds is totally determined. Elastic neutron scattering techniques probe dynamics in the fully degenerate ground state. Quantum entanglement arising from normal coordinates gives rise to quantum interference. With diffraction techniques, the dynamical structure arising from large-scale quantum coherence is observed as ridges of intensity, well separated from Bragg's peaks. The vibrational wave function in the ground state must be regarded as a superposition of non-factorable macroscopic wave function.     

Cysteine adsorption on the Au(111) surface and the electron transfer in configuration of a scanning tunneling microscope: A quantum-chemical approach

Adsorption of two forms, molecule and radical, of amino acid L-cysteine (Cys) on the Au₁₂ cluster that simulates the (111) face of single-crystal gold is studied in the framework of the density functional theory. Effects of solvation of adsorbed Cys particles and lateral interaction in a monolayer are analyzed. The simulation predicts a commensurate adsorption energetics of the molecule and radical, with a difference between the “on-top,” “hollow,” and “bridge” positions. An analysis of lateral electrostatic interactions points to the stability of a cluster comprising six Cys particles, which conforms to the size of a fragment observed experimentally. Adsorption calculations are used to build three-dimensional isosurfaces (STM images), where the tungsten needle of the scanning tunneling microscope is simulated by a tungsten atom or by small clusters. The calculated images are sensitive to both the Cys shape and the orientation of adsorbed Cys particles. Calculation results are compared with fresh in situ submolecular-resolution STM data. Simulated images (with commensurate contributions made by sulfur atom and amino group) built for Cys radical adsorbed in the “on-top” position give best conformance to experiment.     

Theoretical study of proton coupled electron transfer reaction in the light state of the AppA BLUF photoreceptor

The BLUF (blue light sensor using flavin adenine dinucleotide) domain is widely studied as a prototype for proton coupled electron transfer (PCET) reactions in biological systems. In this work, the photo-induced concerted PCET reaction from the light state of the AppA BLUF domain is investigated. To model the simultaneous transfer of two protons in the reaction, two-dimensional potential energy surfaces for the double proton transfer are first calculated for the locally excited and charge transfer states, which are then used to obtain the vibrational wave function overlaps and the vibrational energy levels. Contributions to the PCET rate constant from each pair of vibronic states are then analyzed using the theory based on the Fermi's golden rule. We show that, the recently proposed light state structure of the BLUF domain with a tautomerized Gln63 residue is consistent with the concerted transfer of one electron and two protons. It is also found that, thermal fluctuations of the protein structure, especially the proton donor-acceptor distances, play an important role in determining the PCET reaction rate. © 2018 Wiley Periodicals, Inc.     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

The proton‐coupled proton transfer mechanism,H2O catalysis,and hydrogen tunneling effects in the reaction of HNCH2 with HCOOH in the interstellar medium

The reaction HNCH₂ + HCOOH → H₂NCH₂COOH is supposed to be an important reaction related to the possible origin of amino acids on the early Earth. We find that it has an energy barrier of 87.37 kcal mol⁻¹ obtained with MP2/6‐311+G** in the gas phase, but it is likely enhanced to occur in the interstellar medium (ISM) through a proton‐coupled proton transfer reaction, initiated by HNCH₂ coupled with H₂⁺, H₃⁺, or H₃⁺O. H₂⁺, H₃⁺, and H₃⁺O serve as a donor of energy in the coupled reactions. H⁺, which is a key species to the coupled reactions, further, plays a catalytic role in reducing a barrier up to 14.14 kcal mol⁻¹. In the coupled reaction with H₃⁺O, H₂O, which can seize, transport, and deliver a proton from HCOOH to H₂NCH₂⁺, reduces a barrier up to 14.96 kcal mol⁻¹. A significant hydrogen‐tunneling pathway is predicted by the temperature dependences of k_H^CVT/SCT, calculated using the small curvature tunneling (SCT) approximation and canonical variational transition state theory (CVT). Hydrogen tunneling is another important mechanism to make the reaction happen in the ISM. The achieved results can be applied to discuss the origin of amino acids from the materials of the Earth itself. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Evaluation of the proton transfer kinetics of potential electrolytes in non-aqueous solutions using electrochemical techniques Part 1. Kinetic analysis of the general CE mechanism at stationary and rotating electrodes

The transport and kinetics of potential electrolytes (such as weak organic acids) at stationary and rotating electrodes have been examined in detail. A coherent mathematical analysis enabling the normalised current response to be evaluated has been developed, and various rate limiting scenarios have been identified and examined. Received: 13 March 1998 / Accepted: 31 July 1998     

Enthalpies of proton transfer of hydroxy amines and maltol in methanol-water mixtures by direct calorimetry

Enthalpies of proton transfer in methanol-water mixtures (0–90 wt. % methanol) have been measured by direct calorimetry for the bases 2-amino-2-methyl-1-propanol, 2-amino-2-methyl-1,3-propanediol and 2,2-bis(hydroxymethyl)-2,2,2-nitrilotriethanol at 25°C. Also reported are the data for 3-hydroxy-2-methyl-4-pyrone. The results are discussed in terms of an adaptation of classical electrostatic formula.     

Potential use of capillary zone electrophoresis in size characterization of quantum dots for environmental studies

Quantum dot (QD) nanoparticles (NPs) are of great interest to various researchers due to their wide range of applications, from photovoltaic sensitizers to in vivo fluorescent probes. There is a need to characterize environmental fate, degradation, and ecotoxicity of QDs because these NPs may be introduced into the environment upon disposal of waste products containing QDs following the anticipated increase in their production and use. Because the properties of QDs are defined primarily by their composition and size, it is imperative that QD size be measured accurately and quickly. Current methods for measuring the size of QDs tend to be relatively slow, require large amounts of sample and may not be suitable for environmental or biological samples. Capillary zone electrophoresis (CZE), with its inherently high separation efficiency based on the size-to-charge ratio of analytes, holds promise for efficient size determination of NPs in aqueous samples.This review examines the potential use of CZE in characterizing and separating QDs compared to the conventional methods employed in determining size distribution of NPs. We briefly discuss the advantages and the limitations of commonly used techniques for size characterization.In addition to published literature, we present results from our laboratory using CZE with laser-induced fluorescence (LIF) to examine the effect of natural organic matter and buffer composition on the electrophoretic mobility of QDs. The use of CZE in environmental studies can provide insights into the degradation and the potential impacts of QDs upon exposure to environmental and biological matrices.