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How Water's Properties Are Encoded in Its Molecular Structure and Energies.
How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties
Simulaciones de sistemas acuosos: de la fase gas a la fase condensada
Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Química. Fecha de lectura: 21-11-2017La presente tesis está dedicada a la simulación de sistemas acuosos desde la fase gas hasta la fase
condensada. En la misma, se utilizaron enfoques y métodos complementarios para estudiar sistemas
acuosos homogéneos y heterogéneos. En particular, se ofrece un análisis detallado de las propiedades
estructurales, termodinámicas, espectroscópicas y de transporte en distintas condiciones termodinámicas
para estos sistemas. A lo largo de todo el trabajo, las comparaciones entre el experimento y la teoría
se establecieron sobre la base de la naturaleza de la interacción entre diferentes sistemas: Agua-Agua,
Ion-Agua y hospedador-huésped (agua). Así, el presente trabajo se ha dividido en tres partes principales.
En la primera parte, se realizaron simulaciones de dinámica molecular clásica en función de la temperatura
para estudiar y determinar las propiedades estructurales y de transporte (tanto individuales como
colectivas) del agua líquida. Hasta la fecha, la estimación de viscosidades a partir de simulaciones
representa un problema computacional desafiante ya que se requieren tiempos de simulación largos
para alcanzar precisión estadística, por lo que aquí se compararon varias estrategias de simulación
y también se validan diversos potenciales de interacción disponibles en la literatura. En la segunda
parte, se utilizaron cálculos de estructura electrónica de última generación para diseñar, desde un
enfoque bottom-up, superficies de energías de potencial analíticas de alta precisión. Dichos modelos de
interacción transferibles, son los primeros potenciales de ion-agua polarizables completamente ab-initio
para el estudio de electrolitos en diferentes entornos acuosos, por ejemplo, desde la microsolvatación
de monohidratos a polihidratos, así como soluciones a dilución infinita, y propiedades interfaciales. En
una colaboración con dos grupos experimentales (EEUU y UE), predecimos y validamos la dependencia
de la temperatura en el mecanismo de predisociación de un ion en contacto con dos moléculas de
agua mediante simulaciones de dinámica molecular mixtas clásico-cuánticas. Finalmente en la tercera
parte, estudiamos la encapsulación de átomos y moléculas dentro de las cavidades del clatrato hidrato
sI. Estas investigaciones estuvieron motivadas por la disponibilidad de mediciones experimentales a
partir de difracción de rayos X y espectros IR, así como de transiciones de fase observadas en el bulk.
Para ello, se tomaron como sistemas de referencia el hidrato clatrato de dióxido de carbono, y los
hidrato clatrato de gases nobles. En particular se llevaron a cabo cálculos cuánticos con el método
de “Multiconfigurational Time Dependent Hartree” para las dos cavidades de clatrato CO2@sI, y por
primera vez se presentan resultados sobre los estados traslacionales-rotacionales-vibracionales de dicho
sistema. Además, se comprobó el rendimiento de diferentes modelos de interacción analítica, así como
cálculos de estructura electrónica para describir la orientación rotacional y la anisotropía angular dentro
de ambas cavidades. De igual manera, se llevaron a cabo simulaciones clásicas de “parallel-tempering
Monte Carlo” en el ensamble isobárico-isotérmico (NPT) para agregados tipo clatratos con gases nobles
de tamaño seleccionado y se presentó un análisis detallado de sus diagramas de fase en temperatura y
presión, así como cambios estructurales en un amplio rango de presiones y temperatura.The present thesis is devoted to the simulations of aqueous systems from the gas to the condensed
phase. Here we used complementary approaches and methods to study both homogeneous and
heterogeneous aqueous systems. In particular, we provided a detailed analysis on their, structural,
thermodynamical, spectroscopical and transport properties at different thermodynamic conditions.
Along the whole work, comparisons between experiment and theory were established based on the
nature of the interactions between different systems. It was divided into three main parts corresponding
to: water-water, ion-water and guest-host(water network). In the first part, classical molecular dynamic
simulations were performed as a function of temperature, to study and determine the structural and
transport properties (both single and collective) of liquid water. Nowadays, the estimation of viscosities
from simulations is a challenging computational problem, as long simulation times are required to reach
statistical accuracy. So several simulation strategies were compared being able to validate interaction
model potentials available in the literature. In the second part, state-of-the-art electronic structure
calculations were employed to design, from a bottom-up approach, highly accurate analytical potential
energy surfaces. Such transferable interaction models are the first fully ab-initio polarizable ion-water
potentials for studying electrolytes at different aqueous environments i.e. from the microsolvation of
monohydrates, to polyhydrates, as well as solutions at infinite dilution, and interfacial properties. In a
collaboration with two experimental groups (USA and EU) we predict and validate the temperature
dependence vibrational predissociation mechanism of an ion in contact with two water molecules
by means of mixed quantum-classical molecular dynamic simulations. Finally in the third part, we
studied the encapsulation of atoms and molecules within the cavities of sI type clathrate hydrates.
These investigations were motivated by available experimental measurements from X-ray diffraction
and IR spectra, as well as observed phase transitions in the bulk. For such, we took as reference systems
the carbon dioxide clathrate hydrate and the rare gases (Rg) clathrate hydrates. In particular, we
performed quantum multi-configuration time-dependent Hartree calculations for the two cages of the sI
CO2 clathrate hydrate, and we reported for the first time results on the translational, rotational and
vibrational states. Additionally, we tested the performance of different analytical interaction models, as
well as electronic structure calculations for describing the rotational orientations and angular anisotropy
of the CO2 within both cages. Moreover, classical parallel-tempering Monte Carlo simulations in the
isobaric-isothermic (NPT) ensemble were carried out for size-selected Rg clathrate-like clusters and
we presented a detailed analysis of their temperature-pressure phase diagrams, as well as structural
changes in a wide range of temperatures and pressuresEste trabajo de investigación ha sido posible gracias a la concesión de una beca predoctoral BES2012-054209
enmarcada en el subprograma de ayudas de formación de personal investigador (FPI)
del gobierno español, a través del Ministerio de Economía, Industria y Competitividad, y asociada
al proyecto de investigación FIS2014-51933-P del CSIC
Mapping Free Energy Pathways for ATP Hydrolysis in the E. coli ABC Transporter HlyB by the String Method
HlyB functions as an adenosine triphosphate (ATP)-binding cassette (ABC) transporter that enables bacteria to secrete toxins at the expense of ATP hydrolysis. Our previous work, based on potential energy profiles from combined quantum mechanical and molecular mechanical (QM/MM) calculations, has suggested that the highly conserved H-loop His residue H662 in the nucleotide binding domain (NBD) of E. coli HlyB may catalyze the hydrolysis of ATP through proton relay. To further test this hypothesis when entropic contributions are taken into account, we obtained QM/MM minimum free energy paths (MFEPs) for the HlyB reaction, making use of the string method in collective variables. The free energy profiles along the MFEPs confirm the direct participation of H662 in catalysis. The MFEP simulations of HlyB also reveal an intimate coupling between the chemical steps and a local protein conformational change involving the signature-loop residue S607, which may serve a catalytic role similar to an Arg-finger motif in many ATPases and GTPases in stabilizing the phosphoryl-transfer transition state
An Effective Fragment Method for Modeling Solvent Effects in Quantum Mechanical Calculations
An effective fragment model is developed to treat solvent effects on chemical properties andreactions. The solvent, which might consist of discrete water molecules, protein, or othermaterial, is treated explicitly using a model potential that incorporates electrostatics,polarization, and exchange repulsion effects. The solute, which one can most generally envision as including some number of solvent molecules as well, is treated in a fully ab initio manner, using an appropriate level of electronic structure theory. In addition to the fragment model itself, formulae are presented that permit the determination of analytic energy gradients and, therefore, numerically determined energy second derivatives (hessians) for the complete system. Initial tests of the model for the water dimer and water‐formamide are in good agreement with fully abinitio calculations
The Molecular Electrostatic Potential as a Determinant of Receptor-Drug Recognition
The validity of the concept of the molecular electrostatic .
potential and its applicability in rationalizing drug-receptor interactions
are discussed on hand of examples covering the qualitative
aspects. The computational methods are briefly reviewed with
respect to economy and quality of results
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