Conformation And Properties Of Asparagine And Aspartic Acid In Gas Phase And Solution

The dissertation is devoted to the study of the conformation and properties of amino acids in gas phase and solution. With the progress in density functional theory (DFT) and its numerical methods, DFT based first-principles calculation has been widely used in quantum chemistry, biochemistry and material science. As the building blocks of proteins the conformational behavior of the natural amino acids is of special interest because it determines the functional specificity of proteins and polypeptides. In this dissertation, systematic and extensive conformational search has been performed for asparagine and aspartic acid in gas phase and solution and the properties of the stable conformers are studied using DFT based first-principles calculation. The concerned properties include geometry structure, IR spectra, conformer distributions, intramolecular interactions, ionization energies and electron affinities, protonation and deprotonation processes. Three solution models are also devoted to investigat solution effect to stabilize the zwitterionic structures of aspartic acid.In the first chapter, we introduce some methods and theories for quantum chemistry computation including Hartree-Fork Theory, Semiempirical Method, Density Functional Theory, Perturbation Theory and Coupled Cluster Theory. We also introduce three solution models including Discrete Model, Self-Consistent Reaction Field Model and Hybrid Method. At the end of the chapter, we introduce three software packages we used in the dissertation and the aim of the study.In chapter 2, we through search the stable conformers of asparagine and examine the properties of the stable conformers using ab initio and DFT methods. With an accessional acylamino the H-bond combinations in asparagine are complex and various types of H-bonds may coexist in one conformer. The H-bonds especially two linked H-bonds much improve the stabilization of the asparagine conformers and can also impact other properties of asparagine. The intramolecular interactions of asparagine coformers are characterized using AIM theory.In chapter 3, we systematically explore the conformational potential energy surfaces of aspartic acid in gas phase and calculate the concerned properties including geometry structure, IR spectra, conformer distributions, ionization energies and electron affinities. We explain the reason of conformational dependence of vertical ionization energy. We also study the protonation process of the aspartic acid and calculate the concerned thermodynamics data.In chapter 4, we focus on the properties of aspartic acid in solution. We through search the stable zwitterionic conformers in solution using Self-Consistent Reaction Field Model. We study the properties of aspartic acid such as ionization energies, electron affinities and IR spectra and compare with the results in gas phase. We also employed the discrete/SCRF and discrete models to examine the solvent effects including the vicinal and long range solvent effect in stabilizing the structures of zwitterionic conformers.In chapter 5, we focus on the properties of asparagine ions in gas phase. We calculate the vertical and adiabatic ionization energies, vertical and adiabatic electron affinities of asparagine conformers. The positions where the electron ionizes are quite different as the asparagine conformers with various H-bonds. Different intramolecular hydrogen bond interactions can also significantly change the components of several molecular orbits near HOMO orbital of the asparagine conformers, resulting in a pronounced difference in their vertical ionization energy values. We reveal the protonation and deprotonation processes of gaseous asparagine molecule. The calculated proton dissociation energy, gas-phase acidity, proton affinity and gas-phase basicity are in good agreement with the experiments.Chapter 6 provides a brief summary of the research results in the dissertation.