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Chemical equilibrium amino acid equilibria

QM/EFPl scheme was used for investigating a variety of chemical processes in aqueous environment, including chemical reactions, amino acid neutral/zwitterion equilibrium, solvent effects on properties of a solute such as changes in dipole moment and shifts in vibrational spectrum, and solvatochromic shifts of electronic levels [36, 56, 59-60, 71-79]. Applications of a general QM/EFP scheme were limited so far to studies of electronic excitations and ionization energies in various solvents [56-58]. Extensions of QM/EFP to biological systems have been also developed [80-85]. [Pg.168]

If a sample contains groups that can take up or lose a proton, (N//, COO//), then one must expect the pH and the concentration to affect the chemical shift when the experiment is carried out in an acidic or alkaline medium to facilitate dissolution. The pH may affect the chemical shift of more distant, nonpolar groups, as shown by the amino acid alanine (38) in neutral (betaine form 38a) or alkaline solution (anion 38b). The dependence of shift on pH follows the path of titration curves it is possible to read off the pK value of the equilibrium from the point of inflection... [Pg.60]

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. [Pg.191]

The isolation of an SCP protein from rat liver homogenates has also been reported (S2). This protein has been found to be heat-labile, to be detectable only in the liver, and to have a molecular weight of approximately 50,000 daltons by gel filtration (S2) and 28,000 daltons by sedimentation equilibrium (S3). Although the functional properties of the heat-labile SCP (SI) are similar to the heat-stable SCP (R2, R3), these proteins appear to be different. According to Scallen et al. (S3), their SCP preparation resembles chemically serum LDL this based on the similarity in amino acid composition between these two proteins. In the... [Pg.135]

Recently, a simple solubilization theory has been developed to predict the equilibrium distribution of zwitterionic amino acids from information of the initial conditions of the system. This theory is based on the chemical and electrostatic interactions between the amino acids and active reverse micellar interface. The predictions of the model are in excellent agreement with the experimental results [201]. [Pg.157]

Write the chemical reactions whose equilibrium constants are Kbl and Kb2 for the amino acid proline. Find the values of Kbl and Kbl. [Pg.196]

Scheme 37 Second and third stages of the evolution of amino acid activation (2) The system of Scheme 39 is improved by enzyme catalysis the mixed anhydride is stabilized by binding making the equilibrium with NCA more favorable (3) an alternative pathway for nucleoside triphosphate synthesis has been introduced, the chemical flux has been reverted... Scheme 37 Second and third stages of the evolution of amino acid activation (2) The system of Scheme 39 is improved by enzyme catalysis the mixed anhydride is stabilized by binding making the equilibrium with NCA more favorable (3) an alternative pathway for nucleoside triphosphate synthesis has been introduced, the chemical flux has been reverted...

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See also in sourсe #XX -- [ Pg.148 ]




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