Structural and vibrational study of the Ci(1) conformation of 18-crown-6.
Optimized geometry, vibrational frequencies and IR absorption intensities were calculated for the Ci(1) conformation of 18c6. Optimized geometry was compared with the experimental geometry and that of the Ci(2) conformation of 18c6. The Ci(1) conformation is more planer than the Ci(2) conformation of 18c6, which provides an explanation that free 18c6 in the crystal phase has a Ci(1) conformation. Vibrational frequencies were scaled using an unvaried one-scale-factor scaling of 0.928. Calculated vibrational frequencies were compared with the experimental vibrational frequencies. The root-mean-square deviation of the difference between the calculated and experimental frequencies was only 15 cm(-1). This excellent agreement between the calculated and experimental frequencies is an indication of the proper assignment of the fundamental vibrational frequencies of the Ci(1) conformation of 18c6.
Solvation structures of the 18-crown-6 in carbon tetrachloride as studied by Monte-Carlo simulation based on ab initio potential models
Abstract Solvation structure of the 18-crown-6 molecule in carbon tetrachloride (CCl 4 ) was investigated by Monte-Carlo simulation technique using ab initio pair potential function. The 18-crown-6–CCl 4 and the CCl 4 –CCl 4 pair potential functions were constructed from sets of interaction energies computed quantum mechanically at the HF/D95** and MP2/6-31G** levels. The simulation was carried out for a system consisting of 259 CCl 4 molecules plus one 18-crown-6 molecule. The simulation results reveal that there are two CCl 4 molecules in the first solvation shell; one lies above the ligand plane and other lies below it. The CCl 4 molecules prefer to approach closer to the O atoms than the C atoms of the 18-crown-6 molecule. The entire solvation structure around the 18-crown-6 was observed even the 18-crown-6–CCl 4 interaction is rather weak.
New method for analysis of enzymatic reactions: Thermolysin-catalyzed synthesis of aspartame precursor
Aspartame is an artificial sweetener which is 200 times sweeter than sucrose. A precursor of aspartame, N-(benzyloxycarbonyl)-l-aspartyl-l-phenylalanine methyl ester (Z-APM) was synthesized and hydrolyzed with thermolysin in an aqueous monophasic system. Graphical plotting methods and/or nonlinear regression have been used to analyze enzymatic reactions because of the nonlinearity of enzymatic rate equations. There are, however, problems: the former require accurate measurement of the initial reaction rate and the latter gives no analytical information. The authors proposed a method to solve the nonlinear enzymatic rate equations with respect to the concentration of a product with the Taylor expansion, especially for the reversible Theorell-Chance mechanism (Murakami, Y., Hirata, M., and Hirata, A., J. Ferment. Bioeng., 82, 246–252, 1996). This solution, however, is complex for practical use. In this paper, a simpler solution to explain the thermolysin-catalyzed synthesis of Z-APM is obtained. New methods to determine the kinetic parameters, especially for the reversible Theorell-Chance mechanism, based on the approximated solution are proposed. The results calculated using the obtained kinetic parameters can satisfactorily explain the experimental results, which demonstrates the effectiveness of the proposed analysis. We must know the relation between the concentrations of substrates and a product, and accurately evaluate of the result obtained using Taylor expansion when we determine the kinetic parameters using our newly developed methods. The methods to determine the kinetic parameters of the reversible Theorell-Chance mechanism described in this paper is, however, expected to be applied to general enzymatic reactions.