Application of ‘molecular’ theories to the structure of the crystalline state
Recent years have seen the development of simple molecular orbital based ideas to view the structures of molecular ring, cage and cluster compounds. The principles of these methods are used here to demonstrate in a new way how the structures of solids depend on electronic configuration.
Pyrite, marcasite, and arsenopyrite type minerals: Crystal chemical and structural principles
Quantitative molecular orbital (MO) calculations and qualitative perturbational MO arguments are used to interpret the spectra and structure of transition metal dichalcogenides and related compounds. Competition between pyrite (FeS2), marcasite (FeS2) and loellingite (FeAs2) structure types is explained in terms of the number of electrons occupying a set of MO's obtained from the mixing of dianion (A2) orbitals and metal (M) dσ orbitals. Direct metal-metal d orbital interaction is argued to be small. Attention is focused upon the M - A - M angles which differ substantially among the three structure types as a consequence of varying numbers of electrons in orbitals which closely resemble the pπ* orbitals of the dianions. Variations in M - A and A - A distances can also be understood in terms of the occupations of this set of MO's. Disulfide valence region photo-emission spectra are interpreted in terms of calculations on MS6 and S6 molecular clusters. M3d orbitals are found to progressively approach the S3p orbitals with increasing atomic number of M from Fe to Ni. For CuS2 comparison of calculation and experiment supports an approximate electron configuration of Cu+1 S2−1.
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.