Ligand-induced biphasic thermal denaturation of RNAase A

DSC measurements have been accomplished in aqueous solutions of bovine pancreatic ribonuclease A (RNAase A) in the presence of subsaturating amounts of 3′ cytidine monophosphate (3′ CMP) and 2′ cytidine monophosphate (2′ CMP) atpH 5.0 and 5.5. In these conditions the experimental profiles do not conform to a one-step unfolding process. It can be emphasized, as a general phenomenon, that a strong linkage between the temperature-induced protein unfolding and the ligand binding, when the ligand is less than the saturation level, causes marked distortions from a two-state transition. A purely equilibrium thermodynamic analysis gives a correct account of this behaviour and allows to simulate calorimetric curves. It is thus possible to obtain, in an indirect manner, information about the thermodynamic parameters concerning the binding process, namely the association constant and the binding enthalpy. The values ofKb and Δ_b H for 3′CMP and 2′CMP, so determined, are consistent with the literature data.     

Structure and magnetic properties of nitroxide molecular crystals by density functional calculations employing periodic boundary conditions.

The structure and magnetic properties of one-dimensional chains of representative nitroxides have been studied by a density functional model employing periodic boundary conditions. The optimized geometries are in better agreement with experiments than those obtained from optimizations of model dimeric systems. The spin populations and isotropic hyperfine couplings compare well with the values measured by polarized neutron and electron spin resonance experiments. Magnetic couplings computed by the broken symmetry approach reproduce the ferro- or antiferromagnetic behavior of different nitroxides derived from experiments. These results point out the reliability of the computational model and the significant tuning of all the magnetic properties by intermolecular hydrogen bridges.     

Bounded charges and their ranges

Sunto Si caratterizzano le masse non continue e non atomiche, definite su un campo di insiemi e le masse definite su un -campoA che verificano la proprietá: (A T(A)) è chiuso, per ogni A A. Si dà inoltre, per tali masse, la struttura di (A T(A)), provando che tale insieme numerico è simile al codominio di una misura limitata.

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Research supported by M.P.I. and C.N.R.     

New hints on the Ph-driven tautomeric equilibria of the topotecan anticancer drug in aqueous solutions from an integrated spectroscopic and quantum-mechanical approach

The equilibria between the different forms of the topotecan anticancer drug have been studied at moderately acidic and physiological pH by an integrated computational tool rooted in the density functional theory and its time-dependent extension together with the polarizable continuum model. The results allow an unbiased selection between the different possible tautomeric forms and provide invaluable complements to experimental data. The ultraviolet-visible topotecan spectrum, recorded at moderately acidic pH, is accurately reproduced only by TD-DFT computations including solvent effects. Comparison of the experimental and calculated bands of the UV-vis spectrum at physiological pH indicates the presence of an equilibrium among different forms that is tuned by the microenvironment embedding the drug. The quantitative agreement between TD-DFT/PCM computations and experiments allows the identification of unequivocal spectroscopic signatures for different forms of topotecan.     

Vibrational zero-point energies and thermodynamic functions beyond the harmonic approximation

This paper compares harmonic and anharmonic zero-point energies and thermodynamic functions for a number of molecules of small and medium size. Anharmonic corrections cannot be neglected for quantitative studies, but can be obtained quite effectively by a perturbative treatment including cubic force constants to the second order and semidiagonal quartic constants to the first order. Simple finite difference equations provide all the necessary terms by at most 6N-11 Hessian evaluations, where N is the number of atoms in the system. Accurate values are obtained by this method using the Becke three parameter Lee-Yang-Parr functional, medium size basis sets, and, when needed, proper treatment of internal rotations. The whole model has been completely automated in the Gaussian package.     

A theoretical study of the nitrogen—graphite system

The chemisorption of atomic and molecular nitrogen on the basal plane of graphite has been studied by both semiempirical and non-empirical Hartree—Fock methods employing finite cluster models.

Geometric and energetic parameters obtained by semiempirical and non-empirical methods are generally in good agreement. Among the different chemisorption sites considered, nitrogen atoms are found to prefer a two-fold coordination site, and only in this case has any evidence been found for an exothermic process. No evidence of dissociative chemisorption has been found in the case of molecular nitrogen, whose preferred orientation changes from nearly vertical to nearly horizontal on approaching the surface. Molecular adsorption is strongly endothermic at the Hartree—Fock level, so that the only possible interaction occurs through dispersion forces, which are not taken into account in our computations, but can be well represented by model potentials.     

Hydrophobic effect in aqueous solutions of nonelectrolytes. II. Cross-interactions of alkylureas

The heats of mixing of the aqueous solutions of monomethylurea, monoethylurea, 1,3-dimethylurea, 1,3-diethylurea were determined at 25°C. The excess enthalpies obtained are expressed in virial form, in terms of pair and triplet interaction coefficients. A method previously discussed for predicting the excess enthalpies and the interaction coefficients of ternary solutions has been applied to the alkylureas. The large deviations between the experimental and calculated values, led to the modification of that method, which better accounts for the marked mixed character of these substances.     

An ab initio reinvestigation of the geometric and electronic structure of boron trioxide

In an attempt to resolve the controversy over the structure of the boron trioxide (B₂O₃) molecule an ab initio molecular orbital study employing a minimal STO-3G basis set with complete geometry optimization is reported. Our results indicate that B₂O₃ is a planar “w” shaped molecule with a rather small inversion barrier around the central atom and a quite important coupling between BOB and OBO angles. The computed bond distances are consistent with previous electron diffraction results, whereas the apex angle is in better agreement with the most recent IR study. The results obtained by the MNDO method are in good agreement with the ab initio ones. The electronic structure of B₂O₃ is discussed by means of Walsh diagrams and Mulliken population analysis.