Equilibrium geometries of low-lying isomers of some Li clusters, within Hartree-Fock theory plus bond order or MP2 correlation corrections.

In a recent study by Kornath et al. [J. Chem. Phys. 118, 6957 (2003)], the Li(n) clusters with n=2, 4, and 8 have been isolated in argon matrices at 15 K and characterized by Raman spectroscopy. This has prompted us to carry out a theoretical study on such clusters up to n=10, using Hartree-Fock theory, plus low-order Møller-Plesset perturbation corrections. To check against the above study of Kornath et al., as a by-product we have made the same approximations for n=6 and 8 as we have for n=10. This has led us to emphasize trends with n through the Li(n) clusters for (i) ground-state energy, (ii) HOMO-LUMO energy gap, (iii) dissociation energy, and (iv) Hartree-Fock eigenvalue sum. The role of electron correlation in distinguishing between low-lying isomers is plainly crucial, and will need a combination of experiment and theory to obtain decisive results such as that of Kornath et al. for Li(8). In particular, it is shown that Hartree-Fock theory plus bond order correlations does account for the experimentally observed symmetry T(d) symmetry for Li(8).

Elucidation of the chiral recognition mechanism of cinchona alkaloid carbamate-type receptors for 3,5-dinitrobenzoyl amino acids.

A cinchona alkaloid having extraordinary chiral discriminatory powers (alpha = 32.6 for dinitrobenzoyl leucine) is developed as a chiral stationary phase (CSP) for chromatography. An explanation of how chiral discrimination takes place is presented. Using a soluble analogue of the CSP, we found that NMR spectrometry indicates that 1:1 complexes exist for both optical isomers interacting with the CSP, that the free base form of the CSP exists in an open/closed ratio of 35/65 but that the protonated, bound-state form is exclusively in the anti-open conformation, and that significant intermolecular NOEs exist for the more stable diastereomeric complex but not for the less stable complex. Stochastic molecular dynamics simulations were carried out in solvents of low and high dielectric. The chromatographic retention orders and free energy differences of analyte binding to CSP were reproduced computationally as were the observed intra- and intermolecular NOEs. Data from the simulation were used to evaluate the intermolecular forces responsible for analyte binding as well as to discern fragments of the CSP doing most of the work of holding the complexes together. The enantiodifferentiating forces and the parts of the CSP most responsible for chiral discrimination are described. Moments of distributions of key dihedral angles and distances between centroids were used to assess the relative rigidity of the competing diastereomeric complexes. Simultaneous multiple-contact ion-pairing, hydrogen bonding, and pi-stacking are possible for the longer retained enantiomer only. An X-ray crystallographic study of the more stable complex confirms the conclusions derived from chromatography, NMR spectroscopy, and molecular modeling.

Structural features and molecular assembly of amorphous phosphazenic materials in the bulk--combined theoretical and experimental techniques: Tris-(2,2'-dioxy-1,1'-binaphthyl)cyclotriphosphazen.

The structure and the assembly of tris-(2,2'-dioxy-binaphthyl)cyclotriphosphazene [(+)-[NP3(O2C20H12)3], DBNP, in the solid amorphous state was studied using molecular dynamics (MD) including ad hoc quantum mechanically derived force field (FF) parameters, in combination with the energy dispersive X-ray diffraction (EDXD) technique. The atom-atom radial distribution function (RDF) curve obtained through the EDXD experiment revealed low intensity peaks not attributable to the intramolecular distances of the single molecule, but clearly featuring a low energy state of long-distance three-dimensional assembly. The radial distribution functions (RDF) were calculated for various models of DBNP submitted to theoretical MD simulations. Based on the comparison of theoretically calculated RDFs and those obtained from the EDXD experiment, the predominant structural motif of the material in the bulk was found to have DBNP molecules laid one upon the other to form tubular nanostructures. These contain eight DBNP units each (length ca. 46 A) with two and three of these units aligned in parallel and held together. The material can be represented as a bulk of tubular snake-like chains undergoing distortions with a step of eight DBNP units. The bending angles, that vary randomly, attain limited values sufficient to induce disorder and thus nonperiodic structure. The present application of MD simulations combined with EDXD data appear to be a general approach to solve for the first time otherwise intractable issues concerning structural features and assembly of molecular materials in the bulk.

Effect of diphosphonate binding to collagen upon inhibition of calcification and promotion of spontaneous endothelial cell coverage on tissue valve prostheses.

A noncalcifying drug, aminodiphosphonate, was immobilized on pericardial collagen to inhibit calcification. The parameters of ADP binding were optimized by using labeled tracer. The effects of the detergents, Triton X-100 and sodium dodecyl sulfate, pH, and temperature on ADP binding to collagen in fresh pericardium, and stabilization of ADP-collagen bond by borohydride (BH) reduction, were optimized. These studies indicate that pretreatment of pericardium with SDS and TX-100 increases ADP binding three- to fivefold. Three-step cross-linking via Schiff-base reactions between collagen, ADP, and cross-linking via Schiff-base reactions between collage, ADP, and glutaraldehyde gives a high value of 30 to 35 molecules of ADP per collagen in pericardium. Twenty-five millimeter tissue valves were made with detergent (1% SDS, 1% TX-100) treated ADP-bound pericardium and implanted in the mitral annuli in calves, which were killed at 60 days post implantation. Regional calcium deposition was measured with atomic absorption spectrometry. The calcium level (micrograms/mg of tissue) in components of control and four processed valves were determined. Regional platelet deposition on zones of leaflets and components of tissue valves were quantified with 111In-labeled autologous platelets. The calcium levels in thrombus and flexion zones of treated valves are lower than that in the control valve. SEM studies of leaflet surfaces at 60 days indicate that these treatment processes reduce calcification and promote spontaneous cell coverage on leaflets around the smooth surface of the outflow tract.