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Notes: Monte Carlo simulations are applied to imitate the incoherent electronic energy transfer among identical luminescent molecules in three-dimensional isotropic systems. The interacting molecules are then either rapidly or slowly rotating compared to the time scale of emission and we refer to these as the fast and slow cases, respectively. The time dependence of the excitation probability of the initially excited molecule [Gs(t)] and the mean square displacement [〈R2(t)〉] of the initial excitation are simulated for the slow and fast cases. The results are compared to those obtained from the analytical model of Gochanour, Andersen, and Fayer (the GAF theory) and to some extent to the computer method of Riehl. In the fast case and at a reduced concentration being less than two both Gs(t) and 〈R2(t)〉 are in very good agreement with the so-called three-body GAF theory. The deviations found at higher concentrations are likely due to the omission of higher orders of coupling in the GAF theory. For the slow case the GAF theory is developed exactly only to two-body order, and the agreement between theory and simulations is not so encouraging. However, if an approximative three-body term is included for Gs(t) the agreement becomes excellent for a large range of concentrations. Finally, the emission anisotropy [r(t)] of a donor–donor system is simulated for the slow case. The emission from any donor in the system, i.e., not necessarily the one excited initially, is thereby considered. The simulated r(t) agrees very well with the approximation of r(t)=(2/5)⋅Gs(t) which justifies the so-called Galanin model.

Notes: Using ab initio SCF molecular orbital techniques, the electric field gradients (efg's) at the oxygen and hydrogen nuclei were calculated for water clusters ranging from dimer to pentamer in an attempt to reproduce the shift in 17O and 2H nuclear quadrupole coupling constants (qcc's) that is observed on going from ice to vapor. For 2H, where the qcc shift is due mostly to the change in O–H bond length, excellent agreement with the experimental vapor → ice shift was obtained. For 17O, the change in the qcc is found to be mainly electronic in origin, effectively due to the polarization of the charge associated with the oxygen atom, and approximately 75% of the observed qcc shift was reproduced. On the basis of the calculations, estimates of the 17O and 2H qcc's in liquid water were made that are consistent with the values obtained from an analysis of the available NMR relaxation data, provided that librational motions are properly taken into account. We also present results of SCF calculations on water interacting with a Li+, Na+, or Cl− ion, indicating that the effect of a nearby ion on the 2H and 17O qcc's is similar to that produced by H bonding.

Notes: Energy migration between fluorescent molecules undergoing Brownian rotational motion has been studied by numerically solving the master equation of a many-body system. The molecules interact according to Förster's mechanism of dipole–dipole coupling. An algorithm for solving the master equation, and for calculating the fluorescence anisotropy and the mean-square displacement of the excitation is presented. In this study the molecules rotate like spherical particles in a liquid solvent at a reduced concentration of unity. The time-dependent fluorescence anisotropy was determined and compared to an analytical theory [G. H. Fredrickson, J. Chem. Phys. 88, 5291 (1988)]. At rotational rates less than the rate of fluorescence, the agreement between our results and those predicted by the theory is not satisfactory. The decay of the fluorescence anisotropy predicted by Fredrickson's theory is generally too fast. A modification of his model using parameters obtained in our simulations gives a better agreement.

Notes: Abstract. This paper describes the development of a stable, controlled-release formulation of metronidazole for use in the treatment of periodontal disease. It is formulated as a suspension, which undergoes transformation to a release-controlling, semi-solid on contact with gingival fluid. The system is based on the ability of mixtures of monoglycerides and triglycerides to form liquid crystals, i.e., reversed hexagonals, in contact with water. The reversed hexagonal form was found to have the most favourable sustained release properties, compared with those from the cubic form. The source of metronidazole is the prodrug, metronidazole benzoate, which further helps to slow down the release rate. Product characteristics are assessed by differential scanning calorimetry and viscometry. The release data derive from the results of in vitro dissolution tests. X-ray diffraction, phase diagrams, and polarized light microscopy were used to elucidate the structure of the liquid crystalline phases.

Notes: Electronic energy migration in membrane systems has been studied by means of Monte Carlo (MC) simulations according to an algorithm described previously [S. Engström, M. Lindberg, and L. B.-A(ring). Johansson, Chem. Phys. 89, 204 (1988)]. In the systems investigated, the interacting donor molecules are randomly localized in mono-, bi-, and multilayers and are either oriented with their transition dipoles isotropically or parallel to the layers. The mean-square displacement [〈R(t)2〉] of the excitation and experimental observables in terms of different fluorescence depolarizations were determined. All results are relevant for the "slow case,'' which means that translational and rotational motions of the donors are much slower as compared to the rate of fluorescence. A two-particle approximation for calculating the excitation probability of the initially excited molecule [denoted by Gs(t) ] and the fluorescence depolarizations in two-dimensional systems has been published previously [J. Baumann and M. D. Fayer, J. Chem. Phys. 85, 4087 (1986)]. By using the MC algorithm, we have in this work tested this model extensively.The excitation probability Gs(t) is found to be in excellent agreement with the MC simulations for all of the systems studied. For isotropically oriented donor molecules in multilamellar systems, the simulations show that Gs(t) is very well approximated by that of a monolayer at distances of d≥3 R0 between the layers. At distances of d≤0.5 R0, the function Gs(t) is equal to that of a three-dimensional solution. For the in-plane oriented dipoles in a multilayer system, Gs(t) is very well approximated by that for a single bilayer at d〉2 R0. In general, the depolarizations obtained by the two-particle model and MC simulations differ depending on the particular orientational distribution and the experimental geometry. To obtain a physically correct behavior of the fluorescence anisotropy at long times (i.e., the limiting anisotropy) is not possible within a two-particle approach. We instead propose a phenomenological model for calculating the fluorescence anisotropy which is a function of Gs(t) obtained within the two-particle approximation. This model gives a remarkably good agreement with the MC simulations and has the correct limiting anisotropy. The MC simulations of energy migration show that 〈R(t)2〉 is about twice as large for in-plane oriented donors in mono- and bilayers, as compared to the case of isotropically oriented transition dipoles. For multilayer systems consisting of layers separated by d=R0, we find that 〈R(t)2〉 is only slightly larger for the in-plane oriented transition dipoles as compared to the isotropic orientational distribution.

Notes: Summary H142 is a synthetic decapeptide designed to inhibit renin, an enzyme acting in the regulation of blood pressure. The inhibiting effect of H142 is caused by a reduction of a-Leu-Val-peptide bond (i. e. C(=O)-NH→CH2-NH). The conformational and dynamical properties of H142 and its unreduced counterpart (H142n) was modelled by means of molecular dynamics simulations. Water was either included explicitly in the simulations or as a dielectric continuum. When water molecules surround the peptides, they remain in a more or less extended conformation through the simulation. If water is replaced by a dielectric continuum, the peptides undergo a conformational change from an extended to a folded state. It is not clear whether this difference is a consequence of a too short simulation time for the water simulations, a force-field artifact promoting extended conformations, or if the extended conformation represents the true conformational state of the peptide. A number of dynamic properties were evaluated as well, such as overall rotation, translational diffusion, side-chain dynamics and hydrogen bonding.