The low fluorescence quantum yield of 8-hydroxyquinoline cannot be correctly interpreted without knowing the form that such a compound assumes in different environments. The commonly accepted emission-quenching excited-state proton transfer can follow different reaction paths if 8-hydroxyquinoline is dimeric or monomeric or if it exists in the form of cis and trans conformers; in this light, the knowledge of the compound form in a particular environment is basic. We have performed a spectroscopic and computational investigation aimed at the determination of the form of 8-hydroxyquinoline in different solvents. UV-vis, fluorescence, and IR spectral features have been assigned by ab initio computations based on the density functional theory and time-dependent density functional theory; the density functional theory and MP2 computations have been applied to the determination of the relative stability of the dimeric and monomeric cis and trans forms of 8-hydroxyquinoline in different solvents. Molecular dynamics computations have been used to determine the compound behavior in water solutions. According to our results, 8-hydroxyquinoline shows a clear preference for the cis conformation (as dimer or monomer), but, in water solutions, a small fraction of the trans conformation is also present.

SynopsisThe preferred conformations of model cyclopropylglycine peptides have been investigated by means of ab initio and empirical methods. Empirical computations performed with fixed bond lengths and valence angles using two well-known force fields show that only values of $ I in the ranges k70" k 20" are sterically allowed, and that the C,-conformation corresponds to the absolute energy minimum irrespective of the terminal groups used. Also, ab initio computations give similar results, but suggest greater stabilities for bridge and, especially, extended structures. These discrepancies can be removed, adding to the empirical force field a twofold torsional potential on \c. and using softer steric repulsive potentials. Complete geometry optimization using both ab initio and empirical methods does not affect the relative stabilities of folded conformations, but leads to a further significant stabilization of the fully extended structure via large modifications of some valence angles.

Molecular dynamics simulations at atomistic level have been performed on a metal-porphyrazine complex. Starting from an isotropic state, the system was cooled until transition from isotropic to columnar phase was observed; no nematic phase was encountered. Many tools were utilized to follow the system evolution: order parameter, g(r), g(||)(r(||)), g(c)(r(||)), g(perpendicular)(r(perpendicular)), g(2)(r), also density and energy changes. Very long runs were required to get reliable results, times greater than 40 ns of simulation. The structure of columnar phase was analyzed and the organization of molecules in the columns was investigated, along with the role of conformation of side chains. We found that in columnar phase the molecules are tilted versus the column axis and the conformation of side chains changes during the phase transition to allow this kind of organization; moreover the direction of columns axes is different from that of the director.

The conformational behavior of -aminoisobutyric acid has been investigated by means of ab initio and empirical methods. Empirical computations performed with fixed bond lengths and valence angles using well-known force fields show that C6, C7, and helical structures correspond to energy minima, but the relative stability of different conformers is strongly dependent on the parametrization. Ab initio computations performed to solve these discrepancies suggest that the three structures are essentially isoenergetic. An alternative set of net charges significantly improves the agreement between ab initio and molecular mechanics results. Complete geometry optimization using both ab initio and empirical methods does not affect the relative stabilities of C6 and C7 structures but significantly destabilizes the helical one. Zero point and entropy effects, although slightly destabilizing the C7 structure, do not alter this general trend. The stabilization of helical structures with respect to C5 and, especially, C7 ones in polar solvents has been recovered by varying the dielectric constant governing intramolecular electrostatic interactions. Computations performed for oligomers of alanine and -aminoisobutyric acid up to the octamer confirm the faster onset of helical structures for -aminoisobutyric acid evidenced by experimental observations.

The microscopic characteristics of concentrated aqueous solutions of formamide and urea have been investigated through energy minimization of clusters consisting of one or two solute molecules surrounded by up to 19 water molecules. The computations performed for single solute molecules correctly reproduce the pattern of solvent molecules in the first hydration shell found by molecular-dynamics simulations and lead to reasonable solvation enthalpies. The computations performed for two solute molecules indicate that direct CO---NH bridges are not to be expected in aqueous solution. Solute molecules are, instead, linked by water chains, leading to compact structures which are also compatible with the local tetrahedral environment of the solvent molecules.