RESUMEN
Enantiomeric alanine was covalently grafted onto modified gold electrodes with mercaptopropionic acid and PAMAM dendrimers G4.0 with amine terminal groups. Cyclic voltammetric experiments in the presence of monocarboxylic ferrocene as a probe molecule proved that the alanine (Ala) was immobilized as a monolayer on the gold electrodes. Electron transfer to Ru(NH3)6Cl3 in solutions of different pH was studied by cyclic voltammetry (CV). Changes in solution pH resulted in the variation of the charge state of the terminal group and surface pKa values were estimated on the basis of these results. Because of electrostatic interactions between the positive charged groups on the electrode surface and the Ala, enantioselective recognition was possible. The interaction between enantiomers can be proven with molecular simulation. The electro-oxidation peak current was linearly dependent on Ala concentration over the range 0-10 microM with slopes between 143 and 187 microAcm(-2)/microM. The detection limit (3sigma) was 0.4059 microM for PAMAM G4.0-D(+)Ala-L(-)Ala and 0.4172 microM for PAMAM G4.0-L(-)Ala-D(+)Ala.
Asunto(s)
Alanina/análisis , Técnicas Electroquímicas , Poliaminas/química , Alanina/química , Dendrímeros , Electrodos , Oro , Concentración de Iones de Hidrógeno , Oxidación-Reducción , Rutenio , Soluciones , EstereoisomerismoRESUMEN
Theoretical analysis within the frame of the Topological Theory of Atoms in Molecules confirms the repulsive steric interaction between an axial dimethylphosphinoyl group and the syn-diaxial hydrogens in cyclohexane derivative 2-ax. In seemingly good agreement with experiment, equatorial isomer 2-eq was calculated to be 1.49 kcal/mol more stable than 2-ax. (Experimental energy difference in (diphenylphosphinoyl)cyclohexane, Delta H(o) = 1.96 kcal/mol.) In contrast, axial 2-(dimethylphosphinoyl)-1,3,5-trithiane, 3-ax, was calculated to be 6.38 kcal/mol more stable than 3-eq. (Experimentally, the axial conformer of 2-(diphenylphosphinoyl)-1,3,5-trithiane, was found to be 1.43 kcal/mol more stable than the equatorial conformer, in solvent chloroform.) Theoretical analysis, in particular the electron density at the bond critical point within the C(4,6)-H...O=P bonding trajectory, implies significant bonding in this segment of interacting atoms. By the same token, substantial positive charge is acquired by the C--H bonds adjacent to the sulfonyl groups in disulfone 4. Hydrogen bonding between the phosphoryl group and H(4,6) leads to stabilization of 4-ax, which is estimated to be 5.0 kcal/mol lower in energy than 4-eq. This conclusion is supported by examination of P==O...H--C(4,6) bond trajectories, as well as from evaluation of the critical point properties along those interacting moieties. By contrast, fluorinated derivative 5 is more stable in the equatorial conformation, indicating a repulsive electrostatic interaction of the C--F...O-P entity in 5-ax.
RESUMEN
A variable-temperature NMR study of (cis-4-methylcyclohexyl)methyl sulfide (1), sulfoxide (2), and sulfone (3), as well as (cis-4-methylcyclohexyl)phenyl sulfide (4), sulfoxide (5), and sulfone (6) allowed determination of the thermodynamic parameters, DeltaH degrees and DeltaS degrees, for the title groups. Reproduction of the experimental results with Allinger's MM3 program was successfully accomplished in the case of the sulfoxide and sulfide groups. Nevertheless, modification of the original force field torsional parameters was required in order to adequately reproduce the experimentally observed behavior of the sulfonyl derivatives. Rationalization of the enthalpic and entropic contributions to DeltaG degrees [S(O)(n)()R, n = 0, 1, 2; R = CH(3), Ph] is advanced in terms of the steric characteristics of these sulfur-containing groups and the resulting rotameric populations in the axial and equatorial monosubstituted cyclohexanes.