Kinetic and Mechanistic Studies on [Zn(II)-Gly-Phe]+–Ninhydrin Reaction in Aqueous and Cationic CTAB Surfactant Micelles

Structural and vibrational analysis of glycyl-L-phenylalanine and phase transition under high-pressure

The structural and vibrational properties of the glycyl-L-phenylalanine dipeptide were investigated using vibrational spectroscopy (Raman and infrared) and first-principle calculations. Raman spectroscopy measurements were performed between 100 and 3200 cm-1 and infrared spectroscopy from 100 and 3200 cm-1 under ambient conditions. The conformational analysis of the zwitterionic form of the dipeptide was performed using the B3LYP functional, the 6-311++ base set and the Polarizable Continuum Model of solvation, determining the lowest energy conformation and assigning the vibrational modes. The effect of pressure on the glycyl-1-phenylalanine crystal was investigated using the Raman spectroscopy between 0.0 and -7.1 GPa in the spectral region of 100 - 3200 cm-1. As a result, conformational changes around 1.0 GPa were observed in the lattice modes and in some internal modes, showing a reorganization of the molecule in the crystal. In the decompression process, it was observed that the conformational change is reversible and the original Raman spectrum is recoverd.

Entropy reduction from strong localization - an explanation for enhanced reaction rates of organic synthesis in aqueous micelles

HYPOTHESIS

The underlying mechanism for increased reaction rates in micellar catalysis-based organic synthesis is a reduced entropy barrier for the reaction. A two-dimensional localization of reactants and catalyst in the surfactant micelle reduces the translational entropy of all components. The entropy is reduced less for the reaction intermediate than for the reactants, which leads to the lower barrier.

SIMULATIONS

Quantum chemistry, the COSMO-RS implicit solvent model and statistical thermodynamics were employed to predict the stability of a range of reactants, catalysts and intermediates in a series of surfactant micelles. The localized stability in the linker region between the lipophilic and hydrophilic regions and the resulting decrease in entropy were also calculated.

FINDINGS

The predicted reaction rates for the proposed mechanism show that the entropy reduction leads to a larger prefactor for the reaction. The resulting reaction rate can be significantly higher than conventional organic synthesis in an organic solvent even when the smaller reaction volume and lower reaction temperatures typically needed under micellar catalysis conditions are considered. The results are general across a wide range of types of reactions, reactants and catalysts and a selection of surfactants commonly used in organic synthesis, strongly supporting the hypothesis.

Picolinic Acid Promoted Permanganate Oxidation of D-Mannitol in Micellar Medium

Kinetics of permanganate oxidation of D-mannitol have been investigated spectrophotometrically under pseudo-first-order conditions in aqueous acidic media at 30 °C. The spectral analysis of hydrazone derivative of the product indicates the product to be an aldehyde. The observed rate constant value was found to be relatively slow in the uncatalyzed path, which increases by the presence of four isomeric promoters: 2-picolinic acid (2-PA), 4-picolinic acid (4-PA), 2,3-dipicolinic acid (2,3-diPA) and 2,6-dipicolinic acid (2,6-diPA). The catalytic effect of sodium dodecylbenzene sulfonate (SDBS) surfactant on the permanganate oxidation of D-mannitol has been also studied in the presence of the promoters. The critical micelle concentration (CMC) of SDBS alone and in presence of D-mannitol was determined by conductometry and spectrophotometry. The aggregation and morphological changes during reaction were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The variation of the reaction rates for the different promoters in the presence and absence of SDBS micellar catalyst is discussed qualitatively in the terms of partitioning nature of substrate, charge of surfactant and reactants. 2,3-diPA in association with SDBS as micellar catalyst accelerated the reaction velocity compared to the uncatalyzed path.

Kinetic Study of the Formation of Ruhemann's Purple in Micellar and Microemulsion Phases

The reaction kinetics of ninhydrine (Nin) with glycine–glycine dipeptide (Gly) was studied in aqueous solution. The same reaction was studied in micellar solutions of different surfactants such as sodium dioctylsulfosuccinate (AOT) and sodium dodecyl sulfate (SDS). The reaction was also studied in the L2 microemulsion phase of the three systems AOT/heptane/water, SDS/pentanol/heptane/water, CTAB/pentanol/heptane/water. Spectrophotometric measurements were perfomed at 570 nm. The data show that the reaction is first-order with respect to Nin and Gly in all cases. Values of the observed rate constants (k obs) increase with increasing concentration of the surfactant in micellar media. AOT was more promotional than SDS. The reaction rate in the presence of different micelles could be explained using a pseudo-phase kinetic model. Association constants of Nin and Gly with surfactant micelles are reported. Values of k obs for the reaction rates in the microemulsion phases are reported. AOT/heptane L2 microemulsion system has been clearly found as the most promotional among all the other studied systems. Association constants of Nin and Gly with surfactant in microemulsion were obtained. The rate was increased with the temperature in aqueous, micellar and microemulsion phases. The activation parameters ΔH*, ΔS* and ΔG* have also been obtained. The reaction rate in different microemulsion media is slightly increased with the change of oil (heptane > hexane > pentane), while co-surfactants (butanol, pentanol and hexanol) have no effect on k obs values in AOT, CTAB or SDS microemulsion.