Daniele Casarini

A novel chitosan derivative to immobilize α-L-rhamnopyranosidase from Aspergillus niger for application in beverage technologies

alpha-L-rhamnopyranosidase (Rha, EC 3.2.1.40) is an enzyme of considerable importance to food technology in increasing the aroma of wines, musts, fruit juices and other beverages. The aim of this research is the immobilization of the Rha contained in a commercial preparation already used in the winemaking industry and purified in the manner described in a previous study [1]. The immobilization supports tested were chitin, chitosan and derivatized chitosan, diethylaminoethyl chitosan (DE-chitosan) never previously used for this type of application. Particularly, on DE-chitosan, the Rha was adsorbed and cross-linked with various bifunctional agents (glutaraldehyde, diepoxyoctane, suberimidate and carbodiimide), whose best results (immobilization yields and activity) were obtained with carbodiimide (EDC) that allowed a reduction in the involvement of the enzyme amine groups that are probably important in catalytic mechanism. In addition, the use of rhamnose and a succinimide (NHS) during cross-linking enhanced the action of the EDC and so increased the immobilization yield and activity. The immobilized Rha retained the kinetic parameters (K(m) and V(max)) of the free enzyme and increased stability. Moreover, this biocatalyst allowed an increase in the aroma in a model wine solution containing glicosidic precursors with a marked reduction in specificity toward tertiary monoterpenols as compared to the free enzyme.

Recent trends in conformational analysis

An outlook of the various research fields of organic chemistry where conformational analysis plays a fundamental role is presented. The section Methodologies for Conformational Analysis is focused on the information that can be obtained by various spectroscopic techniques such as nuclear magnetic resonance and optical methods such as electronic circular dichroism and vibrational circular dichroism. A comprehensive screening of the more recent ab initio and density functional theory theoretical approaches to conformational analysis is presented in the section Theoretical Conformational Analysis. The section Application of Conformational Analysis to the Determination of the AC of Organic Molecules shows how the synergic use of experimental and theoretical methods can solve challenging conformational tasks arising from modern organic chemistry.

Immobilization of the glycosidases: α-l-arabinofuranosidase and β-d-glucopyranosidase from Aspergillus niger on a chitosan derivative to increase the aroma of wine. Part II

Abstract The purpose of this paper was to study the immobilization of two glycosidases, α- l -arabinofuranosidase (EC 3.2.1.55) and β- d -glucopyranosidase (EC 3.2.1.21), contained in a commercial preparation and purified as reported in Part I. The procedure which proved to be the best is simple and inexpensive to perform, employing the chitosan derivative, glyceryl-chitosan, especially synthesized and characterized, as a support. The glycosidases were adsorbed on this support and cross-linked with glutaraldehyde to prevent them from being released into the wine. Subsequent reduction of the biocatalyst with sodium borohydride allowed for improved stability. Finally, the immobilized glycosidases were compared with free ones in terms of optimum pH and temperature, stability over time, and kinetics parameters (Km and Vmax) after which they were employed for aromatizing a model wine solution containing aromatic precursors.

The effect of exocyclic conjugation on the inversion of a saturated six-membered ring. A dynamic NMR study of N-substituted morpholines.

Abstract The effect of exocyclic conjugation on the inversion of a saturated six-membered ring is shown in a dynamic nmr study of a series of N-substituted morpholines where the substituents have similar shapes but different conjugating ability. As conjugation increases the ring inversion barrier decreases in contrast to previous observations of an unsaturated ring with conjugating substituents.

Unprecedented Detection of Distinct Barriers Involving Formally Enantiotopic Substituents: Phenyl Rotation in Solid Diphenyl Sulfoxide

The formally enantiotopic phenyl rings of Ph2 SO (Cs symmetry) display different rotation rates about the Ph-S bonds in the crystalline state. For example, at 0°C one phenyl ring rotates (see picture; cylinder represents rotating phenyl ring), whereas the other does not. Two distinct barriers (13.0 and 14.0 kcal mol-1 ) were determined by solid-state NMR spectroscopy.

Conformational studies by dynamic NMR. part 36.: The steric barrier and the π-barrier to rotation in simple enamines: diethylaminocyclohexenes

1-Diethylaminocyclohexene adopts a near-to-coplanar conformation at the N-Csp2bond with a 5.95 Kcalmol π-barrier to rotation through an orthogonal conformation with the lone pair in the π-p1ane. In contrast 1-diethy1aim no-2-methylcyclohexene adopts a non-planar conformation with the lone-pair near to the π-p1ane and a steric barrier to rotation of the diethylamino group through the plane, of 8.1 Kcalmol.

Conformational profile, energy barriers and optical properties of quinquethiophene-S, S-dioxides

Abstract Theoretical calculations, dynamic NMR experiments and absorption and photoluminescence data in solution are reported for a series of quinquethiophene S,S-dioxides substituted with alkyl groups of variable size and steric hindrance. Ab initio B3LYP/6-31G∗ and force field MM3 theoretical calculations show that the energy barriers for rotation around the inter-ring C–C bonds amount to a few kcal/mol even in the presence of very bulky substituents such as the cyclohexyl group. Dynamic NMR data were in agreement with the results of theoretical calculations. It was found that changing the steric hindrance of the substituents leaves the emission and photoluminesce properties unaltered. However, the photoluminesce intensities and wavelengths of all compounds were found to be very sensitive to solvent variations.

Correlated rotations in benzylfluorene derivatives: structure, conformation, and stereodynamics.

Fluorene derivatives, having substituted benzyl groups bonded to position 9, have been investigated by variable temperature NMR spectroscopy. Stereodynamic processes involving restricted rotation about the fluorenyl-CH2 and aryl-CH2 bonds have been observed, leading to conformers and enantiomeric forms, and the corresponding barriers were determined by line shape simulation. These dynamic processes are interpreted as being due to correlated rotation pathways, on the basis of DFT computations that satisfactorily reproduce the experimental barriers. The structures of two such compounds were also determined by single-crystal X-ray diffraction.

CONFORMATIONAL STUDIES BY DYNAMIC NMR. 66. RING INVERSION IN A CYCLIC DISILANE : 1,1,4,4-TETRAMETHYL-1,4-DISILACYCLOHEXANE

Abstract The barrier to ring inversion of the title compound (ΔG≠ = 6.0 kcal mol−1) has been found to be much lower than that (ΔG≠ = 11.1 kcal mol−1) of the corresponding cyclohexane derivative (1,1,4,4-tetramethylcyclohexane).

Conformation and absolute configuration of 2-naphthylalkylsulfoxides by combined use of dynamic NMR, ECD spectroscopy, DFT computations, and X-ray diffraction.

The E/Z conformer ratios of five 2-naphtylalkylsulfoxides were determined by means of dynamic NMR spectroscopy at very low temperatures and this information was used to predict, by means of DFT calculations, the ECD spectra of the R and S enantiomers. The latter were separated by enantioselective HPLC technique and the comparison of theoretical and experimental ECD spectra allowed the absolute configurations to be determined. In the case of 2-naphtyl tert-butylsulfoxide (1), the assignment was independently confirmed by anomalous dispersion using single crystal X-ray crystallography.