“Homogeneous” and “heterogeneous” cellulose tri-esters and a cellulose triurethane: synthesis and structural investigations of the crystalline state

Combining Computational Chemistry and Crystallography for a Better Understanding of the Structure of Cellulose

The approaches in this article seek to enhance understanding of cellulose at the molecular level, independent of the source and the particular crystalline form of cellulose. Four main areas of structure research are reviewed. Initially, the molecular shape is inferred from the crystal structures of many small molecules that have β-(1→4) linkages. Then, conformational analyses with potential energy calculations of cellobiose are covered, followed by the use of Atoms-In-Molecules theory to learn about interactions in experimental and theoretical structures. The last section covers models of cellulose nanoparticles. Controversies addressed include the stability of twofold screw-axis conformations, the influence of different computational methods, the predictability of crystalline conformations by studies of isolated molecules, and the twisting of model cellulose crystals.

A cellulose-based chiral fluorescent sensor for aromatic nitro compounds with central, axial and planar chirality

This is the first example of a powerful chiral fluorescent sensor applicable to a wide range of chiral compounds with central, axial and planar chirality.

Crystal Polymorphism of Curdlan Propionate: 6-Fold versus 5-Fold Helices

The molecular and crystal structures of curdlan propionate (CDPr) were examined by the X-ray fiber diffraction methods combined with energy calculations. CDPr forms two different crystal structures (CDPr forms I and II) depending on annealing conditions: solvent-annealing yields CDPr form I, whereas thermal-annealing gives CDPr form II. In CDPr form I, the 6/1 helix is packed in the hexagonal unit cell with a = b = 1.154 nm, and c (fiber axis) = 2.287 nm. In the case of CDPr form II, the 5/1 helix is packed in the pseudohexagonal cell with a = b = 1.175 nm, and c (fiber axis) = 1.859 nm. The crystal transition from CDPr forms I to II occurs by thermal-annealing at temperatures ≥ 160 °C.

Chiral fluorescent sensors based on cellulose derivatives bearing terthienyl pendants

A chiral fluorescent sensor capable of recognizing different kinds of chirality was synthesized from naturally occurring cellulose, whose sensing ability was mainly based on its one-handed helical conformation.

Comparative HPLC enantioseparation of ferrocenylalcohols on two cellulose-based chiral stationary phases

The direct HPLC enantiomeric separation of several ferrocenylalcohols on the commercially available Chiralcel OD and Chiralcel OJ columns has been evaluated in normal-phase mode. Almost all the compounds were resolved on one or both chiral stationary phases (CSPs) with separation factor (alpha) ranging from 1.06 to 2.88 while the resolution (R(s)) varied from 0.63 to 12.70 In the separation of the alpha-ferrocenylalcohols 1a-e and the phenyl analogues 2a-e, which were all resolved except 1c, a similar trend in the retention behavior for the two series of alcohols was evidenced and the selectivity was roughly complementary on the two investigated CSP. For three ferrocenylacohols, chosen as model compounds, the influence of the mobile phase composition and temperature on the enantioseparation were investigated and additional information on the chiral recognition mechanism were deduced from the chromatographic behavior of their acetylderivatives.

NMR and Computational Studies of Chiral Discrimination by Amylose Tris(3,5-dimethylphenylcarbamate)

Proton NMR and simulations were combined to study the origin of chiral selectivity by a polysaccharide used in a commercial chromatographic stationary phase: amylose tris(3,5-dimethylphenylcarbamate). This material has unusually high enantioselectivity for p-O-tert-butyltyrosine allyl ester, which is activated by the presence of an acid. Proton NMR spectra agreed with the HPLC in showing that the l-enantiomer interacts much more strongly with the polysaccharide and that acidity switches on the selectivity. 2D NOESY spectra revealed which protons of each enantiomer and the polysaccharide were in proximity, and these spectra revealed folding of the l-enantiomer. Computations generated energy-minimized structures for the polysaccharide-enantiomer complexes, independently predicting folding of the l-enantiomer. Molecular dynamics simulations 2 ns in duration, repeated for three different energy-minimized structures, generated pair distribution functions that are in excellent agreement with the 2D NOESY spectra. The modeling studies revealed why acidity switches on chiral selectivity and minimally affects the chromatographic retention time of the unfavored d-enantiomer. The results comprise the first case of a chiral separation by a commercial polysaccharide stationary phase being explained using a combination of 2D NOESY and simulations, providing excellent agreement between experiment and computation and lending detailed molecular insight into enantioselectivity for this system.

Polysaccharide derivatives as useful chiral stationary phases in high-performance liquid chromatography

The chromatographic separation of enantiomers using chiral stationary phases (CSPs) has significantly advanced. The esters and carbamates of polysaccharides coated on silica gel have been extensively studied and widely used as CSPs for high-performance liquid chromatography (HPLC). In order to overcome the strict solvent limitation on these coated CSPs, the preparation of a new generation of CSPs consisting of immobilized polysaccharide derivatives has become increasingly important. The universal solvent compatibility of the new CSPs provides flexibility in both analytical and preparative chromatographies.

Enantioseparation by HPLC using phenylcarbonate, benzoylformate,p-toluenesulfonylcarbamate, and benzoylcarbamates of cellulose and amylose as chiral stationary phases

Phenylcarbonate, benzoylformate, and p-toluenesulfonylcarbamate of cellulose and five new benzoylcarbamate derivatives of both cellulose and amylose were synthesized and their chiral recognition abilities were evaluated as chiral stationary phases (CSPs) for high-performance liquid chromatography (HPLC). Cellulose benzoylcarbamate has a higher chiral recognition ability compared to phenylcarbonate, p-toluenesulfonylcarbamate, and benzoylformate of cellulose. The benzoylcarbamate derivatives exhibited a characteristic chiral recognition for the racemates, which bear a hydrogen atom capable of hydrogen bonding to the carbonyl group of the benzoylcarbamates. The structures of the benzoylcarbamates were investigated by CD spectroscopy.

Enantiomeric separation of a thiazolbenzenesulfonamide compound using packed-column subcritical fluid chromatography

Separation of enantiomers of a thiazolbenzenesulfonamide compound was performed on a Chiralpak AD column using subcritical fluid chromatography. Effects of alcohol modifier and temperature on the separations were studied. The results revealed that while the main adsorbing interactions were between the hydroxyl group of the analyte and the carbamate group of the stationary phase, chiral discrimination was achieved through an inclusion mechanism within the chiral cavity created along the amylose chains. Analogs and synthetic precursors of the thiazolbenzenesulfonamide studied were also investigated so as to understand the effect of functional groups and configuration of the analyte molecule upon chiral recognition.