Abstract

Abstract

ABSTRACT

ABSTRACT

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

A chiral column, with decaproline as the chiral selector, has broad chiral selectivity. To understand the separation mechanism of this chiral column, multiple spectroscopic techniques, including optical rotation, electronic circular dichroism, infrared absorption and vibrational circular dichroism, have been used here to study the conformation of the decaproline oligomer in isopropanol(IPA)/dichloromethane(DCM) mixtures. These studies indicate that decaproline oligomer adopts polyproline II conformation in IPA/DCM solvent system (0% IPA ∼ 100% IPA). Hydrogen bonding interactions between CO groups of decaproline and IPA molecules increase as the content of IPA in the solvent mixture increases up to 60% and become less significant from then onwards. These spectroscopic observations are found to have a good correlation with the enantiomeric separation of racemic 2,2,2-trifluoro-1-[10-(2,2,2-trifluoro-1-hydroxy-ethyl-anthracen-9-yl]-ethanol by the decaproline column. Chirality 2006. © 2006 Wiley-Liss, Inc.

Two of the chiroptical spectroscopic methods, namely optical rotatory dispersion (ORD) and electronic circular dichroism (ECD), have been around for several decades. But their use in determining the absolute configuration and predominant conformation is gaining renewed interest with the availability of quantum mechanical methods for predicting ORD and ECD. Two other methods, namely vibrational circular dichroism (VCD) and vibrational Raman optical activity (VROA), are relatively new and offer convenient approaches for deducing the structural information in chiral molecules. With the availability of quantum mechanical programs for predicting VCD and VROA, these methods have attracted numerous new researchers to this area. This review summarizes the latest developments in these four areas and provides examples where more than one method has been used to confirm the information obtained from individual methods. © 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 125–136; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20117

The availability of sophisticated quantum mechanical methods for predicting molecular optical rotations has revolutionized the determination of molecular stereochemistry. However, the objectives and approaches used for the applications of optical rotation vary from one laboratory to the other. With the number of applications of optical rotation increasing at an unprecedented rate, it is considered necessary to discuss the protocols for the general use of optical rotations predicted with quantum mechanical methods. A summary of the quantum mechanical methods for predicting optical rotations and protocols for the use of predicted optical rotations are presented in this article. Chirality, 2006. © 2006 Wiley-Liss, Inc.

When a limited region of the experimental electronic circular dichroism (ECD) spectrum is subjected to Kramers–Kronig (KK) transformation, the resulting optical rotatory dispersion (ORD) may or may not reproduce the experimentally measured ORD in the long-wavelength nonresonant region. If the KK transform of experimentally measured ECD in a limited wavelength region reproduces the experimentally measured ORD in the long-wavelength nonresonant region, then that observation indicates that the ORD in the long-wavelength nonresonant region should be satisfactorily predicted from the correspondingly limited number of electronic transitions in a reliable quantum mechanical calculation. On the other hand, if the KK transform of experimentally measured ECD in a limited region does not reproduce the experimentally measured ORD in the long-wavelength nonresonant region, then it should be possible to identify the ECD bands in the shorter wavelength region that are responsible for the differences between experimentally observed ORD and KK-transformed ECD. This approach helps to identify the role of ECD associated with higher energy-excited states in the nature of ORD in the long-wavelength nonresonant region. These concepts are demonstrated here by measuring the experimental ECD and ORD for dimethyl-L-tartrate in different solvents. While ECD spectra of dimethyl-L-tartrate in different solvents show little variation, ORD spectra in the long-wavelength nonresonant region show marked solvent dependence. These observations are explained using the difference between experimental ORD and KK-transformed ECD. Quantum mechanical predictions of ECD and ORD are also presented for isolated (R, R)-dimethyl tartrate at the B3LYP/aug-cc-pVDZ level. Chirality, 2006. © 2006 Wiley-Liss, Inc.

Die helicitätsselektive Polymerisation eines achiralen Carbodiimids durch einen chiralen Titankatalysator ergibt regio- und stereoreguläres Poly[N-(1-anthryl)-N′-n-octadecylguanidin] (Poly-1). Untersuchungen zum elektronischen Circulardichroismus und zur optischen Drehung zeigen ein reversibles thermisches und solvensgesteuertes Schalten für Poly-1 (siehe Schema). Die Helicität von Poly-1 wird zugeordnet, und der Mechanismus des Schaltprozesses wird erklärt.

VCD versus ECD spectroscopy. Peptides rich in aromatic residues yield anomalous far-UV electronic circular dichroism (ECD) spectra that preclude secondary structure assignment. The utility of vibrational circular dichroism (VCD) in conformation analysis is demonstrated by using a set of well-defined peptide helices and hairpins containing proximal aromatic residues.

The enantiomers of 3,3,3′,3′-tetramethyl-1,1′-spirobi[3 H,2,1]benzoxaselenole have been separated on a chiral preparative chromatographic column. The experimental vibrational circular dichroism (VCD) spectra have been obtained for both enantiomers in CH₂Cl₂. The theoretical VCD spectra have been obtained by means of density functional theoretical calculations with the B3 LYP density functional. From a comparison of experimental and theoretical VCD spectra, the absolute configuration of an enantiomer with positive specific rotation in CH₂Cl₂ at 589 nm is determined to be R. This conclusion has been verified by comparing results of experimental optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) to predictions of the same properties using the B3 LYP functional for the title compound.

The absolute configuration of C₇₆ has been determined as (+)₅₈₉-(^fC)-C₇₆ , for the first time, by comparing the experimental and predicted optical rotatory dispersion (ORD) patterns. The experimental ORD pattern was derived from the experimental electronic circular dichroism (ECD) spectrum using the Kramers–Kronig (KK) transform. The theoretical ORD spectra were calculated in the resonant region using linear response theory, and also using the KK transform of the theoretical ECD spectrum, at different theoretical levels, namely BHLYP/6-31G*, B3LYP/6-31G*, BLYP/6-31G*, and HF/6-31G*. Good agreement noted between experimental and predicted spectra allows for an unambiguous determination of the absolute configuration.

Enantiopure herbicides (+)-2-(4-chloro-2-methylphenoxy) propanoic acid, (+)-1 and (+)-2-(2,4-dichlorophenoxy) propanoic acid, (+)-2 were investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-1 and (+)-2 in CDCl₃ solution in the 2000–900 cm⁻¹ region were compared with the ab initio predictions of absorption and VCD spectra obtained with density functional theory using the B3LYP/6-31G* basis set for different conformers of (R)-1 and (R)-2. Due to the intermolecular hydrogen bonding, this comparison did not provide unambiguous conclusions. To eliminate intermolecular hydrogen bonding influence, the two acids 1 and 2 were converted to the corresponding methyl esters, namely, (+)-methyl 2-(4-chloro-2-methylphenoxy) propanoate, (+)-3 and (+)-methyl 2-(2,4-dichlorophenoxy) propanoate, (+)-4. The experimental VCD spectra were measured for these esters and ab initio calculations for different conformers of (R)-3 and (R)-4 were carried out. The experimental VCD spectra and corresponding population-weighted theoretical VCD spectra were found to be in excellent agreement, which allowed unambiguous determination of absolute configuration of 3 and 4 as (+)-(R). Since esterification does not invert the configuration, the absolute configuration of the parent acids 1 and 2 is the same as that of corresponding methyl esters. Chirality 17:S1–S8, 2005. © 2004 Wiley-Liss, Inc.

Vibrational absorption and vibrational circular dichroism (VCD) spectra of valinomycin are measured, in different solvents, in the ester and amide carbonyl stretching regions. The influence of cations, namely Li⁺, Na⁺, K⁺, and Cs⁺, in methanol-d₄ solvent is also investigated. Ab initio quantum mechanical calculations using density functional theory and 6–31G* basis set are used to predict the absorption and VCD spectra. A bracelet-type structure for valinomycin that reproduces the experimental absorption and VCD spectra in inert solvents is identified. For the structure of valinomycin in polar solvents, a propeller-type structure was optimized, but further investigations are required to confirm this structure. A symmetric octahedral environment for the ester carbonyl groups in the valinomycin–K⁺ complex is supported by the experimental VCD spectra. The results obtained in the present study demonstrate that even for large macrocyclic peptides, such as valinomycin, VCD can be used as an independent structural tool for the study of conformations in solution. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004

The vibrational absorption and vibrational circular dichroism (VCD) spectra of melittin in D₂O solutions at different pH values, different salt concentrations, or different 2,2,2-trifluoroethanol (TFE) concentrations are recorded in the amide I′ (1850–1600 cm⁻¹) region. Two models are used to simulate this peptide in different conditions, and a coupled oscillator program is used to obtain the calculated absorption and VCD spectra. This study indicates that melittin adopts a mixed structure in D₂O solution at low pH, low salt concentration, or low TFE concentration. With an increase in pH, salt concentration, or TFE concentration, the structure changes to α-helix and further increases lead to aggregation. These results demonstrate the versatility of VCD in probing the conformations of peptides under different environmental perturbations. © 2003 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 70: 614–619, 2003

1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane, compound B, is a product obtained in the degradation of the anesthetic Sevoflurane. Enantiopure (+)-B was investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-B in CDCl₃ solution in the 2,000–900 cm⁻¹ region are compared with the ab initio predictions of absorption and VCD spectra obtained from density functional theory using B3LYP/6-31G* basis set for different conformers of (S)-1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane. This comparison indicates that (+)-B is of the (S)-configuration in CDCl₃ solution, in agreement with previous literature results. Our results also indicate that this compound adopts six predominant conformations in CDCl₃ solution. Chirality 14:618–624, 2002. © 2002 Wiley-Liss, Inc.

The vibrational circular dichroism (VCD) and absorption spectra of gramicidin D in three model membranes (dioctadecyldimethylammonium chloride vesicles, dimyristoyl-phosphatidylcholine vesicles, and sodium dodecyl sulfate micelles) are presented. The absorption and VCD spectra suggest that the stable gramicidin D conformation in the model membranes is different from those in organic solvents. The presence of cations does not change the membrane-bound conformation of gramicidin D. © 2001 John Wiley & Sons, Inc. Biopolymers (Biospectroscopy) 62: 336–340, 2001