Fourier-transform infrared vibrational circular dichroism of simple carbohydrates

Abstract

Vibrational circular dichroism spectra have been measured in [2HH,]DMS0 solutions for simple carbohydrates in the 1650-800 cm-' region. The links between the observed spectral features and configurational and conformational aspects of carbohydrates were investigated. Some useful correlations and difficulties involved in deciphering the spectral content are pointed out. Vibrational circular dichroism (v.c.d.) is a measure of differential absorption of left versus right circularly polarized i.r. radiation due to the vibrational transitions of chiral molecules. The unique nature of v.c.d. spectroscopy results from the combination of a large number of vibrational transitions with the stereochemical sensitivity inherent in circular dichroism (c.d.) The conventional c.d. studies are carried out in the visible spectral region, probing the electronic transitions, and such accessible electronic transitions are usually very few in number. For simple carbohydrates, in particular, there are no electronic transitions in the visible spectral region, which required a major portion of optical activity studies on simple carbohydrates to be limited to studying the optical rotation.' Thus v.c.d. spectroscopy offers two advantages for simple carbohydrates. There are 3n -6 fundamental vibrational transitions where n is the number of atoms in a molecule, and every one of these transitions can, in principle, exhibit c.d.; this provides a very large base of accessible spectral transitions. Secondly, since different vibrations encompass different portions of the molecule, c.d. features associated with different vibrational bands provide three-dimensional structural information in different portions of the molecule. Thus a complete v.c.d. spectrum contains the necessary information about the three-dimensional structure of the entire molecule in the solution phase. In practice, though, the determination of the complete molecular structure from v.c.d. spectra is rather ambitious, with the current state of knowledge, owing to (i) the complicated nature of molecular vibrations, (ii) the overlapping of vibrational bands and (ili) the small v.c.d. signals that are beyond instrumental sensitivity.In this paper we present some of the v.c.d. spectral studies carried out in our laboratory in the last five years in the 1650-800 cm-I region. V.c.d. spectra of lactones, sorbose and some additional simple carbohydrates were also investigated in our laboratory, and they will be reported separately. A preliminary report from our laboratory2 and two other v.c.d. paper^^^^ in the C-H stretching region comprise the previously published v.c.d. studies for carbohydrates. Experimental The v.c.d. spectrometer used for the present studies is based on a commerical Fouriertransform i.r. spectrometer (Nicolet 6000C). The instrumental details for these v.c.d. measurements are given el~ewhere.~ The raw v.c.d. features are usually dominated by the artifacts resulting from the birefingence in the optical path, which makes it difficult to determine the zero line. It is...

Conformational Preference and Chiroptical Response of Carbohydratesd-Ribose and 2-Deoxy-d-ribose in Aqueous and Solid Phases

“…Our conclusions in terms of pyranose forms are in good agreement with those obtained in the literature. [15][16][17][18]20,21,24,67 On the contrary, those references conclude the presence of a non-negligible amount of furanose forms, in different percentages, and our results are not able to reproduce this finding because all the spectral patterns which could be associated to furanose forms are also well reproduced with the presence of pyranose ones.…”

“…The chiral response of the 2-deoxy- d -ribose has not been studied yet, however, some studies exist in the case of the d -ribose. Thus, Tummalapalli et al recorded the IR and VCD spectra of simple carbohydrates, including d -ribose, in DMSO- d 6 solutions in the 1650–800 cm –1 region. They eliminated the presence of artifacts in the VCD spectrum equilibrating a given enantiomer with H 2 O or D 2 O and lyophilizing the equilibrated solution.…”

Feasibility of non-invasive optical blood-glucose detection using overtone circular dichroism

“…The vibrational circular dichroism of glucose has been studied on the fundamental vibrational transitions extensively. The spectral ranges 900 to 1200 cm −1 , 16,17 1175 to 1575 cm −1 , 18 and 2750 to 3050 cm −119 have been reported in the literature. These results are of little interest for non-invasive glucose detection due to the strong absorption of water in this spectral region severely limiting the penetration depth.…”

Vibrational circular dichroism: Chiroptical analysis of biomolecules

“…Probably the introduction of carbonylcontaining groups constitute another possibility to analyze carbohydrates by VCD. Also, deuteration of hydroxyls by lyophilization from D 2 O can afford different useful VCD spectral features [28]. Some hitherto reported VCD spectra exhibited various patterns consistent with the presence of sugar species but structure prediction from spectra has been difficult.…”