Vibrational circular dichroism of carbohydrate films formed from aqueous solutions

Abstract

Vibrational circular dichroism (VCD) spectra in the entire 2000–900 cm −1 region have been recorded, for the first time, for films of carbohydrates prepared from aqueous solutions. Eight different carbohydrates, α- d-glucopyranosyl-(1→4)- d-glucose, cyclomaltohexaose, α- d-glucopyranosyl α- d-glucopyranoside, β- d-glucopyranosyl-(1→6)- d-glucose, β- d-glucopyranosyl-(1→4)- d-glucose, d-glucose, and both enantiomers of 6-deoxygalactose and of allose, were investigated. The VCD spectra obtained for films are found to be identical to the corresponding spectra obtained for aqueous solutions of carbohydrates. These measurements demonstrate several advantages of significant importance. The strong infrared absorption of water has prevented, in the past, the pursuit for routine applications of VCD in determining the structures of carbohydrates in aqueous solutions. This limitation is not present for film studies because water solvent is removed in the process of preparing the films. Also, strong infrared absorption of water at 1650 cm −1 requires the use of very short-pathlength (6 μm) cells for measurements on aqueous solutions. This requirement and concomitant inconveniences (such as laborious assembling of a demountable liquid cell or purchasing an expensive variable pathlength liquid cell) have been eliminated for film measurements. The removal of interfering water absorption in film studies resulted in higher light throughput and better signal-to-noise ratios for VCD measurements. Another point of significance is that the amount of carbohydrate sample required for VCD measurements on films is approximately one to two orders of magnitude smaller than that required for corresponding VCD measurements on aqueous solutions. Since carbohydrate samples can now be studied as films, VCD spectroscopy becomes much more broadly applicable for carbohydrates than previously believed. The present work, in combination with other film measurements in our laboratory, indicate that VCD studies on films can be used more generally, providing a convenient and powerful approach for probing structural information for biologically important compounds.