Raman Optical Activity of Glucose and Sorbose in Extended Wavenumber Range

Abstract

Raman optical activity (ROA) is pursued as a promising method for structural analyses of sugars in aqueous solutions. In the present study, experimental Raman and ROA spectra of glucose and sorbose obtained in an extended range (50–4000 cm−1) are interpreted using molecular dynamics and density functional theory, with the emphasis on CH stretching modes. A reasonable theoretical basis for spectral interpretation was obtained already at the harmonic level. Anharmonic corrections led to minor shifts of band positions (up to 25 cm−1) below 2000 cm−1, while the CH stretching bands shifted more, by ∼180 cm−1, and better reproduced the experiment. However, the anharmonicities could be included on a relatively low approximation level only, and they did not always improve the harmonic band shapes. The dependence on the structure and conformation shows that the CH stretching ROA spectral pattern is a sensitive marker useful in saccharide structure studies.

Use of Raman and Raman optical activity to extract atomistic details of saccharides in aqueous solution

“…Most of the experimental spectral bands are correctly reproduced with comparable accuracy (S > 0.9 for Raman and S > 0.8 for ROA) as in our previous works with monosaccharides. 10,11 Next, we used structures extracted from the biased MD simulations, which efficiently sample local minima of the FES profile. In Figure 4 (bottom), we show the Raman/ROA spectra for each of the conformer regions ( md 1/ md 2/ md 3/ md 4).…”

“…Another example is identifying conformers in flexible sugars such as gluconic acid that contribute to the experimental spectra. 14 It has also been applied to study sorbose/glucose in the aqueous solution in an extended vibrational range (75-3100 cm −1 ), 11 where the effects of small conformational changes of CH 2 OH group were linked to the overall shape of spectra. The method is not limited only to monosaccharides as it has been applied to the study of agarose trisaccharides in the gel phase to gain insight into its packing 15 or as in the study of differently linked mannobioses.…”

“…The microscopic structure of the first solvation layer (3-3.6 Å cutoff) was found to be very important, [10][11][12]14,17,18 while the long-range polarization can be modeled either with continuum solvation methods or using a larger shell of explicit water molecules(10-12 Å cutoff). 13,18 The spectra are then calculated for solute-solvent clusters at the DFT level, usually using the B3LYP or BPW91 functionals, with a sufficiently large basis set, 10,19 e.g., 6-311++G** or aug-cc-pVTZ.…”

Use of Raman and Raman optical activity to extract atomistic details of saccharides in aqueous solution

“…The advantage of Raman/ROA techniques when studying saccharides is a transparent vibrational spectral region between 75 and 3100 cm -1 , as compared to more traditional infrared spectroscopy (IR) coupled with vibrational circular dichroism (VCD), which offer limited information due to the strong water absorption. Moreover, Raman/ROA spectroscopies are very sensitive to conformational changes in saccharides both in the fingerprint [ 10 ] (75–1600 cm -1 ) and CH stretching region [ 11 ] (2700–3100 cm -1 ). Although Raman/ROA spectroscopies are readily accessible experimentally, their experimental spectra alone provide limited structural information due to their complexity and the substantial effects of the surrounding water molecules.…”

“…Another example is identifying conformers in flexible sugars such as gluconic acid that contribute to the experimental spectra [ 14 ]. It has also been applied to study sorbose/glucose in the aqueous solution in an extended vibrational range (75–3100 cm -1 ) [ 11 ], where the effects of small conformational changes of CH 2 OH group were linked to the overall shape of spectra. The method is not limited only to monosaccharides as it has been applied to the study of agarose trisaccharides in the gel phase to gain insight into its packing [ 15 ] or as in the study of differently linked mannobioses [ 16 ].…”