D. K. Buslov

Investigation of stretching vibrations of glycosidic linkages in disaccharides and polysaccarides with use of IR spectra deconvolution

The results are presented for the deconvolution of IR spectra of disaccharides and polysaccharides with alpha and beta configurations of the 1 --> 4 glycosidic linkage (maltose, cellobiose, amylose, and cellulose), as well as of their corresponding monosaccharides (alpha- and beta-D-glucose) in the 1200-920 cm(-1) frequency range. It is established that a characteristic of di- and polysaccharides with 1 --> 4 glycosidic linkage is the appearance of new absorption bands in the 1175-1140 cm(-1) spectral range, as opposed to the IR spectra of monosaccharides. This can be a spectroscopic manifestation of the glycosidic linkage formation. In the 1000-970 cm(-1) frequency range, absorption bands, which are not observed in the monomer spectrum, are separated as a result of the deconvolution of the IR spectra of cellobiose and cellulose. The number of bands in this range remains unchanged for maltose and amylose, as compared to the monomer spectra. It is shown that the application of the method of deconvolution leads to a considerable enhancement in the resolution of the absorption bands in the IR spectra of mono-, di-, and polysaccharides.

A Priori Estimation of the Parameters of the Method of Spectral Curve Deconvolution

A method is proposed for estimating the parameters used in the deconvolution technique. The proposed method is based on the use of the Fourier transform modulus of the spectrum being treated with the purpose of obtaining information about the characteristics of the spectral bands and random measurement errors. Formulas are given that permit estimation of the values of noise variance in the experimental spectrum, the low-pass filter passband, and the contour half-width of the high-pass filter. Results are considered for the application of the proposed formulas for determining the parameters used with the deconvolution of synthetic and experimental spectra. It has been concluded that practical use of the proposed method of a priori estimation of the parameters permits, to a certain extent, the optimization of the process of deconvolution of spectral curves.

Regularized Method of Spectral Curve Deconvolution

A regularized method of spectral curve deconvolution is proposed. This method is based on three fundamental principles: the regularized method of solving the convolution equation; the use, instead of the apodization function, of the digital low-pass filter, which permits exact knowledge of its characteristics; and the use of the Fourier transform modulus of the spectrum being treated for obtaining a priori information about the frequency characteristics of the solution and noise, required for determination of the optimum parameters of the regularizing operator. The regularized method of de-convolution permits the acquisition of an approximately stable solution for the deconvolution problem of spectral curves, which moves toward an exact solution with the decrease of the experimental spectrum error. Examples are given of the application of the regularized method of deconvolution to simulated and experimental IR spectra. A conclusion about the expediency of using the given method for resolution enhancement in complex spectra is made.

Analysis of the results of α-d-glucose Fourier transform infrared spectrum deconvolution: comparison with experimental and theoretical data

Abstract Deconvolution of the Fourier transform infrared (FT–IR) spectrum of α- d -glucose in the 1500–500 cm−1 range has been carried out and the deconvolution results have been compared with IR and Raman spectra of the above compound recorded at room and a low temperature as well as with the data of normal coordinate analysis of the α- d -glucose molecule in the crystalline state. It is shown that deconvolution of the IR spectra recorded at room temperature permits separating absorption bands observed in the vibrational spectra obtained by using cooled samples. Also, as a result of deconvolution, it is possible to improve the resolution of individual components of complex absorption bands without frequency shifts characteristic of low temperature spectra, which can be used for analytical purposes. The results of IR spectrum deconvolution are in good agreement with the data of normal coordinate analysis of α- d -glucose in the crystalline state.

IR investigation of hydrogen bonds in weakly hydrated films of poly(vinyl alcohol)

A system of hydrogen bonds in weakly hydrated PVA films containing up to ≤8.5 wt % water is investigated via IR spectroscopy. It is shown that water molecules bind to only part of the hydroxyl groups of the polymer that are available for hydration and form the first hydrating layer. In a completely dehydrated film, practically every hydroxyl group of PVA forms hydrogen bonds with two other hydroxyl groups and serves as both a proton donor and a proton acceptor. In the hydrated film, one to three water molecules directly bind with one hydroxyl group of PVA.

Deconvolution of Fourier transform infrared spectrum of α-d-galactose: comparison with experimental and theoretical data

Deconvolution of the Fourier transform infrared spectrum of α-d-galactose in the 1500–450 cm−1 range has been performed and a comparison of the deconvolution results with the IR spectrum of the above compound recorded (by using cooled samples as well as with the data of normal coordinate analysis of the α-d-galactose molecule in the crystalline state) has been made. A good agreement has been obtained between the results of the IR spectrum deconvolution and the low temperature spectrum, as well as the theoretical data. The splitting of some fundamental vibrational modes into two components has been found, which can be attributed to the result of intermolecular interactions of four molecules of α-d-galactose in the crystal cell. The experimental values of the observed splittings for the majority of bands are close to the calculated values of the factor group splitting.

Resolution Enhancement in IR Spectra of Carbohydrates by the Deconvolution Method and Comparison of the Results with Low-Temperature Spectra

This paper presents infrared (IR) spectra of β-d-glucose, cellobiose, and cellulose in the 1500–850 cm−1 range as well as the results of their deconvolution. Comparison of the results of deconvolution with the IR spectra of the investigated compounds obtained at room and helium temperatures of the sample as well as with the theoretical data has been made. It is shown that the use of the deconvolution method provides a better—compared to the low-temperature spectra—resolution of individual absorption band components for all the compounds being analyzed, especially for the polysaccharide cellulose. The results of deconvolution of IR spectra of monosaccharide β-d-glucose and disaccharide cellobiose are in good agreement with the data of normal coordinate analysis. It has been found that the minimum values of band halfwidths in the room-temperature IR spectra of β-d-glucose and cellobiose approximately coincide, while for cellulose they increase by a factor of about 1.5.

Analysis of the Structure of the Bands in the IR Spectrum of β-D Glucose by the Regularized Method of Deconvolution

Deconvolution of the IR absorption spectrum of β-D glucose in the spectral range 1500–450 cm−1 has been carried out. The results of the deconvolution were compared with the IR and Raman spectra recorded at room and low temperatures and with the data obtained by theoretical calculations for the frequencies of the normal vibrations of the β-D glucose molecule in the crystalline state. It is shown that deconvolution of the IR spectra recorded at room temperature makes it possible to separate the bands observed experimentally only at a very low temperature of the sample and a number of components that were not resolved earlier. The number of bands separated on deconvolution of the IR spectra of β-D glucose in the spectral range 1500–450 cm−1 is more than twice the number of visible absorption maxima in the usual spectrum. The results of deconvolution of the IR spectrum of β-D glucose are in good agreement with the data of theoretical calculations for the frequencies of the normal vibrations of the β-D glucose molecule in the crystalline state. The existence of the factor-group (Davydov) splitting of a number of frequencies of the nondegenerate fundamental vibrations of molecules in a crystal cell has been revealed in the IR spectrum of β-D glucose. It was concluded that the model of an isolated molecule is insufficient for detailed theoretical interpretation of the vibrational spectra of carbohydrates.

Profile Shape of Absorption Spectra in the IR Spectra of Carbohydrates

In the IR spectra of mono-, di-, and polysaccharides, the profile shapes of eight absorption bands have been investigated. They turned out to be symmetric and very close to the dispersion one for all the bands considered. It has been found that in mono- and disaccharides the minimum halfwidth of the bands is ∼10 cm−1 and in polysaccharides it is higher by a factor of two or more. The halfwidth ratios of different bands in one and the same spectrum can differ severalfold. As is shown, double differentiation of the IR spectra makes it possible to investigate the profile shape of absorption bands when there is marked background absorption or these bands significantly overlap with other bands.

Modification of Derivatives for Resolution Enhancement of Bands in Overlapped Spectra

A method for obtaining modified derivatives based on the multiplication of the Fourier image of a normal derivative of degree n by the function (–iSign(x)) n has been considered. The thus-obtained modified derivatives of even, odd, and fractional degrees of symmetric functions are also even functions, which permits one to use them for obtaining spectra with a better resolution of individual bands. Also, the use of derivatives of various degrees makes it possible to considerably widen the set of functions suitable for resolving individual bands in overlapped spectra. The results of the application of modified derivatives to synthetic and experimental spectra have been considered. The conclusion has been drawn that practical application of such derivatives permits more flexible variation of the resolution in the resulting spectrum compared to the normal derivatives of only even orders.