Elsevier

Carbohydrate Research

Volume 337, Issue 12, 12 June 2002, Pages 1145-1153
Carbohydrate Research

Molecular interactions in bacterial cellulose composites studied by 1D FT-IR and dynamic 2D FT-IR spectroscopy

https://doi.org/10.1016/S0008-6215(02)00102-7Get rights and content

Abstract

Specific strain-induced orientation and interactions in three Acetobacter cellulose composites: cellulose (C), cellulose/pectin (CP) and cellulose/xyloglucan (CXG) were characterized by FT-IR and dynamic 2D FT-IR spectroscopies. On the molecular level, the reorientation of the cellulose fibrils occurred in the direction of the applied mechanical strain. The cellulose-network reorientation depends on the composition of the matrix, including the water content, which lubricates the motion of macromolecules in the network. At the submolecular level, dynamic 2D FT-IR data suggested that there was no interaction between cellulose and pectin in CP and that they responded independently to a small amplitude strain, while in CXG, cellulose and xyloglucan were uniformly strained along the sample length.

Specific strain-induced cellulose interactions in three Acetobacter cellulose composites: cellulose, cellulose/pectin and cellulose/xyloglucan were characterized by FT-IR spectroscopy and dynamic 2D FT-IR.

Introduction

Cellulose is composed of β-d-glucopyranose units joined by (1→4)-glycosidic links, and is the primary structural element of the cell wall: it has a high molecular weight and crystallinity.1 Xyloglucan is the major hemicellulose component in primary cell walls, with chains of (1→4)-β-d-glucan with xylosyl units linked to the glucosyl units in the C-6 position. Pectin is a term for a group of heterogeneous polysaccharides whose backbone consists of (1→4)-linked α-d-galacturonic acid repeating-units. Bacterial (Acetobacter xylinum) cellulose-based composites containing xyloglucan or pectin have been shown to possess organizational features similar to those observed in primary plant cell walls.2., 3., 4.

Infrared (IR) spectroscopy has been extensively used in cellulose research and IR band assignment, orientation data,5., 6. and structural details7., 8. have been produced. Polymer stiffness is considerably enhanced by molecular orientation, and polarized IR spectroscopy can be used to study orientation induced by mechanical strain and to characterize the segmental mobility of polymers under the influence of an external perturbation.9 Dynamic 2D FT-IR spectroscopy has been used to unravel the IR bands affected by deformation and the orientation of submolecular groups in cellulose I in order to probe the relationship between hydrogen bonding and cellulose structure10., 11., 12. in dry spruce-pulp samples. We have recently reported the first application of linear stretching FT-IR microscopy and 2D FT-IR spectroscopy13 to functional cell walls in onion epidermis. However, it is necessary to determine the 2D cross-correlation peak frequencies for pure cellulose and its composites with other cell-wall biopolymers in order to establish an interpretation of cross peaks in hydrated systems.

Here we report the results of an FT-IR study of mechanically strained hydrated cellulose composites and interpret the data in terms of molecular mobility and interactions. Well-characterized model systems were used for detailed studies to allow interpretations to be applied to polymers of intact plant cell walls. Polarized IR spectra allow the determination of molecular alignment and contribute towards an understanding of the structure–function relationship in biopolymer mixtures at different water contents. The analysis of the dynamic 2D FT-IR spectra and two-dimensional correlations are shown for samples measured with a polarized IR source.

Section snippets

Materials

The Acetobacter cellulose samples cellulose (C), cellulose/pectin (CP, containing 20% apple pectin of 67% degree of methyl esterification), and cellulose/xyloglucan (CXG, containing 35% of tamarind xyloglucan) were prepared as described previously.2., 3., 4. Fermentation duration of 24 h gave 10–20 μm thick films, which were still measurable with IR absorbance up to about 2 units in transmission, yet strong enough to be mounted in the clamps of the stretching device. The weight fraction of

Results and discussion

Linear stretching experiments were performed to examine the changes in infrared absorption spectra of samples with applied uniaxial strain. From the polarized FT-IR spectra, dichroic difference, ΔA, was calculated to evaluate the fibril reorientation, and dichroic ratio, R, was used to calculate the segmental orientation, F, of cellulose in order to compare the cellulose orientation as a function of strain in the different composites. Dynamic 2D FT-IR small-amplitude oscillatory strain was

Acknowledgements

This work was funded by a BBSRC competitive strategic grant. The authors gratefully acknowledge Dr E. Chanliaud (Unilever) for sample preparation, Dr N. Wellner (IFR) for technical advice with dynamic 2D FT-IR, Dr M.C. McCann (JIC, Norwich) and Dr K.W. Waldron (IFR) for helpful discussion.

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