Hemicelluloses: Science and Technology - American Chemical Society

Chapter 17

Interaction between Cellulose I and Hemicelluloses Studied by Spectral Fitting of CP/MAS C-NMR Spectra 13

Downloaded by UNIV OF ARIZONA on January 10, 2013 | http://pubs.acs.org Publication Date: October 7, 2003 | doi: 10.1021/bk-2004-0864.ch017

Per Tomas Larsson STFI AB, P.O. Box 5604, SE-114 86 Stockholm, Sweden (email: [email protected])

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A review of our work with spectral fitting of C P / M A S C - N M Rspectra recorded on isolated cellulose I, on a mixtures of cellulose I and xylan and, on kraft pulps is given. The conceptual model for cellulose I used by us is presented. References demonstrating the validity of the spectral fitting model is given. Results from kraft pulps containing hemicelluloses are presented and interpretations discussed based on results obtained from a model system that is a mixture of colloidal sols of isolated cellulose I and xylan. It is concluded that the method used is powerful enough to study, diagnose and monitor interactions between cellulose I and hemicelluloses.

254

© 2004 American Chemical Society

In Hemicelluloses: Science and Technology; Gatenholm, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

255 Studying interactions at the supermolecular level, between cellulose I (the dominating form of native cellulose) and hemicelluloses in wood fibers and in wood fiber based products such as paper pulps and paper is a formidable task. Still, it is an interesting area since current and future commercial use of wood fibers can benefit from a clear understanding of how the supermolecular characteristics of cellulose I and hemicelluloses affect chemical and physical properties of commercial products. In our work C P / M A S C - N M R (Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance) spectroscopy together with spectral fitting has been the method used for studying interactions between cellulose I and hemicelluloses. A necessary prerequisite for this approach is a reliable interpretation of cellulose I spectra.


1 3

The interpretation of cellulose I spectra is based on enriched cellulose I samples, i.e. some sample preparation is necessary before interpretable C P / M A S C - N M R spectra can be recorded on cellulose I. This is equally true for the samples where interactions between cellulose I and hemicelluloses are to be studied. The choice of sample preparation procedure may change the in vivo properties and the in vivo behavior of the polysaccharides to a varying extent. This r^sumi consists of five parts. The first part presents the general characteristics of C P / M A S C - N M R spectra recorded on isolated cellulose I. A presentation of the conceptual model of isolated cellulose I and the vocabulary that we use, are given in part two. Part three discusses the spectral fitting procedure used for isolated cellulose I and give references to results that demonstrate its validity. In part four results from paper pulps containing hemicelluloses are presented and finally, in part five possible interpretations of the results from the paper pulps are discussed by comparison with a model system consisting of a mixture of colloidal sols of isolated cellulose I and xylan. 1 3

1 3

13

CP/MAS C-NMR spectra recorded on cellulose I 1 3

The C P / M A S C - N M R spectra recorded on cellulose I enriched samples exhibit a large variation depending on the origin of the sample and the preparation procedure used (1-17) as shown in Figure 1. Typical C P / M A S C - N M R spectra recorded on isolated cellulose I are made up of four signal clusters originating from the six carbons in the anhydroglucose unit. The CI cluster centered around 105 ppm, the C4 cluster centered around 87 ppm the C2, C3 and C5 clusters centered around 73 ppm and the C6 cluster centered around 63 ppm. The fine structure of each signal cluster is due to the supermolecular structure of cellulose I. A l l chemical shifts reported here are determined by using glycine as an external standard. 1 3

The information content in this fine structure is high, but the accessibility of the information is limited by the severe overlap of the cellulose I signals. In order


256


to obtain quantitative information on the supermolecular structure of cellulose, some post-acquisition processing of the spectra is necessary. The general characteristics of the signal clusters differ substantially, the C4 signal cluster (80-92 ppm) is unique in the sense that it is the most resolved spectral region containing signals from a single atom in the anhydroglucose unit (6,16). This makes the C4 region the preferred target for spectral fitting.

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Figure 1. CP/MAS C-NMR spectra recorded at 7.05 T on cellulose I isolated from different sources and one kraft pulp. From bottom to top the sources are Valonia, Cladophora, Halocynthia, and cotton linters. The top spectrum is bleached birch kraft pulp that contains hemicelluloses. (Reproduced with permission from reference 16. Copyright 1999).

In order to perform spectral fitting, two prerequisites are needed, the signal intensities must be shown to represent amounts present in the sample (quantitativity) and a reasonable model for the structural elements present in the sample is needed for assignments.

A model for isolated cellulose I The polyglucan chains ((3-(l —>4) linked D-glucan units) that make up one cellulose I fibril have all the same sense of direction, all reducing ends points in the same direction. Fibrils have cross-sections of varying shape with widths in the range from a few nanometers to a few tenths of nanometers (18-20). The


257 terms micro fibril and elementary fibril are also found in the literature and are here considered synonymous to the term fibril used herein. Within the cell wall, fibrils may aggregate intofibril aggregates. The fibrils typically aggregate with main axes parallel or anti-parallel. The shape of fibril aggregate cross-sections may be of any form with widths in the range of several tenths of nanometers (18-21).


A fibril aggregate is considered a distinct object with a boundary. The boundary layer consist of fibril surfaces in direct contact with e.g. surrounding water. Due to the direct contact with water the polyglucan chains at the fibril aggregate boundary possess a characteristic behavior. These kinds of surfaces are herein referred to as accessible fibril surfaces (AS) (8,11). The formation of fibril aggregates leads to a situation where some fibril surfaces are in direct contact. At such contact zones, intermolecular bonds may not develop to the same state found in the crystal lattice of cellulose I. Hence, the contact zone is some kind of geometrical discontinuity. The polyglucan chains at such contact zones may possess a characteristic behavior and are herein referred to as inaccessible fibril surfaces (IS) (11). There are other situations that may lead to the formation of inaccessible fibril surfaces, three such situations are discussed below. Direct association of hemicelluloses to accessible fibril surface present at the fibril aggregate boundary transform accessible fibril surface to inaccessible fibril surface with the accompanying change in behavior of the affected polyglucan chains. In the case of non-perfect fibril aggregation, cavities within the fibril aggregate may exist. Such inter-fibril cavities expose fibril surface not directly involved in fibril aggregate formation. If such cavities are filled with hemicelluloses, direct contact between fibril surface and hemicelluloses within the fibril aggregate is possible, leading to the formation of inaccessible fibril surface. Another possibility for the presence of inaccessible surfaces is distortions within the fibril, interior surfaces. Some mechanical treatment of wood fibers shows a strong impact on C P / M A S spectra (22). Distortions of the cellulose I structure within the fibril, distributed along the fibril, may also constitute zones of geometrical discontinuities. Direct contact between fibril surfaces, fibril surfaces in contact with other cell wall polymers (at the fibril aggregate boundary or within the fibril aggregate) and interior surfaces are all considered causes for the presence of inaccessible fibril surfaces. Herein they are all referred to as inaccessible fibril surfaces (IS) (77). In a sense, accessible fibril surfaces and inaccessible fibril surfaces are both geometrical discontinuities compared to the crystal lattice of cellulose I. The polyglucan chains at both kinds of surfaces may exist in conformations distinct


258 from that found in crystalline cellulose. The question then arises whether this difference in conformation is present in the polyglucan chains at the surface layers only, or penetrates deeper into the fibril structure. If it is believed that such a difference in conformation diminishes gradually, going form the surface layer to the fibril interior, yet another form of cellulose may be present. Such cellulose form exist within the fibril, is considered highly ordered but non-crystalline (as judged by N M R ) and herein referred to as para-crystalline cellulose (PC) (10,

11,23)


The remaining polyglucan chains in the fibril constitute the crystalline cellulose (C) (cellulose l a or cellulose 1(3) (1-3) and are located in the fibril interior. We consider the A S , IS, P C and C forms of cellulose the most important features of a model for interpreting C P / M A S C - N M R spectra recorded on a 1 3

Figure 2. Features of the SFAM. A diagrammatic picture showing how the SFAM models a cross-section through a fibril aggregate (16 fibrils surrounded by water). White squares are cross-sections through the individual cellulose I fibrils, gray color represents water. Polyglucan chains (not shown explicitly) are perpendicular to the plane of the paper. Annotations within the figure show some example positions of the different cellulose forms, C=crystalline cellulose, PC=para-crystalline cellulose, AS= accessible fibril surface, IS= inaccessible fibril surface.


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system of aggregated cellulose I fibrils. The Square Fibril Aggregate Model (SFAM) consist of the above mentioned cellulose forms and three simplifying assumptions: 1 The fibrils are assumed to have square cross-sections, 2 the fibril aggregates are assumed to have square cross-sections and, 3 the aggregation is assumed to be perfect, i.e. no cavities exist within the fibril aggregate. The S F A M achieves computational simplicity at the expense of sacrificing the complexity characteristic of real cellulose I systems. The S F A M is, of course, an over-simplification, but it is still a reasonable approximation for interpreting spectra of isolated cellulose I. A diagrammatic representation of the S F A M is shown in Figure 2.

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Fitting CP/MAS C-NMR spectra Our work (10) has substantiated the work on the quantitativity of C P / M A S C - N M R spectra recorded on dry cellulose (4,24). Using polyethylene as an internal standard, spectra recorded on dry and water saturated cellulose samples were shown to be quantitative for a range of contact times (10). Typically, the spectra subjected to fitting are recorded on water saturated cellulose samples (>30 % water w/w) since this improves the quality of the spectra. The mathematical model used for fitting C P / M A S C - N M R spectra recorded on isolated cellulose I use different line-shapes for different forms of cellulose. Signals from crystalline forms of cellulose I are fitted using Lorentzian line-shapes and the remaining signals, from less ordered forms of cellulose, are fitted using Gaussian line-shapes (10). Performing spectral fitting on the C4 region of quantitative spectra recorded on isolated cellulose I, in conjunction with chemical modifications of the cellulose has allowed for the assignment of the less ordered forms of cellulose (77). Apart from the signals from cellulose l a and cellulose IP, signals can be assigned to para-crystalline cellulose (//), accessible fibril surfaces (2,11,13,15,25) and, inaccessible fibril surfaces (11,17). The assignments of the spectral C4 region are shown in Table I and Figure 3. 1 3

1 3

The reason for the presence of two signals originating from accessible fibril surfaces, in the C4 region of spectra recorded on isolated cellulose I, is not known. Two suggestions are that it is due either to the two-fold screw axis of the polyglucan chains at the accessible fibril surfaces or that the cellulose I fibril have two pairs of non-equivalent surfaces. Using the above shown assignments and line-shapes, spectral fitting has been performed on spectra recorded on cellulose I isolated from different sources. Using S F A M , the obtained relative signal intensities have been used to estimate average lateral fibril dimensions and average lateral fibril aggregate dimensions (11,15,21,25,26). The obtained dimensions are in agreement with


260 Table I. Fitting the C4 region of spectra recorded at 7.05 T on cellulose isolated from cotton linters FWHff Assignment

S(ppm)


—i*—*—,

Ict(C) I(cc+p)(C) Para-crystalline (PC) IP ( Q Accessible fibril surfaces (AS) Inaccessible fibril surfaces (IS) Accessible fibril surfaces (AS) a

Relative Intensity (%)

(Hz)

Line-shape

89.6 88.8 88.5

31(2.0) 29 (0.5) 158(1.5)

3.4 (0.4) 19 (0.9) 32(1.9)

Lorentz Lorentz Gauss

88.0 84.3

56 (1.4) 57 (3.3)

20(1.3) 2.6 (0.2)

Lorentz Gauss

84.0

318(8.0)

20 (0.8)

Gauss

83.4

61 (3.3)

3.1 (0.3)

Gauss

c

b

Full Width at Half Height

b

Errors are smaller than 0.05 ppm

c

Values in parentheses are standard errors

NOTE: The results of a non-linear least squares fitting of the C4 region of the CP/MAS C-NMR spectrum recorded on cellulose isolated from cotton linters by HCl(aq) hydrolysis (Reproduced with permission from reference 16. Copyright 1999). 13

94

92

90

88

86

84

82

80

78

76

8 (ppm) 13

Figure 3. Fitting the C4 region of a CP/MAS C-NMR spectrum recorded cellulose I isolated from cotton linters by HCl(aq) hydrolysis (2.5MHCl(aq)

on 100

°C 17h, yield about 80 %. After hydrolysis the cellulose is a colloidal sol). The experimental spectrum is shown as a broken line. The fitted spectral lines and their superposition are shown as solid lines. The broken line of the experimental spectrum is partially hidden by the superimposedfitted curves. See Table I for annotations.


261 dimensions estimated by microscopy (11,21,26). This agreement, between N M R and microscopy results, is only possible if the C P / M A S C - N M R spectra are quantitative and the spectral assignments are correct.


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Despite the coarseness of the S F A M its ability to extract reasonable estimates for average lateral fibril dimensions and average lateral fibril aggregate dimensions is quite surprising (11,21,26). It is worth emphasizing that lateral dimensions can only be estimated by the S F A M in isolated cellulose I samples. In cellulose I systems with large lateral dimensions of fibrils and fibril aggregates the relative intensity of the A S and IS signals are small, presenting a signal-to-noise problem, which makes estimates uncertain. The mathematical formulation of the S F A M (77) also becomes less 'sensitive' at large lateral dimensions. Practical limitations sets an upper bound on the sizes that can be determined by the S F A M at about 50-100 nm. Hence, C P / M A S C-NMR spectroscopy can be considered a tool selective with respect to supermolecular structure. 1 3

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Some difficulties encountered when fitting C P / M A S C - N M R spectra recorded on paper pulps and mixtures of cellulose I and hemicelluloses need to be mentioned. In these spectra additional signals (compared to isolated cellulose I) occur and these signals are assumed to be of Gaussian shape. The cellulose I spectra has been shown to be quantitative for the used contact time (70), but this is not necessarily the case for the hemicelluloses. Signal overlap is more frequent in spectra recorded on paper pulps and mixtures. The fitting procedure is very stable and reproducible in most situations but due to numerical limitations it cannot distinguish closely positioned signals of similar widths in cases where the difference in position is small compared to signal widths. Spectral fitting require spectra of high signal-to-noise ratio. This results in long acquisitions times (typically 'over night' acquisitions). If acquisition is performed without the use of a lock signal compensating for magnetic field-drift a stable magnet is necessary. In spite of these difficulties we have been able to show that spectral fitting of CP/MAS C - N M R spectra can be used to study, monitor and diagnose interactions between cellulose I and hemicelluloses. 1 3

Hemicelluloses in kraft pulps 1 3

CP/MAS C - N M R spectra recorded on kraft pulps contain additional signals as compared to spectra recorded on isolated cellulose I, as shown in Figure 4 and Figure 5.



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94

92

90

88

86

84

82

80

78

76

8 (ppm)

Figure

4. Spectral fitting of the C4 region of a bleached birch haft pulp HCl(aq) hydrolyzed at 100

Hemicelluloses: Science and Technology - American Chemical Society

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