Polysaccharides

The general pathways for pyrolytic degradation of polysaccharides, mainly derived by Shafizadeh et al. (refs. 91, 92) from model studies on cellulose [a ß(l -> 4) glucan], involve splitting of the polymer chain by three basic chemical reaction mechanisms, viz. dehydration, retroaldolisation and decarboxylation. Using these basic mechanisms, Schulten and Görtz (ref. 53) were able to explain the pyrolytic degradation of glycogen [a branched glucan with a(l ■*■ 4) and a(l 6) linkages]. The hexose degradation pathways illustrated for glycogen in Figure 2 result in the formation of furan and pyran-type fragments and smaller acyclic aldehyde and ketone

Figure 2. Some pyrolytic degradation pathways proposed for hexose polymers. This scheme is mainly based on a compilation of pyrolysis products of glycogen as observed by Schulten et al. (refs. 53, 94) using Curie-point pyrolysis-FI-HRMS at various pyrolysis temperatures. The m/z values are indicated in italic numerals. It should be noted that some fragments are hardly present in the Curie-point low voltage-EI spectra of hexose polymers such as those included in the Atlas. Moreover, some molecular ion peaks, regularly observed in the spectra, e.g. at m/z 32, 60, 110, 112 and 124, are not explained by this degradation scheme.

Figure 2. Some pyrolytic degradation pathways proposed for hexose polymers. This scheme is mainly based on a compilation of pyrolysis products of glycogen as observed by Schulten et al. (refs. 53, 94) using Curie-point pyrolysis-FI-HRMS at various pyrolysis temperatures. The m/z values are indicated in italic numerals. It should be noted that some fragments are hardly present in the Curie-point low voltage-EI spectra of hexose polymers such as those included in the Atlas. Moreover, some molecular ion peaks, regularly observed in the spectra, e.g. at m/z 32, 60, 110, 112 and 124, are not explained by this degradation scheme.

fragments. As an example, the largest commonly found cellulose fragment is levogluco-san (m/z 162), whereas a peak at m/z 126 can be attributed to hydroxymethylfurfural (ref. 90) or levoglucosenone (ref. 53) (see Atlas).

Although as yet relatively little work has been carried out to elucidate reaction pathways i n non-hexosyl polysaccharides, e.g. N-acetylhexosaminyl polymers (ref. 93), the pyrolysis mass spectral patterns shown for cellulose and its N-acetylglucosamine analogue chitin (Figure 3) indicate a marked degree of correspondence in basic pyrolysis fragments. As this may also hold for other types of carbohydrates, the hexose model studies should aid considerably in the qualitative interpretation of non-hexosyl polysaccharides.

Notwithstanding these apparent basic similarities in pyrolysis pathways, different types of sugar moieties, e.g. pentoses, amino sugars, N-acetyl aminosugars, hexuronic acids and deoxy- and anhydro sugars, often contribute characteristic fragment series

Figure 3. Pyrolysis mass spectra of cellulose (a) and chitin (b). Note the predominance of even masses in the cellulose spectrum indicating almost complete absence of fragment ions. For the chemical identity of the main ion signals see ref. 53. The characteristic ion series in the chitin spectrum appears to be shifted by 1 or 41 amu, respectively, relative to the cellulose pattern, indicating the presence of NH2 or N-acetyl functional groups instead of an OH group in the fragments. Conditions: samples 10 ug; Tc 510°C; Eel 14 eV (see Atlas).

Figure 3. Pyrolysis mass spectra of cellulose (a) and chitin (b). Note the predominance of even masses in the cellulose spectrum indicating almost complete absence of fragment ions. For the chemical identity of the main ion signals see ref. 53. The characteristic ion series in the chitin spectrum appears to be shifted by 1 or 41 amu, respectively, relative to the cellulose pattern, indicating the presence of NH2 or N-acetyl functional groups instead of an OH group in the fragments. Conditions: samples 10 ug; Tc 510°C; Eel 14 eV (see Atlas).

to the pyrolysis mass spectrum by virtue of their different structures. For typical fragment series obtained from these carbohydrate building blocks we refer to the Atlas.

An example of the sensitivity of Py-MS for structural details was also given by Haverkamp et al. (ref. 49) in the analysis of N-acetyl neuraminic acid polymers (sialopolymers). This study shows that the presence of 0-acetyl substituents as well as of neutral hexosyl moieties in the native polymers is readily detected by Py-MS analysis as simple cleavage reactions give rise to acetic acid from the former and intact ring fragments such as that at m/z 126 (probably levoglucosenone) from the latter. Further, differentiation of a(2 8) and a(2 ->- 9) linkage types between the monomeric units seems to be possible as the primary alcoholic function present in each of the monomeric moieties in the a(2 ->- 8) chain gives rise to an increased amount of methanol as a simple pyrolytic cleavage product.

The noted scarcity of alternative analytical techniques for the rapid chemical characterisation of microgram samples of carbohydrates makes Py-MS a powerful tool, especially for the analysis of insoluble polysaccharides.

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