Lignins absorptivity

Note 5. Insolubles are present when Klason lignin or wood meal is used. After a 40-min ozonation of wood meal, examination of the insoluble residue (in KBr) by FTIR showed the absence of characteristic lignin absorption bands. 

The absorbance at either 205 or 280 nm is the basis of several techniques for the quantitative determination of lignin. The extinction coefficient depends on species and solvent, and varies from 10 to about 26 L g cm. Summaries of extinction coefficients are available in reviews by Dence [18], Lin [2], and Fengel and Wegener [19]. Lin [2] has provided guidelines for measurement of lignin absorption spectra, while Dence [18] provides instructions for determination of acid-soluble lignin and lignin solubilized by the acetyl bromide method. 

The Wiesner Reaction. The reaction of lignified tissue and phloroglucinol—hydrochloric acid gives a visible absorption spectmm with a maximum at 550 nm. This has been attributed to coniferaldehyde units in lignin as the groups responsible for the color formation. 

In solution, lignin is most conveniendy analyzed quaUtatively and quantitatively by uv spectroscopy. Typical absorptivity values, D, at 280 nm for milled wood (MW) lignins and other types of lignins are Hsted ia Table 4. These values are used for quantitative determination of the lignins ia suitable solvents. 

Moreover, the identity of the native and enzymatically liberated lignins from this softwood is further corroborated by their ultraviolet and infrared absorption spectra (79), (99). (See page 88,92 and Fig. 1 and 5.)... 

Repeated methylation of bagasse native lignin with dimethyl sulfate yielded a product with a 29-1 % methoxyl content. Four such methyl-ations did not change this value. However, the infrared spectrum of this derivative still revealed a small absorption band at 3400 cm1. Thus, it appeared that at least one hydroxyl group could not be methylated. Treatment of this methylated product with acetic anhydride and pyridine gave an acetate, which did not exhibit an absorption band at 3400 cm-1. Consequently, a tertiary hydroxyl group, which can be acetylated but not methylated, seems to be present in this lignin. 

The importance of ultraviolet absorption spectroscopy as a tool for the elucidation of the chemical structure of organic compounds is well established. Although it has been extensively used in lignin research, as yet definite conclusions have not been attained. 

Stamm, Seme and Harris 104) in 4932 indicated that hardwood lignins gave a maximum absorption peak at 274-276 mp, and softwood lignins gave maxima at 284-285 mp. 

Later, Patterson and Hibbert (S3) stated that ultraviolet absorption of lignin corresponded with the absorptions of hydroxy derivatives of 4-(4,hydroxy-3,methoxyphenyl)-l-propanone and l-(4,hydroxy-3, 5,dimethoxyphenyl)-4-propanone. These authors, therefore, assumed the presence in lignin of a carbonyl group, or of an ethylenic double bond conjugated with an aromatic nucleus. They ascribed the 280 mp band to m-position freedom in the benzene ring and the 310 mp baud to a masked carbonyl group in conjugation with a benzene nucleus. 

This section will deal with the absorption spectra of the native and enzymatically liberated lignins from various woody tissues. With the exception of aspen native lignin, bagasse native and enzymatically liberated lignins and kiri native lignin, all the native lignins and their derivatives have one maximum at about 280 mp. Some of these results are included in Table 43. The absorption curves of some native and enzymatically liberated lignins are presented in Fig. 4. 

Table 13. Absorption Maxima and Extinction Coefficients of Various Lignins.

The band at 280 mp persists in spite of such alterations in the molecule as are caused by inethylation, acetylation and treatment with sodium hydroxide. However, the absorption of the phenylhydrazones of spruce (33) and white Scots pine native lignins 111) are peculiar in that a second maximum occurs at 352 mp. Glading (33) suggests that the carbonyl group in spruce native lignin is highly enolized, and, therefore,... 

As previously stated, the absorption spectra of the derivatives of many of the native lignins under consideration are very similar to the spectra of the original substance. Due to the unusual absorption band above 300 mp for bagasse native lignin, it was of interest to study the ultraviolet spectroscopic behavior of its derivatives in this region. 

To correlate these changes in absorption with theory, the ultraviolet spectroscopic behavior of model compounds closely related to the degradation products isolated from bagasse native lignin, i.e., p-hydr-oxybenzaldehyde, vanillin and syringaldehyde, was determined. The compounds used were p-hydroxypropiophenone, vanillin, acetovanillone and acetosyringone and their derivatives. 

The ultraviolet absorption spectra of the methylated bagasse native lignins are represented in Fig. 4. Once again absorption at 282 mp. has been magnified relative to that in the region above 300 mp. Furthermore, the completely methylated lignin exhibits an absorption maximum at 310 mp, rather than at 315 mp. It is significant that these spectra are similar to those obtained by Patterson and Hibbert for the 3.4-dimethoxyphenyl-l-propanone compounds (S3). 

Glading, R. E. Ultraviolet Absorption Spectra of Lignin and Lignin Deri-... 

Patterson, R. F., and H. Hibbert Studies on Lignin and Related Compounds. LXXII. The Ultraviolet Absorption Spectra of Compounds Related to Lignin. J. Amer. chem. Soc. 65, 1862 (1943). 

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