Absorption spectra lignin

There is some experimental evidence showing the effects of heme-linked ionization on lignin peroxidase. The redox potential of the Fe +/Fe2+ couple of lignin and Mn-dependent peroxidases is pH-dependent, as is the absorption spectrum of the ferrous form of the lignin and Mn-dependent peroxidases (6). The pKa values determined from both experiments were 6.6. Detailed studies were performed studying O2 binding to the ferrous lignin peroxidase (79). The pKa for the ionization was measured at different... 

Nevertheless, in order to obtain reliable results with lignins, fluorescence, light absorption, and anisotropy must be taken into account. Fluorescence is easily eliminated by the use of an adequate interference filter. It is well known that lignins exhibit an absorption spectrum with a maximum in the near ultra-violet tailing all the way into the visible. 

The acidic fraction, which adsorbs to the resin and must be eluted by acidic extraction, exhibits a different infrared absorption spectrum in the 6-7 micron range. Sharp bands at 6.3 and 6.6 microns indicate absorption assigned to distortion of the aromatic nuclei occurring in lignin breakdown products. 

For an isolated lignin, a 20-mg sample is dissolved in 50ml of ethanol or methyl cellosolve. To 2 ml of this solution are added the same amounts of quinone monochlorimide and sodium hydroxide as were used in the above procedure. The solution is allowed to stand for 1 h at room temperature. Two ml of the solution is diluted to 25 ml with methyl cellosolve or ethanol and the absorption spectrum (/max 640 nm) is recorded spectrophotometrically. 

Ionization of phenolic hydroxyl groups in lignin with alkali causes a batho-chromic shift and a hyperchromic effect in the absorption spectrum. An alkaline ionization difference spectrum is obtained by subtracting the spectrum of the solute in a neutral solution from the corresponding spectrum measured in an alkaline medium. The difference spectrum may be measured directly with a spectrophotometer according to the following procedure ... 

Fig. 5.2.7. Absorption spectrum of softwood kraft lignin in 2% NaOH solution (10%, w/w) and in KBr pellet. The difference spectrum below is obtained by subtraction of the alkaline solution spectrum from the KBr spectrum...

Difference spectroscopy has been useful in interpreting the UV-visible spectra of lignins. In this technique, the absorption spectrum measured before a given chemical treatment is subtracted from that recorded after the treatment (or vice versa). In the simplest case, a compound, C, is transformed to a product P. At any time during the reaction, these quantities are related by mass balance. Equation 3.4, where Cq is the initial concentration of C. The difference in... 

Lundquist et al. [162,165,166] reported that the peak shapes and positions in emission spectra of spruce milled-wood lignin in dioxane/water were independent of excitation wavelength. The emission maximnm occurred at 358 nm. The excitation spec-trnm was consistent with the absorption spectrum of milled-wood lignin, indicating... 

J Polcin and WH Rapson. Spectrophotometric Study of Wood Chromophores In Situ. H. Determination of the Absorption Spectrum of Lignin from Reflectance and Reflectivity... 

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

Lignin is an excellent light absorber its absorption spectrum (Fig. 4.21) extends, with a tail, to over 400 nm. 

Fig. 4.21. Ultraviolet absorption spectrum of lignin. (Reproduced with permission from [981] published by Chapman and Hall, 1981.)...

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. 

Figure 1. Absorption characteristics of lignins in the visible region of the electromagnetic spectrum hardwood kraft lignin (A), HPL (B), and a typical bleached HPL (C).

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