Читать книгу Polysaccharides - Группа авторов - Страница 25

UV-Vis spectroscopy technique is applied for quantitative analyses of glucose and mannose using a wavelength of 490 nm after acid hydrolysis of acemannan with appropriate standards for each monomer [22, 37, 60, 61, 69], as well as for quantitative analyses of acemannan at the wavelength of 540 nm [24, 37, 68, 73]. Eberendu et al. [68, 81] developed a quantitative colorimetric method for measuring glucomannan from A. vera without previous separation or chemical degradation of the polymer. These colorimetric assays are based on the colored complex formed by binding the β (1–4)-linked polysaccharides and dye Congo red (sodium 4,4’-diphenyl-2,2’-diazo-bis-1-naphtlamino-4-sulfonate).

The maximum wavelength of light absorption of Congo red in aqueous solution at 1% (w/v) is approximately 488 nm and maximum absorption of Congo red when conjugated with acemannan shifts the wavelength to 540 nm. The linearity obtained by the plot of absorption readings versus concentration (mg/L) of polysaccharides had a correlation coefficient of 0.999 at concentrations of between 0.9 and 72.7 mg/L. This result confirmed that the colorimetric assay method developed has many advantages over any currently used method for measuring A. vera polysaccharides, and that the assay is accurate for measuring the true amount of glucomannan and is not subject to interference from other components [68].

To date, only a few researchers have documented the quantitative analysis of acemannan by chromogenic formation of Congo red–acemannan [82]. Acemannan was described as the only polysaccharide able to form a characteristic conjugate by reacting with Congo red stain in a basic medium, and the increase in absorption to a wavelength of 540 nm is stoichiometric in relation to acemannan concentration [83].

Ray & Aswatha [24], also using the spectrophotometric technique, detected that the absorbance of the Congo red–acemannan conjugate varied with age of the plant and season of the year. The authors showed that 3-year-old plants harvested in the summer produced mucilaginous gel containing a greater amount of acemannan, given it had higher absorbance of the chromophore compared to 2-year-old plants and was higher than for plants harvested in winter and the rainy season.

The International Aloe Science Council (IASC) has been approved the Congo red assay for quantifying the content of glucomannans from A. vera and its derivates. However, the A. vera gel content other polysaccharides with β(1–4)-linked and could generate confusion or false positive [33]. Metcalfe [84] developed and validated a simple and inexpensive method, where the acetyl groups of the acemannan polysaccharide are converted into a quantified ferric-acetohydroxamic complex using a UV–Vis spectrophotometer at 540 nm.