Carbohydrates, estimation general

There are major differences in the chemical compositions of DOM isolated by XAD resins and ultrafiltration (Table I). In rivers and in the ocean, humic substances (XAD isolation) are depleted in N relative to UDOM. The C/N ratios of UDOM are more representative of bulk DOM than those of humic substances. Most of the functional groups identified by NMR are found in more than one class of compounds, so in most cases specific functional groups are not assigned to a particular group of biochemicals. However, in some circumstances it is possible to estimate the fraction of carbon associated with a biochemical class, such as carbohydrates. Carbohydrates are the most abundant polyalcohols in nature, and the ratio (4-5 1) of areas associated with NMR peaks at specific chemical shifts [e.g., 72 ppm (C—O) -102 ppm (O—C—O)] indicates that carbohydrates are their primary source (see Table I for references). In general, humic substances are depleted in carbohydrates (C—O and O—C—O) and enriched in aromatic and unsaturated C (C=C) relative to UDOM (Table I). As mentioned earlier, humic substances are relatively hydrophobic components of DOM, and it is consistent that they are depleted in N and carbohydrates and enriched in aromatic components. The UDOM fraction includes more hydrophilic components that are relatively enriched in N and carbohydrates. Humic substances from the ocean are enriched in aliphatic C (C—C) relative to UDOM, and this could reflect the more hydrophobic nature of the humic substances. 

Amine oxidases are homodimers with subunit molecular masses generally in the range 70kDa to 95kDa with one copper ion and one organic cofactor per subunit. Eukaryotic amine oxidases are glycoproteins with carbohydrate contents estimated at 3nl0%. Further information on the protein chemistry of amine oxidase can be found in Knowles Yadav (1984) and Pettersson (1985). 

Proton and C-NMR data compare well with each other and suggest that surface ocean HMWDOM has a H C ratio of approximately 1.8—1.9 (Aluwihare, 1999 Benner et al, 1992) and an 0 C ratio between 1 and 1.1. These H C and 0 C ratios are very close to those of a pure carbohydrate with a general hexose structure (e.g., C6H12O6). In comparison, humic substances isolated from seawater have an H C ratio between 1.2 (direct elemental analyses) and 1.4 (based on NMR estimates) and are therefore, relatively C-rich (Hedges et al, 1992). The H C and 0 C composition of phytoplankton as estimated by NMR spectroscopy is approximately 1.7 and 0.3, respectively (Hedges et al, 2002). In comparison to phytoplankton... 

A solids C NMR Spectrum, for a marine plankton sample (from Hedges et o/., 2002). Although NMR only gives a general overview, one can use this spectrum to estimate that this sample contains approximately 65wt% protein, 29wt% lipid and 15 wt% carbohydrate, and has an elemental composition (in Redfield format) of C q6H i 77O37N17. 

The amount of nitrogen fixed by the activity of azotobacter generally is not great as it is estimated that azotobacter requires 100 lbs. of carbohydrate for each pound of nitrogen fixed. 

In general, all the above-mentioned methods, employing concentrated sulphuric acid, have common drawbacks which may lead both to under- and over-estimation of carbohydrate content. During dehydration of individual sugars, non-detectable by-products such as 7-pyrones or benzene derivatives may be formed. No distinction is made between mono- and polysaccharides and since the different methods have different responses towards the various sugars, comparison of results obtained by the different authors is difficult. 

The total carbohydrates remaining in beer can be estimated as the colour produced by anthrone in 85% sulphuric acid [30]. For a range of beers, values between 0-89-5-98 % w/v as glucose were found [31]. Fully attenuated low carbohydrate lite beers, which have been brewed in the past for diabetic patients, are now generally available with carbohydrate contents between 0-4-0 9% w/v as glucose. 

Generally speaking, about 75% of the total biomass produced belongs to the class of carbohydrates. However, only 3.5% of these compounds are actually used by mankind (Tschan et al. 2012). Clearly, the theoretical availability of biomass does not mean that it is economically feasible or environmentally viable to collect it for industrial use. Parikka (2004) estimated the sustainable worldwide biomass energy potential to be about 100 EJ/year. Only 40% of this biomass is currently used according to Parikka (2004). 

Several other methods of polymer quantification in biological material have been used. In general, a polymer is usually isolated from biological material by a deproteini-zation procedure, extraction with organic solvents, etc., and then analyzed. Elemental analysis, viscosimetryturbidimetryand complexation with iodine (for PVP) in extracts were used in early studies in this field. These methods are now rarely used and have been replaced by labelling methods. Among the classical analytical techniques, the anthrone reaction for the estimation of carbohydrate polymers (e.g. dextran inulin >) has remained useful. 

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