The nature of a substituent on cellulose has a large effect on the properties of the cellulosic derivative [1,2]. Thus, HEC is soluble in water at all temperatures, but methylcellulose starts to gel at about 45 °C, and hydroxypropylcellulose precipitates from water at 40–45 °C. An example of the effect of polar groups on solubility has been shown for acetylation of HEC [15] (Figure 1). The reaction was carried out by suspending HEC in N,N-dimethylacetamide (DMAc) and adding Lipase P (from Pseudomonas cepacia, Amano Enzyme USA, Elgin, Illinois). After thorough mixing, a slurry was obtained, which could be isolated to give the product. HEC is water-soluble, but when acetylated, the resulting polymer becomes much less soluble in water.

Another example is the graft polymerization of ε-caprolactone onto HEC through the catalysis of lipase (Figure 2) [18]. In this case HEC film was incubated with ε-caprolactone and porcine pancreatic lipase at 60 °C for 3–5 days. The film was extracted with chloroform to remove any homopolymer that might have formed. The product was found to be HEC grafted with poly(ε-caprolactone).

For some applications, the introduction of an electrically charged group to a polysaccharide can be highly desirable. Enzymes can be used to catalyze the addition of charged substituents onto cellulosic derivatives. For example, succinic anhydride was grafted onto HEC in the presence of a lipase (Figure 3) [15]. For this reaction HEC was suspended in DMAc, and succinic anhydride and Lipase P were added and then heated at 60 °C for 16 h. A slurry was obtained from which the product was precipitated with acetone. Whereas this reaction can also proceed without the use of enzymes, the enzymatic process has the advantage of lower reaction temperature and higher degree of substitution.

An alternative method of placing an anionic charge on HEC is to oxidize the HEC. An example was reported by Hu, et al. [19], using in-situ generation of hypobromite catalyzed by lipase and coupled with oxidation with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). In this way, the chain-end alcohols of both HEC and poly(ethylene glycol) were converted to carboxylic acids.

An example of the addition of a cationic charge to a polysaccharide is shown for the following reactions (Figure 4) with CMC and a diamine, where X = (CH₂)_m, and m > 2. The reaction with CMC (where R = H) was found to proceed at 5%–20% efficiency for subtilisin Carlsberg, papain, and protease from Aspergillus saitoi in N,N-dimethylformamide (DMF) solvent. The reaction with CMC ester (R = methyl or ethyl) was, however, more facile, and a higher efficiency was achieved [20].

Hydrophobic modification is a common method to synthesize surfactants or rheology modifiers. The hydrophobes associate with one another in aqueous solutions at low concentrations, and thereby increase the viscosity of the solution at low shear frequencies [21]. Examples of commercial associative thickeners include hydrophobically modified HEC (HMHEC) [22], ethoxylated urethane (HEUR) [23,24], and alkali-soluble emulsion (HASE) [25,26,27]. Enzyme catalysis can be used to put a hydrophobic unit onto a cellulose derivative. For example, stearic ester was grafted onto HEC through the use of vinyl stearate (Figure 5) [28]. Since vinyl stearate is expensive, a more cost-effective route was reported using methyl stearate instead [15]. The latter reaction entailed a slower rate of reaction, but the rate could be accelerated by pulling a vacuum on the reaction.

Alkyl ketene dimers (AKD) are well known materials, prepared by the 2+2 addition reaction of ketenes from long-chain fatty acids (Figure 6). They are widely used as sizing agents on paper [29,30].

An enzymatic method was developed to graft AKD onto polysaccharides [31,32]. An example of the reaction for HEC is shown in Figure 7. The reaction can also be achieved chemically, but the enzyme approach has the advantage of mild reaction conditions and high efficiencies. The resulting product is a hydrophobically modified water-soluble polymer and has interesting surface-active and rheological properties.

In a typical synthesis [31], HEC with degree of substitution of about 2.5 was suspended in an organic solvent. AKD and a lipase (e.g., from Pseudomonas sp. or Pseudomonas fluorescens) were added. Several AKD-modified HEC esters were reported (Table 2) at 1:100 AKD/HEC weight ratio (DS < 0.01), and the viscosities of the AKD/HEC esters were significantly improved as compared to the starting HEC. It was found that the viscosities remain unchanged after 3 days at pH 6.5 or pH 8.5.

In addition to the above reactions, sometimes it is necessary to graft a specific functionality to a polysaccharide. An example is the grafting of a vinyl group to HEC (Figure 8). Lipase from P. cepacia (Lipase PS, Amano) and Celite-immobilized bacterial alkaline protease (Alcalase, Novozymes A/S) can be used to catalyze this reaction [28,33]. The grafted HEC formed a strong crosslinked gel in DMAc at 5% upon heating to 80 °C, indicating covalent bonding of acrylate to HEC.

Another useful functionality is the acetoacetonate group, which is known to complex with metal ions. The reaction of HEC with diketene in the presence of a lipase was shown [34] to produce the requisite structure and the expected properties (Figure 9).

A different approach is to graft galactose onto HEC using lactose and β-galactosidase. The galactose moiety is transferred to the hydroxyl group in HEC while glucose is discarded (Figure 10). As indicated earlier, the position of the galactose additive is only shown schematically in Figure 10; in actuality galactose is substituted statistically on available OH units. Moreover, the C-6 hydroxy of galactose can be oxidized to an aldehyde via galactose oxidase in a subsequent reaction [35,36,37].

In many applications, polysaccharides with lower molecular weights are useful. In commercial processes, two chemical methods are used, viz., hydrolysis with mineral acids and oxidation with hydrogen peroxide [38,39,40]. Enzymes pose an alternative method and have the advantage of milder reaction conditions, less colored products and reduced undesirable by-products. Most cellulosic derivatives are amenable to enzymatic hydrolysis especially when adjacent glucose residues are not substituted [41,42]. An example of CMC hydrolysis is given in Figure 11.

Several of the bonds in the guar structure are susceptible to enzymatic action, as shown in a simplified scheme in Figure 12.

It is known from the literature that in the presence of mannanase, the molecular weight of guar can be reduced [43,44,45]. In a combinatorial experiment [46], the degradation of guar was reported in the presence of four enzymes (lipase, hemicellulase, pectinase, and protease). As expected, hemicellulase catalyzed a large decrease in molecular weight. The combination of hemicellulase and protease gave the lowest molecular weights achieved. This result is perhaps not surprising in view of the fact that guar gum contains up to 7% proteins. Thus, the protein in guar is a likely contributor to the observed molecular weight of guar.

A good example of side-chain hydrolysis is the removal of galactose from guar using α-galactosidase [47,48], pullulanase [49,50], and pectinase [51]. In this way, the mannose: galactose ratio can be decreased with attendant changes in its gelling tendency. A study of guar rheology in the presence of three enzymes was reported; decreased viscosity was found and a rheokinetic model was used to model the synergistic hydrolysis reactions [52]. In another study [53] guar was treated with α-galactosidase to lower galactose content, oxidized with galactose oxidase, and epimerized using mannuronan C5-epimerases to obtain pseudo-alginates.

An example of the addition of an anionic group to guar is shown for the following acylation reaction (Figure 13), yielding randomly substituted products. Lipase AK (Pseudomonas sp., Amano) was found to have excellent activity for this reaction, giving maleated guar [15].

The same reaction worked equally well with succinic anhydride in the presence of a lipase to produce succinated guar [46]. Three lipases, Lipase AK, Lipase CC (from Candida cylindracea, Sigma) and Lipase OF-360 (from C. cylindracea, Meito Sangyo, Tokyo), were screened for this modification reaction. Lipase AK showed the highest catalytic activity for the acylation of polysaccharide, and Lipase CC the lowest activity.

Hydrophobic modification of cationic guar was achieved in the same way as cellulosic derivatives (Figure 14) [28]. Enzymatic reaction was found to perform very well (conversion >90%) when the starting material was dissolved or swollen in DMAc. However, when the same enzymatic reaction was applied to guar gum instead of its cationic guar derivative, the ester formation was very low.

The reaction shown in Figure 14 could also proceed, albeit at lower efficiency, using palmitic acid and cationic guar in an aqueous buffer. The efficiency could be improved by adding a small amount of DMF to water [14]. In contrast, underivatized guar gave no reaction under the same reaction conditions [14]. This synthesis entailed the use of unmodified fatty acid and was carried out in water. Thus, this represented a normally unfavorable case: there was no convenient leaving group and water was present in abundance. This reaction was possible due to the interaction of the enzyme with the cationic charge on guar, which favored the ester formation even in the presence of water.

The AKD reaction (Figure 7) mentioned in the previous section was also attempted on guar [32]. Novozym^® 435 lipase was found to be an appropriate enzyme with t-butyl methyl ether as reaction medium. The product was obtained at 88% efficiency. The resulting AKD/guar ester even at low DS (less than 0.01) showed increased viscosity in water relative to unmodified guar.

Guar can readily acquire, through galactose oxidase, a reactive aldehyde functionality in the C6 position [54,55,56,57,58,59]. This structure was shown to improve the dry strength of paper [56,57,58,59]. Other enzymes have also been reported to carry out this oxidation [60,61].