Enzymatic hydrolysis of guar galactomannan solutions and gels: Molecular structure and rheology.

摘要

Enzymes offer a powerful tool to degrade and tailor-make water-soluble polymers with enhanced functional performance. This thesis examines the use of enzymes to degrade guar galactomannan--a water-soluble polysaccharide of significant importance in many applications. Specifically, we seek to elucidate the link between molecular and rheological property changes during enzymatic hydrolysis using an array of rheological measurements in conjunction with Gel Permeation Chromatography.; GPC studies on enzymatically degraded guar solutions show that the reciprocal of weight average molecular weight, M{dollar}rmsb{lcub}w{rcub},{dollar} is proportional to the time of degradation, with the proportionality constant varying inversely with polymer concentration. This result indicates that enzymatic degradation of guar follows zeroth-order kinetics in guar concentration--a finding in contrast to prevailing results on polymer degradation. Additionally, the viscosity-time profiles for different enzyme concentrations can be collapsed on to a single curve by shifts along the time axis; enabling a priori prediction of guar solution viscosity as function of degradation time and enzyme concentration.; Dynamic rheological moduli (G{dollar}spprime,{dollar} G{dollar}sp{lcub}primeprime{rcub}){dollar} are very sensitive to the microstructure of guar-borax gels and reduce significantly upon degradation by endo-mannanase, the enzyme cleaving the polymer backbone. The rate of reduction shows three distinct zones: an initial large decrease followed by a slower reduction rate at intermediate times, and, an accelerated reduction at longer degradation times. A synergistic increase in the degradation rate is also observed on using a combination of endo-mannanase and {dollar}alpha{dollar}-galactosidase (side chain cleaving) enzymes as compared to using only endo-mannanase.; Computer simulations, in conjunction with molecular weight distributions obtained from GPC, are used to obtain physical insights on the mechanistics of guar degraded by ultrasound and enzymatic degradation. Sonication of guar results in random hydrolysis of chains, so that all bonds have equal probability of cleavage. In contrast, enzymatic degradation did not follow any of the typical modes of scission (random, gaussian, and central), suggesting that enzymatic hydrolysis of guar does not obey a single scission pathway. Rather, features such as the development of a low molecular weight peak with time and the asymptotic rise of polydispersity to a constant value, are indicative of a multiple scission pathway.