@article{eissa_puhl_kadla_khan_2006, title={Enzymatic cross-linking of beta-lactoglobulin: Conformational properties using FTIR spectroscopy}, volume={7}, ISSN={["1526-4602"]}, DOI={10.1021/bm050928p}, abstractNote={In this study, we use FTIR spectroscopy to probe the conformational changes of beta-lactoglobulin (beta-LG)-the main constituent of whey proteins-as subjected to enzymatic cross-linking by transglutaminase. We investigate both the amide I region (1600-1700 cm(-1)) and the C-H stretching region (2800-3100 cm(-1)). In the amide I region, spectra of denatured conformations of beta-LG, known to be necessary for cross-linking, differ according to the denaturation procedure, i.e., chemical or thermal treatment. Denaturation by chemical denaturants, dithiothreitol (DTT) or beta-mercaptoethanol, show no effect on the alpha-helix, while shifting the monomer dimer equilibrium toward higher monomer concentration. On the other hand, denaturing by thermal treatment dissociates the beta-sheets in the native structure, leading to new intermolecular beta-sheets being formed. Preheated then enzyme cross-linked beta-LG molecules show very similar spectra in the amide I region to the molecules with no cross-linking, indicating minimal effects of the cross-links on the carbonyl stretching mode. However, chemically denatured (using beta-mercaptoethanol) then enzyme cross-linked beta-LG molecules show noticeable diminution in the alpha-helix band and formation of strong hydrogen-bonded intermolecular beta-sheets. In the C-H stretching region, preheated then enzyme cross-linked beta-LG molecules exhibit a different degree of exposure of aliphatic amino acids due to the enzyme action. The same behavior is observed for DTT-treated then enzyme cross-linked beta-LG molecules. Generally, the changes in the C-H stretching region clearly indicate that hydrophobic interactions are altered upon enzymatic cross-linking.}, number={6}, journal={BIOMACROMOLECULES}, author={Eissa, Ahmed S. and Puhl, Christa and Kadla, John F. and Khan, Saad A.}, year={2006}, month={Jun}, pages={1707–1713} }
 @article{eissa_khan_2006, title={Modulation of hydrophobic interactions in denatured whey proteins by transglutaminase enzyme}, volume={20}, ISSN={["1873-7137"]}, DOI={10.1016/j.foodhyd.2005.07.005}, abstractNote={The role of enzyme crosslinking in mediating formation of hydrophobic association in chemically denatured whey protein isolates (WPI) is examined. WPI samples denatured with dithiothreitol (DTT) and incubated at 50 °C with and without transglutaminase enzyme show dramatic differences in viscosity, with the viscosity of the sample exposed to enzyme being lower by several orders of magnitude than the sample without enzyme. Upon further exposure of both samples to sodium dodecyl sulfate (SDS) to eliminate hydrophobic interactions, we observe no change in the viscosity of the sample previously treated with enzyme, suggesting this sample to have minimal hydrophobic associations. In contrast, the sample without enzyme shows a dramatic drop in viscosity indicating it to have had substantial hydrophobic associations. A similar trend but to a lesser extent is observed at a higher WPI concentration. These results taken together suggest that the formation of enzyme catalyzed ε-(γ-glutamyl)lysine bonds attenuates hydrophobic interactions through steric hindrance and formation of compact molecules that limits exposure of the hydrophobic moieties.}, number={4}, journal={FOOD HYDROCOLLOIDS}, author={Eissa, AS and Khan, SA}, year={2006}, month={Jun}, pages={543–547} }
 @article{eissa_khan_2005, title={Acid-induced gelation of enzymatically modified, preheated whey proteins}, volume={53}, ISSN={["1520-5118"]}, DOI={10.1021/jf047957w}, abstractNote={Low-pH whey protein gels are formulated using a sequential protocol of heat treatment, enzyme incubation, and cold-set acidification. The heat-induced disulfide and enzyme-catalyzed epsilon-(gamma-glutamyl)lysine linkages, both at neutral pH, produce a polymerized protein solution. The molecular weights of these samples show an exponential increase with protein concentration. The additional enzyme-catalyzed cross-links cause little change in molecular weight from that of heat-treated samples at low protein concentrations, indicating predominant intramolecular cross-linking. Enzyme treatment at higher protein concentration however causes increase in molecular weight, possibly due to formation of intermolecular cross-links. Acidification of the polymerized protein solutions through glucono-delta-lactone acid leads to gel formation at pH 4. The elastic (G') and viscous (G' ') moduli of gels with and without enzyme treatment show similar frequency dependence, indicating comparable microstructures, consistent with all samples exhibiting similar fractal dimensions of approximately 2 obtained independently using rheology and confocal microscopy. A substantial increase in fracture strain and stress of the gel is achieved by enzyme treatment. However, the elastic modulus (G') is only slightly larger after enzyme treatment compared with heat-treated samples. These results indicate that factors responsible for fracture properties may not be apparent in the gel microstructure and linear viscoelastic properties.}, number={12}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Eissa, AS and Khan, SA}, year={2005}, month={Jun}, pages={5010–5017} }
 @article{eissa_bisram_khan_2004, title={Polymerization and gelation of whey protein isolates at low pH using transglutaminase enzyme}, volume={52}, ISSN={["1520-5118"]}, DOI={10.1021/jf0355304}, abstractNote={Dynamic and steady shear rheology is used to examine the synthesis of low-pH (approximately 4) whey protein gels obtained through a two-step process. The first step involves cross-linking of whey proteins at pH 8 and 50 degrees C using transglutaminase enzyme, while the second step entails cold-set acidification of the resulting solution using glucono-delta-lactone (GDL) acid. During the first step, the sample undergoes enzyme-catalyzed epsilon-(gamma-glutamyl)lysine bond formation with a substantial increase in viscosity. Acidification in the second step using GDL acid leads to a rapid decrease in pH with a concomitant increase in the elastic (G') and viscous (G' ') moduli and formation of a gelled network. We examine the large strain behavior of the gel samples using a relatively new approach that entails plotting the product of elastic modulus and strain (G'gamma) as a function of increasing dynamic strain and looking for a maximum, which corresponds to the yield or fracture point. We find the enzyme-catalyzed gels to have significantly higher yield/fracture stress and strain compared to cold-set gels prepared without enzyme or conventional heat-set gels. In addition, the elastic modulus of the enzyme-catalyzed gel is also higher than its non-enzyme-treated counterpart. These results are discussed in terms of the gel microstructure and the role played by the enzyme-induced cross-links.}, number={14}, journal={JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY}, author={Eissa, AS and Bisram, S and Khan, SA}, year={2004}, month={Jul}, pages={4456–4464} }