The pH threshold in the dissolution of beta-lactoglobulin gels and aggregates in alkali.
The existence of a practical minimum pH for the dissolution of heat-induced whey gels in alkaline solutions has been studied using beta-lactoglobulin (betaLg) as a model protein. A sharp transition in solubility was observed between pH 11 and 12; this transition shifts to higher pHs for gels formed at higher temperatures and for longer gelling times. The breakdown reactions of heat-induced aggregates in alkali were monitored with size exclusion chromatography. The destruction of large aggregates was faster at higher pH and also showed a transition between pH 11 and 12. Using tryptophan fluorescence and near- and far-UV circular dichroism, this transition was assigned to the base-induced denaturation observed in solutions of aggregates (pK 11.53). It is suggested that the high protein repulsion caused by the large number of charges at pH > 11.5 drives the unfolding of the protein and the disruption of the intermolecular noncovalent bonds. Concentrated urea and GuHCl were found to be less effective than a pH 12 solution in destroying large aggregates. Aggregates formed for a long time (80 degrees C for 24 h) contained a larger number of intermolecular disulfide bonds that hinder the dissolution process. Gels formed at low temperatures (65 degrees C for 60 min), with fewer intermolecular noncovalent bonds, showed a similar solubility-pH profile to that observed for the base-induced denaturation of unheated beta-lactoglobulin (betaLg) (pK 10.63).
Mercadé-Prieto
Ruben
R
Department of Chemical Engineering, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, UK.
Paterson
William R
WR
Wilson
D Ian
DI
D000468:Alkalies / Q000737:chemistry*; D005782:Gels / Q000737:chemistry; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007782:Lactoglobulins / Q000737:chemistry*; D012995:Solubility; D012996:Solutions / Q000737:chemistry; D013997:Time Factors
D000468:Alkalies; D005782:Gels; D007782:Lactoglobulins; D012996:Solutions