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Thermal stability of β-lactoglobulins A and B: effect of SDS, urea, cysteine and N-ethylmaleimide

Published online by Cambridge University Press:  04 May 2004

Joyce I Boye
Affiliation:
Food Research and Development Centre, Agriculture & Agri-Food Canada, 3600 Casavant Blvd W., St. Hyacinthe, QC, Canada J2S 8E3
Ching Y Ma
Affiliation:
Department of Botany, University of Hong Kong, Pokfulam Road, Hong Kong
Ashraf Ismail
Affiliation:
Department of Food Science and Agricultural Chemistry, Macdonald Campus, McGill University, 21, 111 Lakeshore Road, St. Ann de Bellevue, QC, Canada, H9X 3V9

Abstract

Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were used to monitor changes in the secondary structure and thermal stability of β-lactoglobulin A and B in the presence of sodium dodecyl sulphate (SDS), N-ethylmaleimide (NEM), urea and cysteine. An increase in the thermal stabilities of both proteins was noted in the presence of 10 mM-SDS. In the presence of 50 mM-SDS, there was extensive denaturation of both variants. In general, the β-strand/β-sheet regions in the secondary structure of both variants were very susceptible to denaturation by SDS and cysteine, suggesting that these regions may be held by hydrophobic and disulphide bonds. At ambient temperature and physiological pH, a notable difference was observed in the 1636 and 1627 cm−1 regions of the FTIR spectra of the two β-lg variants. The results suggest possible differences in the nature of the β-sheet/β-strand distribution/content of the two proteins. Urea and NEM at a concentration of 50 mM, had little effect on the secondary structure and denaturation of both variants. New findings are presented which further indicate that although the β-lg B variant showed greater thermal stability than the A variant in all the cases studied, its denaturation temperature and secondary structure were affected to a greater extent by the protein perturbants than β-lg A.

Type
Research Article
Copyright
Proprietors of Journal of Dairy Research 2004

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