The pH dependence of tryptophan fluorescence of goat alpha-lactalbumin with particular reference to the effect of binding of an impurity from milk.

Two processes giving rise to changes in tryptophan fluorescence of goat alpha-lactalbumin below pH 7 have been studied. Both of these spectral transitions are strongly influenced by the presence of an impurity which binds to the protein if its contact at acid pH with the milk mother liquor is prolonged during its preparation. Transition II, the U leads to N conversion, follows the same course at both 2 and 25 degrees C, i.e. a minimum of three abnormally titrating groups are normalized during the conformational change at both temperatures. The difference in this conclusion from that made earlier (Kronman, M.J., Jeroszko, J. and Sage, G.W. (1972) Biochim. Biophys. Acta 285, 145-166) that groups are normalized at 25 and at 2 degrees C was found to be due to binding of the impurity to the protein used in the earlier study. Transition I, which we detect between pH 7 and 4.4 and which appears to be due to perturbation of tryptophans by vicinal ionizing groups, overlaps Transition II, the U leads to N conversion. However, the fluorescence above approx. 340 nm in the emission spectrum appears to be free of contributions from the former process and can be used in a valid analysis of the thermodynamics of the latter process.