Tyrosine group behaviour in bovine alpha-lactalbumin as revealed by its Raman effect

Side group behaviour is often used for conformational studies of proteins. We have performed Raman spectroscopic measurements on the tyrosine groups of bovine alpha-lactalbumin. The 850/830 cm-1 doublet intensity ratio is a direct measure of the negative charge state of the phenolic oxygen and of the tyrosine environment. pH measurements confirm the existence of an acid conformer of BLA, that is comparable to, but clearly distinguishable from the apo-conformer. Following the Siamwiza theory, the Tyr groups in this partially unfolded state are situated in a more hydrophobic environment. Observation of Tyr groups behaviour in the denaturated states obtained by thermal or chemical treatment leads us to the same conclusion. However, the behaviour of tryptophan groups is quite different. In an unfolded state, the Trp residues are mostly exposed to the solvent. The stabilizing role of Ca2+ and Na+ ions in BLA is also investigated.