Molecular conformation and fluorescence properties of lactalbumin from four animal species

The quantum yield of tryptophan fluorescence of guinea-pig α lactalbumin is more than twice as large as found for the bovine, goat, or human proteins. This difference is due to the absence of Trp 60 from the guinea-pig amino acid sequence. It is proposed that the presence of this residue results in marked quenching of a second tryptophan residue for the other three proteins. Reference to the “lysozyme analogy model” suggests that the quenched residue is most likely Trp 104, which lies within 7Å of Trp 60, the latter lying within 6Å of two disulfide bridges. It is suggested that transfer of the excited state energy takes place from Trp 104 to 60 with subsequent quenching by the two vicinal disulfide bridges. These observations provide support for the validity of the “lysozyme analogy model”.