Lys is a small globular protein, consisting of 129 amino acid res idues with four disulfide bonds. The importance of Lys relies on its extensive use as a model system to understand the underlying principles of protein structure, function, dynamics, and folding through theoretical and experimental studies (1.21. High natural abundance is also one of the reasons for choosing Lys as a model protein for studying protein-ligand interaction. Another important aspect of Lys is its ability to carry drug or biological activity sub- stances 13], and the effectiveness of them depends on their binding ability. Therefore, studies on the interaction between Lys and drugs or biological activity substances are of importance in view of real- izing disposition. transportation and metabolism of drugs or bio logical activity substances as well as efficacy process. Lys contains six tryptophan (Trp) and three tyrosine (Tyr) residues [4]. Three of Trp residues are located at the substrate binding sites. two in the hydrophobic matrix box. while one is separated from the others (4.5l. Trp62 and Trp108 are the most dominant fluoro- phores [6], both being located at the substrate binding sites. Investigation of binding mode between drug and protein under various pH conditions, especially at lower pH values. would