The glucan binding domain (GBD) of the glucan binding protein-A (GBP-A) from the cariogenic bacterium Streptococcus mutans was studied using circular dichroism (CD) analysis, Chou-Fasman-Rose secondary structure prediction, and absorption and fluorescence spectroscopy. Our data show that the binding domain undergoes a conformational shift upon binding to the ligand dextran. The CD spectrum shows two positive bands at 280 nm and 230 nm which were assigned to aromatic residues. The 230-nm band was seen at 20 degrees C and 30 degrees C, lost intensity at 40 degrees C, and was eliminated at 45 degrees C coinciding with complete denaturation. The protein was stable at physiological pH, but precipitated at pH 5. A pH of 10 changed the secondary structure but had no effect on the 230-nm band. Analysis of the CD data in the far UV using the SELCON computer program revealed a high content of beta-sheets and a lack of alpha-helical structures. Secondary structure prediction based on the amino acid sequence of GBD agreed with the CD analysis. The fluorescence emission maximum at 339 nm suggested that the majority of the tryptophans were located in the interior of the protein. This maximum shifted to higher energy upon binding to the ligand dextran.