Exploring the fluorescence quenching interaction of amino acids and protein with natural organic matter by a multi-spectroscopic method

The main objective of this research was to explore the fluorescence quenching mechanism of a humic substance (Suwannee River natural organic matter (SWNOM)) to amino acids (tryptophan, tyrosine) and protein (bovine serum albumin, (BSA)) by multi-spectroscopic methods. The locations of the peaks of tryptophan, tyrosine, and BSA from Parallel Factor Analysis were at Ex/Em 280/356 nm, 275/302 nm, and 280/344 nm, respectively. For SWNOM, two peaks appeared at Ex/Em of 240/448 nm, and 350/450 nm. Static quenching was the dominant quenching mechanism between BSA and SWNOM, whereas no quenching was observed between tryptophan or tyrosine and SWNOM. Fourier-transform infrared spectroscopy and thermodynamic calculation demonstrated that hydrogen bonding and van der Waals force are the potential binding forces of the BSA-SWNOM complex, as a result of rearrangement in the secondary polypeptide carbonyl hydrogen bonding network of BSA. This rearrangement led to the conformational change in BSA that induced quenching of BSA fluorescence by SWNOM.