Fluorescence quenching and time-resolved fluorescence studies on Trichosanthes dioica seed lectin

Abstract

Fluorescence quenching and time-resolved fluorescence studies have been carried out on the Trichosanthes dioica seed lectin (TDSL). The emission λ_max of native TDSL, seen at 328 nm, shifts to 343 nm upon denaturation with 6 M guanidinium chloride. Quenching titrations were performed with neutral (acrylamide and succinimide) and ionic (I⁻ and Cs⁺) quenchers in order to probe the exposure and accessibility of tryptophan residues of the protein. Maximum quenching was observed with acrylamide, followed by succinimide, iodide and Cs⁺. Dramatic increase in the extent of quenching and other quenching parameters by all the quenchers were observed upon denaturation of TDSL, suggesting that all the tryptophan residues in native TDSL are buried in the hydrophobic core of the protein. Increase in the extent of quenching upon denaturation of TDSL was maximum with I⁻ and minimum with Cs⁺, suggesting the presence of positively charged residue(s), near at least one tryptophan residue. Addition of saccharide ligands such as methyl-β-D-galactopyranoside and lactose led to a small, but reproducible decrease in the fluorescence intensity of the lectin. The presence of lactose provided a partial protection against quenching by I⁻, Cs⁺ and succinimide, but not acrylamide. In time-resolved fluorescence measurements the fluorescence decay curves could be best fitted to biexponential patterns with lifetimes of 4.09 and 1.53 ns for native lectin, 3.40 and 1.65 ns for the lectin in presence of 0.1 M lactose and 3.50 and 1.40 ns for denatured lectin.