Denaturation of bovine spleen galectin-1 in guanidine hydrochloride and fluoroalcohols: structural characterization and implications for protein folding.

Guanidine hydrochloride (GdnHCl)-induced unfolding of bovine spleen galectin-1 (Gal-1) exhibits three-state mechanism involving exclusive, structured tertiary monomer in 0.5 M GdnHCl. Gal-1 has one tryptophan residue (Trp 68) per subunit. Phosphorescence spectra of both Gal-1 dimer and intermediate monomer at 77 K show single (0,0) band at 405.6 nm, characteristics of free tryptophan environment as of N-acetyl-l-tryptophanamide. Unfolded Gal-1 in 4 M GdnHCl gives (0,0) band at longer wavelength (408.6 nm) signifying that Trp 68 is less solvent exposed, being localized in an environment of residual structure. Trifluoroethanol (TFE)- and hexafluoroisopropanol (HFIP)-induced structural changes of Gal-1 dimer and monomer have been investigated by far-UV CD and FTIR. CD results show reversible nature of β-sheet to α-helix transition, with 30% helix in 80% TFE or 40% HFIP for Gal-1 dimer. Temperature-dependent studies show that induced helix entails reduced thermal stability. FTIR results reveal partial β-sheet to α-helix conversion but with quantitative yield. At intermediate TFE concentration, both Gal-1 dimer and monomer aggregate as evidenced by FTIR band at ∼1617/cm, however, the onset of aggregation and stability of aggregates for monomer differ from those of dimer. The results may provide important insights into perturbed folding problem of Gal-1.