Understanding the interaction of 5-(tert-Butyl)-2-hydroxy-1,3-isophthaldehyde (5-tBHI), a single benzene-based fluorophore, with albumin and albumin-SDS aggregates

We report the pH dependent spectroscopic study of the solvent mediated intermolecular proton-transfer and binding characteristics of 5-( tert -butyl)-2-hydroxy-1,3-isopthalaldehyde (5- t BHI), a unique single benzene-based fluorophore, with Human Serum Albumin (HSA). The study has been performed at pH 7.4 and 5.6, respectively, to assess the impact of the protein conformational flexibility during the N→F transition. Despite ground state heterogeneity, comparable fluorescence responses suggest that the ligand binding is predominantly unaffected by N→F transition. Computational study indicates that all possible forms of 5- t BHI, in the respective pHs (pH 7.4 and 5.6), namely enolic, anionic, tautomeric and the protonated forms, attach with the subdomain IIA of HSA. The incorporation of sodium dodecyl sulphate (SDS), at pH 5.6, results in significant spectral alterations, indicating cooperative protein-surfactant interactions, partial unfolding of HSA, and the development of necklace and bead model like structures. These structural reorganisations alter local polarity and hydrophobicity, disrupting the excited state proton transfer process. Overall, 5- t BHI is found to be an effective environment-sensitive probe for connecting proton transfer process to protein conformational states and surfactant-induced structural alterations.