Exploring albumin-sulfonephthalein dyes interactions by a chemometric-assisted and multi-technique equilibria analysis in solution

Albumin is undoubtedly the most studied protein thanks to its widespread diffusion and biochemistry; despite its binding ability towards different dyes, provoking dye’s colour change, has been exploited for decades for quantification purposes, the joint effect of working pH, ionic strength, and dye’s pKa still remains only sporadically discussed. In the present study, the interaction of Bovine Serum Albumin (BSA) with five dyes belonging to the sulfonephthalein group, Bromophenol Blue (BPB, pKa = 3.75), Bromocresol Green (BCG, pKa = 4.42), Chlorophenol Red (CPR, pKa = 5.74), Bromocresol Purple (BCP, pKa = 6.05) and Bromothymol Blue (BTB, pKa = 6.72), is investigated at four working pH values (3.5, 6.0, 7.5 and 9.0) and two ionic strength conditions by UV–Vis spectroscopy. Principal Component Analysis is then applied to rationalize dye behavior upon BSA addition at each pH value and to summarize the protein effect on dyes’ spectral features, identifying three general behaviors. The most relevant systems are then submitted to further characterization involving a solution equilibria study aimed at determining conditional binding constants for the selected DSA-dye adducts and fluorescence, CD, and 1H NMR spectroscopy to evaluate the binding effect on the species involved.