Noncovalent Association Thermodynamics of Turn-On Fluorescent Probes with Human Serum Albumin: Dual-Concentration Ratio Method

Efficient quantification of the affinity of a drug and the targeted protein is critical for strategic drug design. Among the various molecules, turn-on fluorescent probes are the most promising signal transducers to reveal the binding strength and site-specificity of designed drugs. However, the conventional method of measuring the binding ability of turn-on fluorescent probes by using the fractional occupancy under the law of mass action is time-consuming and a massive sample is required. Here, we report a new method, called dual-concentration ratio method, for quantifying the binding affinity of fluorescent probes and human serum albumin (HSA). Temperature-dependent fluorescence intensity ratios of a one-to-one complex (L ⋅ HSA) for a turn-on fluorescent probe (L), e. g., ThT (thioflavin T) or DG (dansylglycine), with HSA at two different values of [L]0 /[HSA]0 under the constraint [HSA]0 >[L]0 were collected. The van't Hoff analysis on these association constants further resulted in the thermodynamic properties. Since only two samples at different [L]0 /[HSA]0 are required without the need of [L]0 /[HSA]0 at a wide range, the dual-concentration ratio method is an easy way to greatly reduce the amounts of fluorescent probes and proteins, as well as the acquisition time.

Hydrophobic interaction chromatography of proteins: Studies of unfolding upon adsorption by isothermal titration calorimetry

Heat of adsorption is an excellent measure for adsorption strength and, therefore, very useful to study the influence of salt and temperature in hydrophobic interaction chromatography. The adsorption of bovine serum albumin and β-lactoglobulin to Toyopearl Butyl-650 M was studied with isothermal titration calorimetry to follow the unfolding of proteins on hydrophobic surfaces. Isothermal titration calorimetry is established as an experimental method to track conformational changes of proteins on stationary phases. Experiments were carried out at two different salt concentrations and five different temperatures. Protein unfolding, as indicated by large changes of molar enthalpy of adsorption Δh_ads , was observed to be dependent on temperature and salt concentration. Δh_ads were significantly higher for bovine serum albumin and ranged from 578 (288 K) to 811 (308 K) kJ/mol for 1.2 mol/kg ammonium sulfate. Δh_ads for β-lactoglobulin ranged from 129 kJ/mol (288 K) to 186 kJ/mol (308 K). For both proteins, Δh_ads increased with increasing temperature. The influence of salt concentration on Δh_ads was also more pronounced for bovine serum albumin than for β-lactoglobulin. The comparison of retention analysis evaluated by the van't Hoff algorithm shows that beyond adsorption other processes occur simultaneously. Further interpretation such as unfolding upon adsorption needs other in situ techniques.