Dual-Sensor Fluorescent Probes of Surfactant-Induced Unfolding of Human Serum Albumin

Two extrinsic fluorescent probes, 3-(dimethylamino)-8,9,10,11-tetrahydro-7<i>H</i>-cyclohepta­[<i>a</i>]­naphthalen-7-one (<b>1</b>) and 7-(dimethylamino)-2,3-dihydrophenanthren-4­(1<i>H</i>)-one (<b>2</b>), are used to probe the unfolding of human serum albumin by sodium dodecyl sulfate (SDS). These probes respond separately to the polarity and H-bond-donating ability of their surroundings. Competitive binding experiments show that fluorophore <b>1</b> binds to site I (domain IIA) and <b>2</b> binds to site II (domain IIIA). The local acidity of <b>1</b> in site I is out of the sensing range of <b>1</b>, whereas the local acidity of <b>2</b> in site II is calculated to be nearly zero on Catalan’s solvent acidity index. Both probes show that the first two equivalents of bound SDS result in a decrease in the local polarity of the binding sites. Each subsequent equivalent of SDS gives rise to a dramatic increase in polarity until HSA is saturated with seven molecules of SDS at the end of the specific binding domain. Compound <b>2</b> experiences an increase of acidity of 0.10 on Catalan’s solvent acidity index through seven equivalents of SDS, but the local acidity for <b>1</b> is still out of range. The increase in acidity experienced by <b>2</b> is greater than the increase in polarity. This result is consistent with greater exposure of the carbonyl group in <b>2</b>, but not the bulk of <b>2</b>, to the aqueous solvent in site II of the SDS-saturated HSA complex