Effect of the Structure of Bile Salt Aggregates on the Binding of Aromatic Guests and the Accessibility of Anions

The binding of naphthalene (Np), 1-ethylnaphthalene (EtNp), acenaphthene (AcN), and 1-naphthyl-1-ethanol (NpOH) as guests to the aggregates of sodium cholate (NaCh), taurocholate (NaTC), deoxycholate (NaDC), and deoxytaurocholate (NaTDC) was studied with the objective of determining how the structure of the bile salts affects the binding dynamics of guests and quenchers with the bile salt aggregates. Time-resolved and steady-state fluorescence experiments were used to determine the binding efficiency or the guests with the aggregates and were also employed to investigate the quenching of the singlet excited state of the guests by iodide anions. Quenching Studies of the triplet excited states using laser flash photolysis were employed to determine the accessibility to the aggregate of nitrite anions, used its quenchers, and the dissociation rate constants of the guests from the bile salt aggregates. The binding efficiency of the guests to NaDC and NaTDC is higher than for NaCh and NaTC, and (lie protection efficiency is also higher for NaDC and NaTDC, in line with the larger aggregates formed for the latter bile salts. The formation of aggregates is in part driven by the structure of the guest, where all increased protection efficiency and residence time can be achieved by the introduction of short alkyl substituents (AcN or EtNp vs Np). NpOH was shown to be located in a very different environment in all four bile salts when compared to AcN, EtNp, and Np, Suggesting that hydrogen bonding plays an important role in the formation of the aggregate around NpOH.