Probing Bile Salt Aggregates by Fluorescence Quenching

Abstract

Abstract— The combination of steady‐state and time‐resolved quenching experiments was employed to study the aggregation behavior of sodium cholate at concentrations below 50 mAf. Naphthalene, anthracene and pyrene were used as fluorescent probe molecules, and protection by the aggregates from aqueous quenchers, as well as the onset of aggregation at low sodium cholate concentrations, was dependent on the shape of the probes. Protection from aqueous quenchers was inferred by comparing the efficiency for dynamic quenching in the absence and presence of sodium cholate and was best for naphthalene followed by pyrene and anthracene. Static quenching was observed, suggesting that probe molecules are located in an aggregate environment that also contains iodide. The incorporation of pyrene at low sodium cholate concentrations, as well as the small degree of static quenching observed, suggest that the shape complementarity, i.e. hydrophobic surface and packing, between pyrene and sodium cholate is optimum for aggregate formation.

Dynamics of Probe Complexation to Bile Salt Aggregates

“…13 The dynamic studies were performed at NaC and NaTC concentrations above 20 mM, where primary aggregates were fully formed. In the case of NaC, this was confirmed by the observation that the protection of excited singlet naphthalene by the bile salt aggregates did not increase for NaC concentrations above 20 mM.…”

“…Previous fluorescence studies indicated that when naphthalene was incorporated into the hydrophobic sites of primary aggregates, it was well-protected from quenchers which primarily reside in the aqueous phase, as attested by the decrease of the rate constant for the dynamic quenching of this naphthalene population. 13 In the case of the triplet xanthone the hydrophobic and hydrophilic solubilization sites (Scheme 1) were possible candidates for the incorporation of this probe, and our studies indicate that this probe is located in a different environment than naphthalene.…”

“…13 This excited singlet naphthalene population was very efficiently protected from quenching by iodide. At NaC concentrations SCHEME 2 above 20 mM, the dynamic quenching efficiency decreased by a factor of 50 and remained constant for higher bile salt concentrations.…”

“…The polarity sensed by polycyclic aromatic hydrocarbons, such as pyrene, was much smaller than in SDS, the microviscosity was higher, and aggregation did not occur at a well-defined critical micellar concentration. Bile salt aggregates are quite polydispersed, 8,19 and we recently showed 13 that the onset of aggregation at low sodium cholate concentrations was dependent on the shape of the probe molecule being employed.…”

Synthesis and the Stimuli‐Responsive Characteristics of Cyclic Cholamide Nicotinamide Adenine Dinucleotide (NAD+) Derivative with Aggregation in Basic Aqueous Solution

“…[10] Under different conditions, such as different solvent polarity,p H, and temperature, the behavior and conformation of cholate derivatives would be different. [11] Early studies on structural aspects have shown that cholate aggregates progressively to form micelles, [12] leading to thef ormation of larger structures as the concentrationo fc holate monomers increases. The primary/ secondary aggregationm odel proposes the association of the hydrophobic faces, in which as mall number of monomerslead to primarya ggregation at al ow concentration.A th igh monomer concentrations, the primary aggregation agglomerates into larger structures called secondary aggregates.…”