Complexation of probe molecules to the different binding sites of bile salt aggregates
Date
2018-11-08
Authors
Rinco, Olga
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Abstract
In order to gain an understanding of the interaction of probe molecules with NaCh aggregates, steady-state and time-resolved photophysical methods were used. By employing several probe molecules, an understanding of how the structure of the probe affected binding to the two distinct sites in the aggregates was investigated. The first half of the thesis examined the reactivity of benzophenone (Bp) and 4,4-dimethylbenzophenone (DMBp) with NaCh aggregates by studying the kinetics of both the ketone triplet excited states and the ketone ketyl radicals. There were three species of triplet excited states observed in the presence of primary aggregates. One of the ketone triplet excited state species was located within the primary aggregate, and it reacted to form ketyl radicals. Other triplet states included in the primary aggregate were found to be long-lived, while a third species of triplet states was present in the aqueous phase. At higher bile salt concentrations, and in the presence of secondary aggregates, a layer of complexity was added. The binding dynamics for the triplet excited state with the secondary binding sites were much faster than those observed for the primary binding site. Hydrogen abstraction did not compete with other deactivation pathways in the presence of secondary aggregates, and thus only self-quenching and exit of the excited state probe from the secondary site were observed. Ketyl radical recombination took place in water and in the secondary sites. The second half of the research focused on the study a series of naphthalene (Np) derivatives in order to look at the effects of shape and hydrophobicity of probe molecules on the interactions between these probes and the host NaCh aggregates. 1-Ethylnaphthalene, 2-ethylnaphthalene, 1-acetonaphthone, 2-acetonaphthone, 1-naphthyl-1-ethanol and 2-naphthyl-1-ethanol were studied. 1-Ethylnaphthalene and 2-ethylnaphthalene were Ethylnaphthalene, 2-ethylnaphthalene, 1-acetonaphihone, 2-acetonaphthone, 1-naphthyl-
1-ethanol and 2-naphthyl-1-ethanol were studied. I-Ethylnaphthalene and 2-
ethylnaphthalene were contained within the primary binding site, while 1-naphthyl-1-ethanol,
2-naphthyl-1-ethanol, 1-acetonaphthone and 2-acetonaphthone were contained
within the secondary binding site. The effect of the position of the substituent was only
noticed when the probe molecules formed weak interactions with the outside of the
primary aggregate, and not when the probe was complexed to one of the binding sites
present in the NaCh system.
The naphthalene probe molecules were also used to study the effect of ionic
strength on NaCh aggregate formation. It was found that primary aggregation occurred at
lower NaCh concentration as the ionic strength was increased. No effect of ionic strength
was observed on the formation of secondary aggregates.
All the findings in this study are consistent with an aggregation model in which
two distinct binding sites are present. The shape of the probe as well as its hydrophobicity
are critical to its interaction with the NaCh aggregates. From these dynamic studies it was
found that only a small number of NaCh monomers (6-13) are needed to define both the
primary and secondary binding sites.
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Keywords
Molecules, Bile salts, Probe molecules, Complexation