The synthesis and characterization of diphenylacetylene containing ion channels

Field: Chemistry (Physical Sciences)

This Thesis presents the synthesis, characterization and mechanistic explorations into a series of diphenylacetylene-containing oligoester ion channels. Eighteen final compounds were synthesized and tested for ion transport activity utilizing both vesicle and planar bilayer assays. The oligomers varied in length, hydrophobicity and the nature of the aromatic moiety. Compounds incorporating a modified diphenylacetylene (‘Dip’), or a novel phenyl-extended fluorophore (‘Trip’) were made using a reliable, modular synthesis. The final compounds were prepared in a total of 5 to 11 steps from commercial materials in yields ranging from 10 to 40%. \nThe compounds’ activity varied considerably; both highly active and completely inactive compounds were discovered. The differences in activity are controlled by structure via the influence of structural variables on the aqueous phase aggregation and the ability of the compound to insert into the bilayer membrane. These structure-activity studies uncovered two highly-active ion transporters, HO2C-Hex-Dip-Hex-Hex-OH and –OPO32- (Hex = 6-hydroxyhexanoyl) which exhibited activity almost 10-fold higher than the fully-saturated oligoesters developed in previous work. In some cases, the transport activity is initially high but declines over a period of 20-30 minutes following compound addition. This suggests that the compound slowly transitions to an environment where it cannot form active channels. \nIn the bilayer clamp, a variety of behaviours including highly-conducting openings were observed. An apparent voltage-gated response was exhibited by one of the Trip compounds (HO2C-Trip-G(E3)-OH), a property rarely seen for synthetic ion channels. \nThe Dip and Trip molecules exhibited environment-sensitive fluorescence. The observed Dip excimer-like emission is the second reported instance of this in solution. The Trip compounds are solvatochromic; this property was used to infer their location in the membrane. Partitioning into the membrane was followed by a blue-shifting and increased intensity of the fluorescence emission for both series of compounds. For the Trip isomers, which are significantly more emissive than the Dip molecules, this enhancement in intensity could be visualized by eye. \nFor the Dip oligomers, the excimer emission is a broad band with variable shape and intensity; it is time-dependent under some conditions. The excimer emission has a sub-nanosecond lifetime in homogenous solution that is significantly prolonged in the presence of vesicle bilayers, in which a number of lifetimes could be detected. Both monomer and excimer emissions can be quenched by aqueous copper, the excimer emission is more efficiently quenched than is the monomer. \nThe photophysical characteristics of these molecules allowed for a variety of experiments designed to probe their membrane partitioning and localization behaviours. The results indicate the formation of a complex mixture of interconverting monomeric and aggregate species as the compounds move from water to the bilayer. The slow evolution of the mixture is consistent with the times noted for loss of membrane activity in transport assays. From these data a new model that describes the transport process is proposed. The key feature of this model is that transport must occur via a species that forms quickly upon the mixing of the components. Possible structures of the intermediates formed are discussed.