Hydraphile Channels:  Structural and Fluorescent Probes of Position and Function in a Phospholipid Bilayer

Abstract

Three novel tris(macrocycle)s having fluorescent residues at their distal termini have been prepared and studied. The compounds are of the form 2), and dansyl (3). Compounds 2 and 3 were found to transport Na + at rates similar to those of other tris(macrocyclic)s but 1 was not an ionophore in the bilayer as assessed by 23 Na NMR analysis. The latter failure may be due to a hydrogen-bond blockade leading to a globular conformation adopted by the hydraphile. The fluorescence maximum of 3 was determined in a variety of solvents and in a phospholipid bilayer. The polarity experienced by the dansyl group in the bilayer was intermediate between that observed in methanol and ethanol. Fluorescence depth quenching using doxylsubstituted lipids showed that the dansyl headgroups of 3 were 14 Å from the bilayer's midplane or separated by about 28 Å. Fluorescence energy transfer between 2 and 3 showed that these two hydraphiles were not appreciably aggregated in the bilayer.

Coordination and transport of alkali metal cations through phospholipid bilayer membranes by hydraphile channels

“…The dansyl residue of 13 absorbs light at this wavelength and fluoresces at λ = 516 nm. This fluorescence resonance energy transfer (FRET) experiment indicates that at least for these two compounds, the aggregation number is 1.1 [59]. This supports the initial unimolecular design criteria.…”

“…The fluorescence maximum for 13 was measured when the channel was in the membrane. Its shift suggested that it experienced an environment similar to but slightly more polar than ethanol [59]. If the tris(macrocycle) portion of the molecule spans the insulator regime and the dansyl groups are in the midpolar regime, the observed polarity is as expected.…”

Activity of synthetic ion channels is influenced by cation–π interactions with phospholipid headgroups

“…28 We reported that the dansyl headgroup of 7 experienced a local dielectric similar to that expected for the midpolar regime of the bilayer. 29 The latter measurements were confirmed using fluorescence depth quenching.…”

Tryptophan in Membrane-Active Synthetic Antimicrobials

“…36, 37 When R in the structure of Scheme 3 is 3-indolylmethyl, the channel fails to transport cations. 38 It seems unlikely that protonation of the indole nitrogen is an issue, as its pK A is about 2.3. 39 Further, the six aliphatic nitrogen atoms are more basic and will certainly protonate first.…”