Helical aggregate

Recently, a variety of natural peptides that form transmembrane channels have been identified and characterized. Melittin (Figure 10.35) is a bee venom toxin peptide of 26 residues. The cecropins are peptides induced in Hyalophora cecropia (Figure 10.36) and other related silkworms when challenged by bacterial infections. These peptides are thought to form m-helical aggregates in mem-... 

Reference [33] describes recent progress on cyanine probes that bind noncova-lently to DNA, with a special emphasis on the relationship between the dye structure and the DNA binding mode. Some of the featured dyes form well-defined helical aggregates using DNA as a template. This reference also includes spectroscopic data for characterizing these supramolecular assemblies as well as the monomeric complexes. 

They developed a continuum elastic-free energy model that suggests these observations can be explained as a first-order mechanical phase transition. In other recent work on steroids, Terech and co-workers reported the formation of nanotubes in single-component solutions of the elementary bile steroid derivative lithocholic acid, at alkaline pH,164 although these tubules do not show any chiral markings indicating helical aggregation. 

Figure 6.12 Self-assembly process of 24 from single molecules into helical columns via nonhelical intermediates. The average number of molecules, (IV), participating in one column in n-butanol at three different concentrations is given as function of temperature. Arrows mark transition from achiral to helical aggregates.

While membrane anchors normally contain nonpolar residues, not all transmembrane a-helices act as membrane anchors. Some transmembrane helices have a more complex functional role than serving as a simple tether. Some helices aggregate within a membrane to create a channel. The channel allows the passage of ions across an otherwise impermeable membrane. Many proteins form ion channels, and most use helices to form the breach in the cell membrane. 

Irradiation at X>460 nm gave a fast cis-trans isomerization, with a concomitant increase in Tg. The sol-gel phase transition could be controlled in this way by light, while read-out of the states could be performed by measuring the modulation of the transmission or CD. For instance, the traras-azobenzene gelators displayed a CD effect, presumably due to the formation of helical aggregates, whereas the cis-isomers did not. 

Keywords Chiral amplification Chirality sensing Helical aggregates ... 

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