Chemistry Reference
In-Depth Information
acidic solution, undergo a color change from blue to red upon the addition of
ammonia. Notably, addition of HCl to the red solution, which reprotonates the term-
inal nitrogen on the hydrazide, regenerates a blue solution. This is one of the few
examples reported of a reversible chromatic transition of PDA sensors (Ahn et al.
2003; Kim, Lee, Choi, et al. 2005). Furthermore, the hydrazides form gels in
organic solution at low temperatures, and even dilute organic solutions can be poly-
merized, indicative of ordered self-assembly even under dilute conditions. IR and
computational studies suggest that this self-assembly is mediated by hydrogen
bonding within the hydrazide head groups.
The chromatic transition can also be triggered by changes in pH or the addition of
cations. Titration of vesicles composed of 5 with sodium hydroxide induces a chro-
matic transition (Kew and Hall 2006). The color changes can also be induced using
calcium, cesium, and potassium as counterions, with higher CR values obtained with
the larger cations cesium and potassium. Smaller cations such as lithium and mag-
nesium afford lower CR values. The chromatic transition was accompanied by a
transformation in vesicle morphology from globular to sheetlike structures, as deter-
mined by transmission electron miscroscopy (TEM). Similar results were observed
for the binding of various metal cations to PDA/phospholipid vesicles enriched in
anionic phospholipids (Rozner et al. 2003). Incorporation of metal-binding ligands
and ionophores into the vesicles sensitizes them to different cations, depending on
the cation-binding selectivities of the ligands. PDA/phospholipid vesicles containing
the ionophore valinomycin undergo chromatic transitions upon the addition of alkali
metal cations (Kolusheva, Shahal, et al. 2000). The magnitude of the resulting CR is
cation dependent and parallels the known relative affinities of the cations for valino-
mycin itself. Similar results were also obtained with the ionophores monensin and
A23187. Fluorescence anisotropy and
13
C-NMR studies suggest that the chromatic
transition is induced by increasing the fluidity of the phospholipids upon cation
binding to the ionophore.
12.2.5. Membrane-Binding Peptides
A variety of antibacterial peptides and related toxins function by binding to phos-
pholipid membranes and weakening them by forming transmembrane pores or
other structural perturbations. Vesicles prepared from a mixture of PDA lipids and
phospholipids have been used extensively to detect the binding of these peptides
and toxins. a-Helical membrane-binding peptides such as melittin, magainin, and
alamethicin induce a chromatic transition when added to PDA/DMPC liposomes
(Kolusheva, Boyer, et al. 2000). The assay is sensitive enough to distinguish point
mutants of the peptides that do not have antibacterial activity or do not adopt the
proper fold upon membrane binding. Nonspecific binding to the liposomes can be
a potential problem, but it can be determined and corrected for by using vesicles
lacking phospholipids. Other peptides and toxins that have been examined include
indolicidin, an antibiotic derived from bovine neutrophils (Halevy et al. 2003),
defensins, found in mammalian immune cells (Satchell et al. 2003), and synthetic
lysine-rich amphipathic a-helical polypeptides (Oren et al. 2002; Sheynis et al. 2003).
