3 research outputs found
Unquenchable Surface Potential Dramatically Enhances Cu<sup>2+</sup> Binding to Phosphatidylserine Lipids
Herein,
the apparent equilibrium dissociation constant, <i>K</i><sub>Dapp</sub>, between Cu<sup>2+</sup> and 1-palmitoyl-2-oleoyl-<i>sn-</i>glycero-3-phospho-l-serine (POPS), a negatively
charged phospholipid, was measured as a function of PS concentrations
in supported lipid bilayers (SLBs). The results indicated that <i>K</i><sub>Dapp</sub> for Cu<sup>2+</sup> binding to PS-containing
SLBs was enhanced by a factor of 17 000 from 110 nM to 6.4
pM as the PS density in the membrane was increased from 1.0 to 20
mol %. Although Cu<sup>2+</sup> bound bivalently to POPS at higher
PS concentrations, this was not the dominant factor in increasing
the binding affinity. Rather, the higher concentration of Cu<sup>2+</sup> within the double layer above the membrane was largely responsible
for the tightening. Unlike the binding of other divalent metal ions
such as Ca<sup>2+</sup> and Mg<sup>2+</sup> to PS, Cu<sup>2+</sup> binding does not alter the net negative charge on the membrane as
the CuÂ(PS)<sub>2</sub> complex forms. As such, the Cu<sup>2+</sup> concentration within the double layer region was greatly amplified
relative to its concentration in bulk solution as the PS density was
increased. This created a far larger enhancement to the apparent binding
affinity than is observed by standard multivalent effects. These findings
should help provide an understanding on the extent of Cu<sup>2+</sup>–PS binding in cell membranes, which may be relevant to biological
processes such as amyloid-β peptide toxicity and lipid oxidation
What Is the Preferred Conformation of Phosphatidylserine–Copper(II) Complexes? A Combined Theoretical and Experimental Investigation
Phosphatidylserine
(PS) has previously been found to bind Cu<sup>2+</sup> in a ratio
of 1 Cu<sup>2+</sup> ion per 2 PS lipids to form
a complex with an apparent dissociation constant that can be as low
as picomolar. While the affinity of Cu<sup>2+</sup> for lipid membranes
containing PS lipids has been well characterized, the structural details
of the Cu–PS<sub>2</sub> complex have not yet been reported.
Coordinating to one amine and one carboxylate moiety on two separate
PS lipids, the Cu–PS<sub>2</sub> complex is unique among ion–lipid
complexes in its ability to adopt both <i>cis</i> and <i>trans</i> conformations. Herein, we determine which stereoisomer
of the Cu–PS<sub>2</sub> complex is favored in lipid bilayers
using density functional theory calculations and electron paramagnetic
resonance experiments. It was determined that a conformation in which
the nitrogen centers are <i>cis</i> to each other is the
preferred binding geometry. This is in contrast to the complex formed
when two glycine molecules bind to Cu<sup>2+</sup> in bulk solution,
where the <i>cis</i> and <i>trans</i> isomers
exist in equilibrium, indicating that the lipid environment has a
significant steric effect on the Cu<sup>2+</sup> binding conformation.
These findings are relevant for understanding lipid oxidation caused
by Cu<sup>2+</sup> binding to lipid membrane surfaces and will help
us understand how ion binding to lipid membranes can affect their
physical properties
Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling
Strong
coupling interactions between plasmon and exciton-based
excitations have been proposed to be useful in the design of optoelectronic
systems. However, the role of various optical parameters dictating
the plasmon-exciton (plexciton) interactions is less understood. Herein,
we propose an inequality for achieving strong coupling between plasmons
and excitons through appropriate variation of their oscillator strengths
and spectral widths. These aspects are found to be consistent with
experiments on two sets of free-standing plexcitonic systems obtained
by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying
sizes through silane coupling and (ii) electrostatic binding of cyanine
dyes on polystyrenesulfonate-coated Au nanorods of varying aspect
ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate
remains in monomeric state, and its high oscillator strength and narrow
spectral width enable us to approach the strong coupling limit. In
contrast, in the presence of polystyrenesulfonate, monomeric forms
of cyanine dyes exist in equilibrium with their aggregates: Coupling
is not observed for monomers and H-aggregates whose optical parameters
are unfavorable. The large aggregation number, narrow spectral width,
and extremely high oscillator strength of J-aggregates of cyanines
permit effective delocalization of excitons along the linear assembly
of chromophores, which in turn leads to efficient coupling with the
plasmons. Further, the results obtained from experiments and theoretical
models are jointly employed to describe the plexcitonic states, estimate
the coupling strengths, and rationalize the dispersion curves. The
experimental results and the theoretical analysis presented here portray
a way forward to the rational design of plexcitonic systems attaining
the strong coupling limits