Transport
Across the
Bilayer. Most current topics of interest have emerged
from our
long-standing interest in new structural motifs that can transport ions
across lipid bilayer membranes. Our main contributions to this
collection are rigid-rod molecules as reliable transmembrane scaffolds,
as well as artificial β-barrels. Artificial β-barrels have been
prepared in many different forms for many different purposes;
responsiveness to chemical stimulation (blockage, ligand gating) was
particularly important because it opened the door to sensing
applications. However, there are also π-stack
architectures
(important for artificial photosystems) or push-pull
rods (for
voltage gating). Cation-π slides
are not
further worthwhile and have been made long ago, at a time when
cation-π
interactions were also suspected to account for the selectivity of
biological potassium channels. However, anion-π
slides deserved a
closer look because they clearly go beyond what is offered by nature,
anion-π interactions have never been
used before for transport or
function in general, some even doubt that they exist. As a
result, they provided innovative approaches to electroneutral
photosystems and stimulated the perception of synthetic transport
systems as analytical tools to elaborate on exotic interactions that
are otherwise difficult to explore (anion-π
interactions, halogen
bonds). Polyion-counterion transporters have been studied
in parallel for a long time because of their importance biological
importance (cell-penetrating peptides, biological voltage gating,
non-viral gene transfection) and their
applicability to sensing.
Introductory Reviews: Acc. Chem. Res. 2005, 38, 79; 2008, 41, 1354; 2013, 10.1021/ar400014r.
Artificial Photosystems.
Studies on artificial photosystems also
started in lipid bilayer membranes. Emphasis was on
multifunctional architectures that combine active electron transport
with passive ion transport in different ways. Examples include
twisted π-stacks that open up into ion channels in response to
the
intercalation of ligands, or anion-π slides for electroneutral
photosynthesis. To increase complexity, however, we soon moved on
and became interested to learn how to grow multicomponent architectures
directly on solid surfaces. This is important to preserve the
directionality needed to install oriented gradients, a bit like in
biological photosystems.
Zipper assembly, a sticky-end-layer-by-layer method, was developed
first. To obtain similar complexity with less synthetic effort,
SOSIP (self-organizing surface-initiated polymerization) was introduced
next. Building on SOSIP, templated self-sorting (TSS) and
templated stack exchange (TSE) are currently developed as new synthetic
methods for the construction of complex multicomponent surface
architectures. This is important because supramolecular
architectures of highest sophistication account for biological
function, and many wonder what we would get if we could prepare organic
materials with the same level of sophistication. Current
objectives include the construction of surface architectures with
co-axial channels that are equipped oriented antiparallel gradients to
drive electrons and holes far apart before they can recombine, a bit
like in biological photosystems (so-called OMARG SHJs, see current
projects). Moreover, lessons from SOSIP-TSE are applied to
develop conceptually innovative approaches to cellular uptake
(substrate-initiated cell-penetrating polydisulfides) or fluorescent
membrane probes (planarizable push-pull oligothiophenes).
Introductory Review: Org. Biomol. Chem. 2013, 11, 1754.