20 research outputs found
C vs N: Which End of the Cyanide Anion Is a Better Hydrogen Bond Acceptor?
The ability of the C and N ends of
the cyanide anion (CN<sup>ā</sup>) as acceptors of hydrogen
bonds, an experimentally difficult problem,
has been computationally examined in this study. Structures obtained
in our previous work involving cyanide binding within the cavity of
a triazolophane macrocycle (<i>Chem.īøEur. J</i>. <b>2011</b>, <i>17</i>, 9123ā9129) were used to
analyze the problem. Three different approaches involving (a) breakdown
of the triazolophane into smaller components, (b) population analyses,
and (c) ionādipole analyses helped demonstrate that the N terminus
of cyanide is a slightly better hydrogen bond acceptor than the C
terminus even though it is not the site of protonation or covalent
bond formation. This outcome reflects a competition between the preference
for noncovalent interactions at the nitrogen and covalent bond formation
at the carbon
Hydrophobic Collapse of Foldamer Capsules Drives Picomolar-Level Chloride Binding in Aqueous Acetonitrile Solutions
Aqueous media are competitive environments
in which to perform
hostāguest chemistry, particularly when the guest is highly
charged. While hydrophobic binding is a recognized approach to this
challenge in which apolar pockets can be designed to recognize apolar
guests in water, complementary strategies are required for hydrophilic
anions like chloride. Here, we present evidence of such an alternative
mechanism, used everyday by proteins yet rare for artificial receptors,
wherein hydrophobic interactions are shown to be responsible for organizing
and stabilizing an aryl-triazole foldamer to help extract hydrophilic
chloride ions from increasingly aqueous solutions. Therein, a double-helical
complex gains stability upon burial of ā¼80% of the Ļ
surfaces that simultaneously creates a potent, solvent-excluding microenvironment
for hydrogen bonding. The chlorideās overall affinity to the
duplex is substantial in 25% water v/v in acetonitrile (logĀ Ī²<sub>2</sub> = 12.6), and it remains strong (logĀ Ī²<sub>2</sub> = 13.0) as the water content is increased to 50%. With the rise
in predictable designs of abiological foldamers, this water-assisted
strategy can, in principle, be utilized for binding other hydrophilic
guests
Hydrophobic Collapse of Foldamer Capsules Drives Picomolar-Level Chloride Binding in Aqueous Acetonitrile Solutions
Aqueous media are competitive environments
in which to perform
hostāguest chemistry, particularly when the guest is highly
charged. While hydrophobic binding is a recognized approach to this
challenge in which apolar pockets can be designed to recognize apolar
guests in water, complementary strategies are required for hydrophilic
anions like chloride. Here, we present evidence of such an alternative
mechanism, used everyday by proteins yet rare for artificial receptors,
wherein hydrophobic interactions are shown to be responsible for organizing
and stabilizing an aryl-triazole foldamer to help extract hydrophilic
chloride ions from increasingly aqueous solutions. Therein, a double-helical
complex gains stability upon burial of ā¼80% of the Ļ
surfaces that simultaneously creates a potent, solvent-excluding microenvironment
for hydrogen bonding. The chlorideās overall affinity to the
duplex is substantial in 25% water v/v in acetonitrile (logĀ Ī²<sub>2</sub> = 12.6), and it remains strong (logĀ Ī²<sub>2</sub> = 13.0) as the water content is increased to 50%. With the rise
in predictable designs of abiological foldamers, this water-assisted
strategy can, in principle, be utilized for binding other hydrophilic
guests
Programmed Negative Allostery with Guest-Selected Rotamers Control AnionāAnion Complexes of Stackable Macrocycles
A new
rotamer-based strategy for negative allostery has been used
to control hostāhost interactions and product yield upon anion
complexation. Coassembly of anion dimers as guests inside two cyanostar
macrocycles drives selection of one rotamer in which all ten steric
groups get directed outward to destabilize triply stacked macrocycles.
A large entropy penalty (Ī<i>S</i>) is quantified
upon anion binding when the multiple dynamic rotamers collapse down
to one
Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion
The
existence of two rings in [3]Āpseudorotaxanes presents opportunities
for those rings to undergo double switching and cooperative mechanical
coupling. To investigate this capability, we identified a new strategy
for bringing two rings into contact with each other and conducted
mechanistic studies to reveal their kinetic cooperativity. A redox-active
tetrazine ligand bearing two binding sites was selected to allow for
two mobile copperĀ(I) macrocycle ring moieties to come together. To
realize this switching modality, ligands were screened against their
ability to serve as stations on which the rings are initially parked,
ultimately identifying 5,5ā²-dimethyl-2,2ā²-bipyridine.
The kinetics of switching a macrocycle in a <i>single</i>-site [2]Āpseudorotaxane between bipyridine and <i>single</i>-site tetrazine stations were examined using electrochemistry. The
forward movement was rate-limited by the bimolecular reaction between
reduced tetrazine and bipyridine [2]Āpseudorotaxane. Two bipyridines
were then used with a <i>double</i>-site tetrazine to verify
double switching of two rings. Our results indicated stepwise movements,
with the first ring moving 4 times more frequently (faster) than the
second. While this behavior is indicative of <i>anti</i>cooperative kinetics, positive thermodynamic cooperativity sets the
two rings in motion even though just one tetrazine is reduced with
one electron. Double switching in this [3]Āpseudorotaxane uniquely
demonstrates how a series of independent thermodynamic states and
kinetic paths govern an apparently simple mechanical motion
Polarized Naphthalimide CH Donors Enhance Cl<sup>ā</sup> Binding within an Aryl-Triazole Receptor
The dipolar character of 1,8-naphthalimide together with polarization of the C<sup>4</sup>āH and C<sup>5</sup>āH donors has been utilized in receptor <b>1</b> to effectively bind chloride alongside triazole and phenylene units. The Cl<sup>ā</sup> binding strength of <b>1</b> shows that the naphthalimide provides greater anion stabilization than an unactivated phenylene, and DFT calculations show that its collinear donor array can be a āurea-likeā analog for CHĀ·Ā·Ā·anion interactions
Selective Anion-Induced Crystal Switching and Binding in Surface Monolayers Modulated by Electric Fields from Scanning Probes
Anion-selective (Br<sup>ā</sup> and I<sup>ā</sup>) and voltage-driven crystal switching between two differently packed phases (Ī± ā Ī²) was observed in 2D crystalline monolayers of aryl-triazole receptors ordered at solutionāgraphite interfaces. Addition of Br<sup>ā</sup> and I<sup>ā</sup> was found to stimulate the Ī± ā Ī² phase transformation and to produce ion binding to the Ī² phase assembly, while Cl<sup>ā</sup> and BF<sub>4</sub><sup>ā</sup> addition retained the Ī± phase. Unlike all other surface assemblies of either charged molecules or ion-templated 2D crystallization of metal-ligand or receptor-based adsorbates, the polarity of the electric field between the localized scanning tip and the graphite substrate was found to correlate with phase switching: Ī² ā Ī± is driven at ā1.5 V, while Ī± ā Ī² occurs at +1.1 V. Ion-pairing between the countercations and the guest anions was also observed. These observations are supported by control studies including variation of anion species, relative anion concentration, surface temperature, tip voltage, and scanning time
ArginineāPhosphate Recognition Enhanced in Phospholipid Monolayers at Aqueous Interfaces
Due
to the growing world population, there is an ever-increasing
need to develop better receptors to recover and recycle phosphate
for use in agricultural processes. This need is driven by agricultural
demand and environmental concerns because phosphate eutrophication
has a damaging effect on fresh water supplies by fueling algal blooms.
The air/water interface provides a unique region with a dielectric
constant (Īµ) that diminishes from high in bulk water (Īµ
= 80) to significantly lower (e.g., Īµ < 40) near the monolayer
surface to potentially enhance affinities during molecular recognition.
The work presented here uses a model system of phosphate binding to
an amino acid, arginine, and utilizes the interfacial properties of
the phospholipid monolayer, 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphatidic
acid, as the phosphate source to quantify binding. Employing arginine
as a probe molecule allows for the evaluation of its guanidinium moiety
for phosphate chelation. Surface pressureāarea isotherms from
Langmuir monolayer studies, and the corresponding infrared reflection
absorption spectroscopy, were used along with Brewster angle microscopy
for in situ determination of molecular binding interactions and the
surface binding constants of the phosphateāguanidinium complex,
which are shown here to be greater than 10<sup>3</sup> M<sup>ā1</sup>. The binding constant in bulk solution, determined by NMR titrations
of phosphate and arginine, is determined to be on the order of 0.1
M<sup>ā1</sup>. The greater than 10ā000-fold increase
from the bulk aqueous solution to the air/water interface reveals
that the interface provides a region of enhanced binding affinity
Cyanostar: CāH Hydrogen Bonding Neutral Carrier Scaffold for Anion-Selective Sensors
Cyanostar,
a pentagonal macrocyclic compound with an electropositive
cavity, binds anions with CH-based hydrogen bonding. The large size
of the cyanostarās cavity along with its planarity favor formation
of 2:1 sandwich complexes with larger anions, like perchlorate, ClO<sub>4</sub><sup>ā</sup>, relative to the smaller chloride. We
also show that cyanostar is selective for ClO<sub>4</sub><sup>ā</sup> over the bulky salicylate anions by using NMR titration studies
to measure affinity. The performance of this novel macrocycle as an
anion ionophore in membrane ion sensors was evaluated. The cyanostar-based
electrodes demonstrated a Nernstian response toward perchlorate with
selectivity patterns distinctly different from the normal Hofmeister
series. Different membrane compositions were explored to identify
the optimum concentrations of the ionophore, plasticizer, and lipophilic
additive that give rise to the best perchlorate selectivity. Changing
the concentration of the lipophilic additive tridodecylmethylammonium
chloride was found to impact the selectivity pattern and the analytical
dynamic range of the electrodes. The high selectivity of the cyanostar
sensors and their detection limit could enable the determination of
ClO<sub>4</sub><sup>ā</sup> in contaminated environmental samples.
This novel class of macrocycle provides a suitable scaffold for designing
various anion-selective ionophores by altering the size of the central
cavity and its functionalization
An Overlooked yet Ubiquitous Fluoride Congenitor: Binding Bifluoride in Triazolophanes Using Computer-Aided Design
Despite its ubiquity during the binding
and sensing of fluoride,
the role of bifluoride (HF<sub>2</sub><sup>ā</sup>) and its
binding properties are almost always overlooked. Here, we give one
of the first examinations of bifluoride recognition in which we use
computer-aided design to modify the cavity shape of triazolophanes
to better match with HF<sub>2</sub><sup>ā</sup>. Computational
investigation indicates that HF<sub>2</sub><sup>ā</sup> and
Cl<sup>ā</sup> should have similar binding affinities to the
parent triazolophane <i>in the gas phase</i>. Evaluation
of the binding geometries revealed a preference for binding of the
linear HF<sub>2</sub><sup>ā</sup> along the northāsouth
axis with a smaller Boltzmann weighted population aligned eastāwest
and all states being accessed rapidly through in-plane precessional
rotations of the anion. While the <sup>1</sup>H NMR spectroscopy studies
are consistent with the calculated structural aspects, binding affinities <i>in solution</i> were determined to be significantly smaller
for the bifluoride than the chloride. Computed geometries suggested
that a 20Ā° tilting of the bifluoride (stemming from the cavity
size) could account for the 25-fold difference between the two binding
affinities, HF<sub>2</sub><sup>ā</sup> < Cl<sup>ā</sup>. Structural variations to the triazolophaneās geometry and
electronic modifications to the network of hydrogen bond donors were
subsequently screened in a stepwise manner using density functional
theory calculations to yield a final design that eliminates the tilting.
Correspondingly, the bifluorideās binding affinity (<i>K</i> ā¼ 10<sup>6</sup> M<sup>ā1</sup>) increased
and was also found to remain equal to chloride <i>in the gas
and solution phases</i>. The new oblate cavity appeared to hold
the HF<sub>2</sub><sup>ā</sup> in a single eastāwest
arrangement. Our findings demonstrate the promising ability of computer-aided
design to fine-tune the structural and electronic match in anion receptors
as a means to control the arrangement and binding strength of a desired
guest