4 research outputs found
A Chiral <sup>19</sup>F NMR Reporter of Foldamer Conformation in Bilayers
Understanding and
controlling peptide foldamer conformation
in
phospholipid bilayers is a key step toward their use as molecular
information relays in membranes. To this end, a new 19F
âreporterâ tag has been developed and attached to dynamic
peptide foldamers. The (R)-1-(trifluoromethyl)ethylamido
((R)-TFEA) reporter was attached to the C-terminus
of α-amino-iso-butyric acid (Aib) foldamers.
Crystallography confirmed that the foldamers adopted 310 helical conformations. Variable temperature (VT) NMR spectroscopy
in organic solvents showed that the (R)-TFEA reporter
had an intrinsic preference for P helicity, but the
overall screw-sense was dominated by a chiral âcontrollerâ
at the N-terminus. The 19F NMR chemical shift of the CF3 resonance was correlated with the ability of different N-terminal
groups to induce either an M or a P helix in solution. In bilayers, a similar correlation was found.
Solution 19F NMR spectroscopy on small unilamellar vesicle
(SUV) suspensions containing the same family of (R)-TFEA-labeled foldamers showed broadened but resolvable 19F resonances, with each chemical shift mirroring their relative positions
in organic solvents. These studies showed that foldamer conformational
preferences are the same in phospholipid bilayers as in organic solvents
and also revealed that phospholipid chirality has little influence
on conformation
Disorder and Sorption Preferences in a Highly Stable Fluoride-Containing Rare-Earth <i>fcu</i>-Type MetalâOrganic Framework
Rare-earth (RE) metalâorganic
frameworks (MOFs) synthesized
in the presence of fluorine-donating modulators or linkers are an
important new subset of functional MOFs. However, the exact nature
of the REaXb core of the molecular building block (MBB) of the MOF, where X is
a Ό2 or 3-bridging group, remains unclear.
Investigation of one of the archetypal members of this family with
the stable fcu framework topology, Y-fum-fcu-MOF (1), using a combination of experimental
techniques, including high-field (20 T) solid-state nuclear magnetic
resonance spectroscopy, has determined two sources of framework disorder
involving the Ό3-X face-capping group of the MBB
and the fumarate (fum) linker. The core of the MBB of 1 is shown to contain a mixture of ÎŒ3-Fâ and (OH)â groups with preferential occupation
at the crystallographically different face-capping sites that result
in different internally lined framework tetrahedral cages. The fum
linker is also found to display a disordered arrangement involving
bridgingâ or chelatingâbridging bis-bidentate modes
over the fum linker positions without influencing the MBB orientation.
This linker disorder will, upon activation, result in the creation
of Y3+ ions with potentially one or two additional uncoordinated
sites possessing differing degrees of Lewis acidity. Crystallographically
determined hostâguest relationships for simple sorbates demonstrate
the favored sorption sites for N2, CO2, and
CS2 molecules that reflect the chemical nature of both
the framework and the sorbate species with the structural partitioning
of the Ό3-groups apparent in determining the favored
sorption site of CS2. The two types of disorder found within 1 demonstrate the complexity of fluoride-containing RE-MOFs
and highlight the possibility to tune this and other frameworks to
contain different proportions and segregations of Ό3-face-capping groups and degrees of linker disorder for specifically
tailored applications
Disorder and Sorption Preferences in a Highly Stable Fluoride-Containing Rare-Earth <i>fcu</i>-Type MetalâOrganic Framework
Rare-earth (RE) metalâorganic
frameworks (MOFs) synthesized
in the presence of fluorine-donating modulators or linkers are an
important new subset of functional MOFs. However, the exact nature
of the REaXb core of the molecular building block (MBB) of the MOF, where X is
a Ό2 or 3-bridging group, remains unclear.
Investigation of one of the archetypal members of this family with
the stable fcu framework topology, Y-fum-fcu-MOF (1), using a combination of experimental
techniques, including high-field (20 T) solid-state nuclear magnetic
resonance spectroscopy, has determined two sources of framework disorder
involving the Ό3-X face-capping group of the MBB
and the fumarate (fum) linker. The core of the MBB of 1 is shown to contain a mixture of ÎŒ3-Fâ and (OH)â groups with preferential occupation
at the crystallographically different face-capping sites that result
in different internally lined framework tetrahedral cages. The fum
linker is also found to display a disordered arrangement involving
bridgingâ or chelatingâbridging bis-bidentate modes
over the fum linker positions without influencing the MBB orientation.
This linker disorder will, upon activation, result in the creation
of Y3+ ions with potentially one or two additional uncoordinated
sites possessing differing degrees of Lewis acidity. Crystallographically
determined hostâguest relationships for simple sorbates demonstrate
the favored sorption sites for N2, CO2, and
CS2 molecules that reflect the chemical nature of both
the framework and the sorbate species with the structural partitioning
of the Ό3-groups apparent in determining the favored
sorption site of CS2. The two types of disorder found within 1 demonstrate the complexity of fluoride-containing RE-MOFs
and highlight the possibility to tune this and other frameworks to
contain different proportions and segregations of Ό3-face-capping groups and degrees of linker disorder for specifically
tailored applications
AtomAccess: A Predictive Tool for Molecular Design and Its Application to the Targeted Synthesis of Dysprosium Single-Molecule Magnets
Isolated
dysprosocenium cations, [Dy(CpR)2]+ (CpR = substituted cyclopentadienyl), have
recently been shown to exhibit superior single-molecule magnet (SMM)
properties over closely related complexes with equatorially bound
ligands. However, gauging the crossover point at which the CpR substituents are large enough to prevent equatorial ligand
binding, but small enough to approach the metal closely and generate
strong crystal field splitting has required laborious synthetic optimization.
We therefore created the computer program AtomAccess to predict the
accessibility of a metal binding site and its ability to accommodate
additional ligands. Here, we apply AtomAccess to identify the crossover
point for equatorial coordination in [Dy(CpR)2]+ cations in silico and hence predict a cation that is
at the cusp of stability without equatorial interactions, viz., [Dy(Cpttt)(Cp*)]+ (Cpttt = C5H2tBu3-1,2,4, Cp* = C5Me5). Upon synthesizing this cation, we found that
it crystallizes as either a contact ion-pair, [Dy(Cpttt)(Cp*){Al[OC(CF3)3]4-Îș-F}],
or separated ion-pair polymorph, [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4]·C6H6. Upon characterizing these complexes, together with their precursors,
yttrium and yttrium-doped analogues, we find that the contact ion-pair
shows inferior SMM properties to the separated ion-pair, as expected,
due to faster Raman and quantum tunneling of magnetization relaxation
processes, while the Orbach region is relatively unaffected. The experimental
verification of the predicted crossover point for equatorial coordination
in this work tests the limitations of the use of AtomAccess as a predictive
tool and also indicates that the application of this type of program
shows considerable potential to boost efficiency in exploratory synthetic
chemistry