Investigation of Amide and Nucleotidebased Molecule SelfAssembly by Vapor Pressure Osmometry

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Investigation of Amide and Nucleotide-based
MoleculeSelf-Assembly by Vapor Pressure Osmometry

Melissa A. Kelly GLAC/ACM ORSS - Macalester
College Chemical and Analytical Sciences
Division Bruce A. Moyer, Konstantinos
Kavallieratos
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Problem The functional groups of anion receptors
that allow binding also allow the receptor
molecules to assemble with each other, a process
competitive with anion binding. Objective Our
goal is to investigate molecular aggregation to
further our understanding of the thermodynamics,
selectivity and equilibria involved in ion-pair
extraction. Approach To accomplish our
objective, we will use vapor pressure osmometry
to examine molecular association in solution.
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Self-Assembly
Through hydrogen bonding, electrostatic
interactions or other intermolecular forces, a
wide variety of molecules assemble into more
complex structures in solution. The resulting
aggregates can be as complex as DNA, or as simple
as four guanosines that form a tetramer through
eight hydrogen bonds.
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The complexes that form in extraction processes
are another form of self-assembly that is of
particular interest to our group. Selectivity
and binding ability are highly dependent on the
receptor structure, and an understanding of
association is important to fully characterize
the binding process.
This is a crown ether highly selective for
cesium. Nitrate is co-extracted for charge
balance. A second receptor, for the anion, is
able to enhance the extraction. The association
of anion receptor molecules will be investigated.
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Vapor Pressure Osmometry
The pressure of vapor over a liquid is constant
at any particular temperature. If some solute is
dissolved in the liquid, the overall vapor
pressure changes depending on the interactions
between solvent and solute molecules. Vapor
pressure osmometry (VPO) uses this pressure
disparity to gain information about an unknown.
Inside the instrument chamber, a drop of solvent
is placed on one thermistor, solution on the
other. The solution is not at equilibrium with
the surrounding pure solvent vapor environment,
causing molecules of solvent vapor to condense
onto the solution. This exothermic reaction
increases the solution temperature until its
vapor pressure equals that of the pure solvent.
Since thermistor resistance is incredibly
sensitive to temperature, even a small change is
read as a bridge imbalance, with the subsequent
voltage (or current) difference as measurable
output.
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A primary use of VPO is molecular weight
determination. Starting with the
Clausius-Clapeyron equation and relationships
between mole fraction and partial pressure, one
can simplify to ?T K1C/m, where K1 is a
constant, C is the concentration of the solution
and m is the solute molecular weight. Since the
observed voltage change is proportional to the
temperature difference, ?V K2 ?T, one can
substitute and combine constants to yield ?V
KC/m. K becomes a calibration factor determined
experimentally for each set of conditions.
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VPO and Self-Assembly
Once the calibration factor is found using a
known molecular weight standard, the molecular
weight of a sample can be determined. If the
experimental weight is quite different from its
actual molecular weight, one can infer that some
degree of association may be occurring, and an
aggregation factor can be calculated. If that
factor is close to an integer value, it indicates
that the molecule forms a distinct polymer in
solution. If not an integer, the species may
form a number of different aggregates, which can
be characterized by other methods.
nA (A)n (n 1, 2,)
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Amide and Nucleotide-based Molecules for
Investigation