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Scott M. Husson, Ph.D. -- Research Activities
Thrust 3: Spectroscopic Studies of Biomolecule-Polymer Surface Interactions
The primary objectives in this thrust area are to use our methodology for preparing
well-defined polymer surfaces as platforms for measuring biomolecule-polymer
interaction strengths and to use these measurements to predict peptide adsorption
properties.
Well-Defined Polymer Surfaces for Thermodynamic Measurements
Overview: Designing surfaces to interface with biological systems requires knowledge
of, and better still, control of how biomolecules adsorb to such surfaces.
Since the early 1990s, surface plasmon resonance (SPR) spectroscopy has been
used routinely to measure the binding properties of proteins and peptides on
functionalized surfaces. SPR is an optical technique that measures changes
in the refractive index of the medium (in our case a polymer film) near a metal
(sensor) surface. Changes in refractive index are correlated to changes in
adsorbed peptide mass.
Opportunity: Careful film preparation is the most important factor for SPR studies
on polymer films. Polymer surface chemistry must be reproducible and free of
defects for meaningful SPR analysis. Prior to our work, polymer films were either
(plasma) deposited, spin-coated, physisorbed, or covalently grafted to the underlying
SPR sensor surface. We believe that using ATRP to grow polymer films from gold
sensor surfaces provides advantages over conventional spin-coating and grafting
to approaches in the design of model polymer films for biomolecule adsorption
studies. Notably, our approach leads to more uniform films.
Research activities and findings: SPR requires the use of a metal substrate such
as gold. Here again, we use atom transfer radical polymerization (ATRP) to grow
polymer films from the surface. To prepare the surface for polymerization, we
exploit the ability of alkanethiols to form self-assembled monolayers (SAMs)
on gold by depositing a SAM of hydroxyl-terminated thiols. Next, we anchor a
polymerization initiator group to the surface by covalent reaction with the SAM
hydroxyl groups. Finally, the polymer film grows from these initiator sites.
Figure 5 presents data that follow the process of polymer film fabrication. FTIR
confirms successful formation of the SAM, coupling of the initiator, and growth
of poly(2-vinylpyridine) from a gold surface. Ellipsometry tracks the growth
rate. SPM illustrates the high degree of film uniformity post polymerization.
 
Figure
5. (Upper left) External Reflectance FTIR spectra on gold substrate:
(a) 11-Mercapto-undecanol
SAM; (b) grafted (4-chloromethyl)benzoyl chloride initiator
on SAM; (c) grafted poly(2-vinylpyridine) layer (55Å).
(Upper right) Growth of surface-confined poly(2-vinylpyridine) measured by ellipsometry.
(Lower left) SPM topographical image (1 ?m square * 20 nm) of poly(2-vinylpyridine)
layer with 55 Å thickness.
Predictions of Peptide Adsorption on Polymer Surfaces
Overview: Much has been learned and written about protein adsorption to materials
surfaces. For applied purposes, the scientific challenges include understanding
how to predict under what conditions proteins will adsorb, and, in many cases,
how to control or to minimize their adsorption. For example, the adsorption of
proteins on the surfaces of biomedical devices and heat exchangers in the food
and dairy industries disrupts their function. For these and other applications,
understanding biomolecule-surface interactions would allow better a priori design
of materials surfaces to achieve the desired response.
Opportunity: At a fundamental level, we believe that adsorption depends on interactions
between individual amino acid units on the biomolecule surface and the functional
groups of the material surface. Our hypothesis is this: For short-chain biomolecules,
where adsorption can be considered reversible, applying the principle of additivity
to known submolecular level interaction energies may allow predictive estimates
for the adsorption energies of whole biomolecules. Specifically, since adsorption
of proteins onto synthetic surfaces is driven thermodynamically, it should be
possible, theoretically, to predict adsorption behavior for a peptide by knowing
values of ?Gad for the amino acid residue units that comprise it.
Research activities and findings: In
a recent study, we prepared uniform, nanothin poly(2-vinylpyridine)
films
using our graft polymerization methodology
and
measured the adsorption energies of a homologous series of tyrosine
(Y) homopeptides on
these films in order to determine sub-molecular level interaction energies.
Using SPR, adsorption isotherms (Figure 6) were measured for YY and
YYY peptides; analysis
of these isotherms provided ?Gad,residue data for mid-chain and chain-end
tyrosine units; they were –0.75 ± 0.07 kcal/mol and –2.12 ± 0.04
kcal/mol, respectively. Combining the thermodynamic contributions for adsorption
of individual tyrosine units allowed a predictive estimate of –5.12 ± 0.32
kcal/mol for the adsorption energy for YYYYYY; this estimate deviated by only
3 % from its measured value of –5.24 ± 0.06 kcal/mol.
Within the experimental uncertainty values, it can be argued that the
estimated
and measured
values are the same. We have also determined interaction energies for
other amino acid residue units and extended the predictions to mixed
residue
peptides.
 Figure
6. Adsorption isotherms for tyrosine peptides (2, 3 and 6 units) adsorbing
on the poly(2-vinylpyridine) surface at 25 °C from pH 7 HEPES buffer
solutions. Symbols represent experimental data. Curved lines represent
predictions for YYYY (— —) and YYYYY (- - -).
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