Entropic and Electrostatic
Effects on the Folding
Free Energy of a Surface-Attached Biomolecule: An Experimental and
Theoretical Study
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Abstract
Surface-tethered biomolecules play key roles in many
biological
processes and biotechnologies. However, while the physical consequences
of such surface attachment have seen significant theoretical study,
to date this issue has seen relatively little experimental investigation.
In response we present here a quantitative experimental and theoretical
study of the extent to which attachment to a chargedbut otherwise
apparently inertsurface alters the folding free energy of
a simple biomolecule. Specifically, we have measured the folding free
energy of a DNA stem loop both in solution and when site-specifically
attached to a negatively charged, hydroxylalkane-coated gold surface.
We find that whereas surface attachment is destabilizing at low ionic
strength, it becomes stabilizing at ionic strengths above ∼130
mM. This behavior presumably reflects two competing mechanisms: excluded
volume effects, which stabilize the folded conformation by reducing
the entropy of the unfolded state, and electrostatics, which, at lower
ionic strengths, destabilizes the more compact folded state via repulsion
from the negatively charged surface. To test this hypothesis, we have
employed existing theories of the electrostatics of surface-bound
polyelectrolytes and the entropy of surface-bound polymers to model
both effects. Despite lacking any fitted parameters, these theoretical
models quantitatively fit our experimental results, suggesting that,
for this system, current knowledge of both surface electrostatics
and excluded volume effects is reasonably complete and accurate