Determination of Peptide<b>–</b>Surface
Adsorption Free Energy for Material Surfaces Not Conducive to SPR
or QCM using AFM
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Abstract
The interactions between peptides and proteins with material
surfaces
are of primary importance in many areas of biotechnology. While surface
plasmon resonance spectroscopy (SPR) and quartz crystal microbalance
(QCM) methods have proven to be very useful in measuring fundamental
properties characterizing adsorption behavior, such as the free energy
of adsorption for peptide–surface interactions, these methods
are largely restricted to use for materials that can readily form
nanoscale-thick films over the respective sensor surfaces. Many materials
including most polymers, ceramics, and inorganic glasses, however,
are not readily suitable for use with SPR or QCM methods. To overcome
these limitations, we recently showed that desorption forces (<i>F</i><sub>des</sub>) obtained using a standardized AFM method
linearly correlate to standard-state adsorption free energy values
(Δ<i>G°</i><sup><i></i></sup><sub>ads</sub>) measured from SPR in phosphate buffered saline (PBS: phosphate
buffered 140 mM NaCl, pH 7.4). This approach thus provides a means
to determine Δ<i>G°</i><sup><i></i></sup><sub>ads</sub> for peptide adsorption using AFM that can be
applied to any flat material surface. In this present study, we investigated
the <i>F</i><sub>des</sub>–Δ<i>G°</i><sup><i></i></sup><sub>ads</sub> correlation between AFM
and SPR data in PBS for a much broader range of systems including
eight different types of peptides on a set of eight different alkanethiol
self-assembled monolayer (SAM) surfaces. The resulting correlation
was then used to estimate Δ<i>G°</i><sup><i></i></sup><sub>ads</sub> from <i>F</i><sub>des</sub> determined by AFM for selected bulk polymer and glass/ceramic materials
such as poly(methyl methacrylate) (PMMA), high-density polyethylene
(HDPE), fused silica glass, and a quartz (100) surface. The results
of these studies support our previous findings regarding the strong
correlation between <i>F</i><sub>des</sub> measured by AFM
and Δ<i>G°</i><sup><i></i></sup><sub>ads</sub> determined by SPR, and provides a means to estimate Δ<i>G°</i><sup><i></i></sup><sub>ads</sub> for peptide
adsorption on macroscopically thick samples of materials that are
not conducive for use with SPR or QCM