Probing the Orientation of β‑Lactoglobulin
on Gold Surfaces Modified by Alkyl Thiol Self-Assembled Monolayers
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Abstract
The adsorption of a globular protein
on chemically well controlled
surfaces was investigated in order to correlate its orientation to
the surface properties. To this end, three different alkyl thiols,
differing by their end group (−COOH, −CH<sub>3</sub>, and −NH<sub>2</sub>), were used to build up self-assembled
monolayers (SAMs) on gold substrates. β-Lactoglobulin (βLG)
was then adsorbed on these SAMs by immersion in a phosphate buffer
solution. The surface modification with alkyl thiols and the subsequent
adsorption of proteins were characterized <i>ex situ</i> by polarization modulated infrared reflection–absorption
spectroscopy (PM-IRRAS) and X-ray photoelectron spectroscopy (XPS).
The adsorption behavior of proteins was also monitored <i>in
situ</i> using quartz crystal microbalance with dissipation measurements
(QCM-D). Direct evidence regarding the protein orientation in the
adsorbed state was obtained by means of time-of-flight secondary ion
mass spectrometry (ToF-SIMS). Principal component analysis (PCA),
performed on the ToF-SIMS results, enables to separate the samples
and shows that the proteins display different distributions of amino
acids at the surface depending on the conditioning thiol layer. Our
results revealed that the adsorption mode of the protein is influenced
by the thiol end groups, and specific orientations of the protein
on the surface are proposed for the different substrates