Biomedical Engineering Reference
In-Depth Information
mit the functional activity of incorporated proteins.
30
Such a func-
tionality was recovered only by mixing the thiolipopeptide with a
hydrophilic thiol, such as mercaptohexanol.
A structural and functional characterization of a DPTL mono-
layer tethered to gold was recently reported by Vockenroth et al.
167
using neutron reflectivity (NR) and EIS. The tetraethyleneoxy
moiety was found to be only partly hydrated at the more positive
potentials. However, at -0.600 V vs. Ag/AgCl(0.1 M KCl) a pro-
nounced increase in the neutron scattering length density of the
spacer was observed, denoting an increased amount of water trans-
ferred into this region. Leitch et al.
168
drew similar conclusions
using polarization modulation infrared reflection absorption spec-
troscopy (PM-IRRAS). Thus, the fraction of nonhydrated C=O of
the lipoic acid ester group was found to be about 50% at the more
positive potentials and to attain a value of about 30% at -0.600 V,
which denotes an increasing hydration of the spacer at these nega-
tive potentials. Analogous conclusions were also drawn by McGil-
livray et al.
35
by using NR, EIS and Fourier-transform IRRAS (FT-
IRRAS) to investigate a gold-supported thiolipid monolayer simi-
lar to DPTL, with a hydrophilic spacer moiety consisting of a hex-
aethyleneoxy chain directly bound to a sulfhydryl group. FT-
IRRAS revealed a significant disorder in the spacer region and a
substantial order in the hydrocarbon tail region. Moreover, NR
showed that the spacer region had a thickness smaller than its fully
extended length and only 5 vol% exchangeable water, despite its
significant disorder. Since the incorporation of proteins with ex-
tramembrane domains requires a significant hydration of the spac-
er, the thiolipid monolayer was then diluted with short E-
mercaptoethanol (EME) molecules. This permitted water mole-
cules to be accommodated in the more spacious thiolipid-EME
mixture. By self-assembling a lipid monolayer on top of this mixed
monolayer, McGillivray et al.
35
obtained a tBLM with a differen-
tial capacitance comparable with that of conventional BLMs.
Moreover, NR data revealed the presence of an appreciable
amount of exchangeable water in the spacer moiety of this tBLM.
Thiolipid-based tBLMs, when anchored to solid supports such
as gold or silver, do not meet the requirement of fluidity and lateral
mobility. The thiolipid molecules are rigidly bound to the metal
surface atoms. In principle, the lipid molecules on top of the thio-
lipid monolayer may be free to move laterally. In practice, howev-
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