2 research outputs found
Monitoring the Orientational Changes of Alamethicin during Incorporation into Bilayer Lipid Membranes
Antimicrobial
peptides (AMPs) are the first line of defense after
contact of an infectious invader, for example, bacterium or virus,
with a host and an integral part of the innate immune system of humans.
Their broad spectrum of biological functions ranges from cell membrane
disruption over facilitation of chemotaxis to interaction with membrane-bound
or intracellular receptors, thus providing novel strategies to overcome
bacterial resistances. Especially, the clarification of the mechanisms
and dynamics of AMP incorporation into bacterial membranes is of high
interest, and different mechanistic models are still under discussion.
In this work, we studied the incorporation of the peptaibol alamethicin
(ALM) into tethered bilayer lipid membranes on electrodes in combination
with surface-enhanced infrared absorption (SEIRA) spectroscopy. This
approach allows monitoring the spontaneous and potential-induced ion
channel formation of ALM in situ. The complex incorporation kinetics
revealed a multistep mechanism that points to peptide–peptide
interactions prior to penetrating the membrane and adopting the transmembrane
configuration. On the basis of the anisotropy of the backbone amide
I and II infrared absorptions determined by density functional theory
calculations, we employed a mathematical model to evaluate ALM reorientations
monitored by SEIRA spectroscopy. Accordingly, ALM was found to adopt
inclination angles of ca. 69°–78° and 21° in
its interfacially adsorbed and transmembrane incorporated states,
respectively. These orientations can be stabilized efficiently by
the dipolar interaction with lipid head groups or by the application
of a potential gradient. The presented potential-controlled mechanistic
study suggests an N-terminal integration of ALM into membranes as
monomers or parallel oligomers to form ion channels composed of parallel-oriented
helices, whereas antiparallel oligomers are barred from intrusion
Determination of the Local Electric Field at Au/SAM Interfaces Using the Vibrational Stark Effect
A comprehensive
understanding of physical and chemical processes
at biological membranes requires the knowledge of the interfacial
electric field which is a key parameter for controlling molecular
structures and reaction dynamics. An appropriate approach is based
on the vibrational Stark effect (VSE) that exploits the electric-field
dependent perturbation of localized vibrational modes. In this work,
6-mercaptohexanenitrile (C5CN) and 7-mercaptoheptanenitrile (C6CN)
were used to form self-assembled monolayers (SAMs) on a nanostructured
Au electrode as a simple mimic for biomembranes. The Cî—¼N stretching
mode was probed by surface enhanced infrared absorption (SEIRA) spectroscopy
to determine the frequency and intensity as a function of the electrode
potential. The intensity variations were related to potential-dependent
changes of the nitrile orientation with respect to the electric field.
Supported by electrochemical impedance spectroscopy, molecular dynamics
simulations, and quantum chemical calculations the frequency changes
were translated into profiles of the interfacial electric field, affording
field strengths up to 4 × 10<sup>8</sup> V/m (C6CN) and 1.3 ×
10<sup>9</sup> V/m (C5CN) between +0.4 and −0.4 V (vs Ag/AgCl).
These profiles compare very well with the predictions of a simple
electrostatic model developed in this work. This model is shown to
be applicable to different types of electrode/SAM systems and allows
for a quick estimate of interfacial electric fields. Finally, the
implications for electric-field dependent processes at biomembranes
are discussed