Thermodynamics and structure of peptide-aggregates at membrane surfaces

This thesis aimed at improving our understanding of the thermodynamic and structural
aspects of peptide aggregation processes at membrane surfaces. For this purpose we
investigated a class of model peptides, which form a [beta]-sheet structure upon binding to
membrane surfaces. Binding of peptides with the repeating sequence of KIGAKI to
anionic membrane surfaces was chosen as model system to characterize the transition
from a random coil to [beta]-sheet structure. Evidence is brought that the process of
intermolecular [beta]-sheets formation by the KIGAKI peptides is a suitable model system for
a peptide aggregation process at membrane surfaces.
In order to understand this aggregation process, thermodynamic parameters of (KIGAKI)3
binding to lipid membranes were determined directly by isothermal titration calorimetry.
For a description of the peptide binding data a theoretical binding model was developed
and evaluated with the drug verapamil. It is shown that the binding model, which is based
on the Gouy-Chapman theory, can be used in a general way to describe electrostatic
attraction and repulsion of charged molecules to lipid membranes under a variety of
environmental conditions. Nevertheless, binding of peptides to lipid membranes is more
complex as simply considering electrostatic attraction of the peptide to the membrane.
Thermodynamic binding parameters of (KIGAKI)3 to lipid membranes, obtained by ITC,
combines mainly two reactions, the intrinsic binding and [beta]-sheet folding process.
Separation of both subprocesses from the overall thermodynamic binding process could be
achieved by varying the extent of [beta]-sheet formation due to substitution of two adjacent D
amino acids within the peptide sequence. Double D amino acid substitution leads to a
local disturbance of the [beta]-sheet structure, where the extent of the [beta]-sheet formation is
dependent on the number and position of the double D amino acid substitution. With this
approach it was possible to determine for the first time a full thermodynamic profile of the
random coil to [beta]-sheet transition for a peptide in a membrane environment and
concomitantly these parameters are the first clearly defined parameters of a peptide
aggregation reaction.
Beta sheet folds in proteins tend to be distinctively smaller than current models predict for
[beta]-sheets in protein and peptide aggregates. To reveal differences between the [beta]-sheet
folding reaction in a native and aggregated protein, we extended the study and determined
the length dependence of the [beta]-sheet folding reaction. Thermodynamic parameters of the
[beta]-sheet folding reaction for KIGAKI peptide with different lengths were determined in
analogy to (KIGAKI)3. A linear length stabilization effect could be demonstrated for
KIGAKI [beta]-sheet structure. Furthermore, for [beta]-sheets shorter than 10 residues the folding
reaction is driven by entropy, whereas for longer [beta]-sheets the folding reaction is driven by
enthalpy. Underlying length dependence of the thermodynamic driving forces of [beta]-sheet
folding reaction is therefore the most important finding of this work since it reveals an
important difference in the folding reaction between native and aggregating [beta]-sheets.
Furthermore, the double D amino acid substitution strategy opens a new way to
systematically resolve the characteristic [beta]-sheet-aggregation at membrane surfaces, as for
example for the Alzheimer peptide.
Beside thermodynamics of the [beta]-sheet folding process we also studied the dynamics and
size of the extended [beta]-sheet structure of KIGAKI at the membrane surface by deuterium
solid state NMR. It is revealed that the [beta]-sheet structure formed by the (KIGAKI)3 peptide
is large and rigid and therefore inevitably extended at the membrane surface. Perturbation
of the membrane integrity, due to peptide binding and [beta]-sheet formation are not observed.
In turn, these findings weaken the theories that peptide aggregates at the membrane
surface mediating cell death by disrupting the cell membrane.
As a new approach to study extended [beta]-sheet structures at membrane surfaces, the
(KIGAKI)3 and Alzheimer peptide were encapsulated in reverse micelles and dissolved in
a low viscosity solvent. Within the reverse micelles KIGAKI peptides adopted their
characteristic [beta]-sheet structure. Promising NMR results show that the reverse micelles
technique is an interesting alternative for the structure analysis of membrane peptide and
protein aggregates.
The last part of this thesis dealt with the partitioning process of xenon in lipid membranes.
NMR spectroscopic analysis of the chemical shift behavior of 129Xe in lipid suspension
offered a new method to determine partitioning coefficients of xenon in lipid membrane
samples, like blood and tissue samples, which are of particular interest for various medical
applications of xenon. Additionally, our data provide new aspects of the anesthetic
properties of xenon. In particular, we demonstrated that lipid molecules maintained their
structure upon xenon partitioning, which suggests that structural changes of the lipid
molecules are not necessary to mediate anesthesia.