3 research outputs found
Stereochemical Effect Revealed in Self-Assemblies Based on Archaeal Lipid Analogues Bearing a Central Five-Membered Carbocycle: A SAXS Study
The relative stereochemistry (cis or trans) of a 1,3-disubstituted
cyclopentane unit in the middle of tetraether archaeal bipolar lipid
analogues was found to have a dramatic influence on their supramolecular
self-assembly properties. SAXS studies of two synthetic diastereomeric
archaeal lipids bearing two lactosyl polar head groups at opposite
ends revealed different lyotropic behaviors. The cis isomer led to
L<sub>c</sub>āL<sub>Ī±</sub>āQ<sub>II</sub> transitions
whereas the trans isomer retained an L<sub>Ī±</sub> phase from
20 to 100 Ā°C. These main differences originate from the conformational
equilibrium (pseudorotation) of 1,3-disubstituted cyclopentanes. Indeed,
this pseudorotation exhibits quite similar orientations of the two
substituents in a trans isomer whereas several orientations of the
two alkyl chains are expected in a <i>cis</i>-1,3-dialkyl
cyclopentane, thus authorizing more conformational flexibility in
the lipid packing
Experimental Observation of Double-Walled Peptide Nanotubes and Monodispersity Modeling of the Number of Walls
Self-assembled nanoarchitectures based on biological
molecules
are attractive because of the simplicity and versatility of the building
blocks. However, size control is still a challenge. This control is
only possible when a given system is deeply understood. Such is the
case with the lanreotide acetate, an octapeptide salt that spontaneously
forms monodisperse nanotubes when dissolved into pure water. Following
a structural approach, we have in the past demonstrated the possibility
to tune the diameter of these nanotubes while keeping a strict monodispersity,
either by chemical modification of one precise amino acid on the peptide
sequence or by changing the size of the counterions. On the basis
of these previous studies, we replaced monovalent counterions by divalent
ones to vary the number of walls. Indeed, in the present work, we
show that lanreotide associated with a divalent counterion forms double-walled
nanotubes while keeping the average diameter constant. However, the
strict monodispersity of the number of walls was unexpected. We propose
that the divalent counterions create an adhesion force that can drive
the wall packing. This adhesion force is counterbalanced by a mechanical
one that is related to the stiffness of the peptide wall. By taking
into account these two opposite forces, we have built a general model
that fully explains why the lanreotide nanotubes formed with divalent
counterions possess two walls and not more
Structural Role of Counterions Adsorbed on Self-Assembled Peptide Nanotubes
Among noncovalent forces, electrostatic ones are the
strongest
and possess a rather long-range action. For these reasons, charges
and counterions play a prominent role in self-assembly processes in
water and therefore in many biological systems. However, the complexity
of the biological media often hinders a detailed understanding of
all the electrostatic-related events. In this context, we have studied
the role of charges and counterions in the self-assembly of lanreotide,
a cationic octapeptide. This peptide spontaneously forms monodisperse
nanotubes (NTs) above a critical concentration when solubilized in
pure water. Free from any screening buffer, we assessed the interactions
between the different peptide oligomers and counterions in solutions,
above and below the critical assembly concentration. Our results provide
explanations for the selection of a dimeric building block instead
of a monomeric one. Indeed, the apparent charge of the dimers is lower
than that of the monomers because of strong chemisorption. This phenomenon
has two consequences: (i) the dimerādimer interaction is less
repulsive than the monomerāmonomer one and (ii) the lowered
charge of the dimeric building block weakens the electrostatic repulsion
from the positively charged NT walls. Moreover, additional counterion
condensation (physisorption) occurs on the NT wall. We furthermore
show that the counterions interacting with the NTs play a structural
role as they tune the NTs diameter. We demonstrate by a simple model
that counterions adsorption sites located on the inner face of the
NT walls are responsible for this size control