Organization and Structure of Branched Amphipathic Oligopeptide Bilayers

Abstract

A class of self-assembling Branched Amphiphilic Peptide Capsules (BAPCs) was recently developed that could serve as a new drug delivery vehicle. BAPCs can encapsulate solutes up to ~12 kDa during assembly, are unusually stable and readily taken up by cells with low cytotoxicity. Coarse-grained simulations have supported that BAPCs are defined by bilayers that resemble those formed by diacyl-phospholipids. Here, atomistic simulations were performed to characterize the structure and organization of bilayers formed by three branched amphiphilic peptides (BAPs), bis(Ac-FLIVIGSII)-K-K4-CO-NH2, bis(Ac-CHA-LIVIGSII)-K-K4-CO-NH2, and bis(Ac-FLIVI)-K-K4-CO-NH2, respectively. The results show BAPs form a network of intra- and intermolecular backbone hydrogen bonds within the same leaflet in addition to hydrophobic sidechain interactions. The terminal residues of two leaflets form an interdigitation region locking two leaflets together. The phenyl groups in bis(Ac-FLIVIGSII)-K-K4-CO-NH2 and bis(Ac-FLIVI)-K-K4-CO-NH2 are tightly packed near the bilayer center, but do not formed ordered structures with specific π-π stacking. Replacing phenyl groups with the cyclo-hexane sidechain only slightly increases the level of disorder in bilayer structures, and thus should not significantly affect the stability, consistent with experimental results on bis(Ac-CHA-LIVIGSII)-K-K4-CO-NH2 BAPCs. Self-assembly simulations further suggest that leaflet interdigitation likely occurs at early stages of BAPC formation. Atomistic simulations also reveal that the BAPC bilayers are highly permeable to water. This prediction was validated using fluorescence measurements of encapsulated self-quenching dye upon transferring BAPCs to buffers with different salt concentrations. Improved understanding of the organization and structure of BAPC bilayers at atomic level will provide a basis for future rational modifications of BAP sequence to improve BAPC properties as a new class of delivery vehicle.

Branched Amphipathic Peptide Capsules: Different Ratios of the Two Constituent Peptides Direct Distinct Bilayer Structures, Sizes, and DNA Transfection Efficiency

“…Perhaps at temperatures above the range we tested, differences in thermal stability will become apparent. The π–π stacking interactions of the phenylalanines that populate the bilayer interface do not appear to be involved based on atomistic simulations previously reported …”

“…The π−π stacking interactions of the phenylalanines that populate the bilayer interface do not appear to be involved based on atomistic simulations previously reported. 28 The observation that all of these mixed and more homogeneous structures support assembly and temperature stability imply that these structural arrangements have to be stabilized in different ways. The extended random coil structures would have to form bilayers with a longer crosssectional distance or as random coils they could have a shorter cross-sectional distance if they interdigitated.…”

Synthesis and Characterization of Multifunctional Branched Amphiphilic Peptide Bilayer Conjugated Gold Nanoparticles

“…The distance between 2 bis­(Ac-FLIVIGSII)-K-K 4 -C-CONH 2 -bound AuNPs, measured for ∼80 pairs of AuNPs, was found to be ∼1.9 nm with a SD of 0.5 nm. The bis­(Ac-FLIVIGSII)-K-K 4 -C-CONH 2 BAP bilayers are tightly packed with a packing density of 0.85 and are known to form smaller BAPCs as compared to those of bis­(Ac-FLIVI)-K-K 4 -CONH 2 -only bilayer with a packing density of 0.68 . Thus, when the distance between 2 AuNPs bound by bis­(Ac-FLIVI)-K-K 4 -C-CONH 2 peptides was measured, the average size of the bilayer was found to be ∼2.92 nm with a SD of 0.3 nm for 80 pairs.…”

“…The bis(Ac-FLIVIGSII)-K-K 4 -C-CONH 2 BAP bilayers are tightly packed with a packing density of 0.85 and are known to form smaller BAPCs as compared to those of bis(Ac-FLIVI)-K-K 4 -CONH 2 -only bilayer with a packing density of 0.68. 35 Thus, when the distance between 2 AuNPs bound by bis(Ac-FLIVI)-K-K 4 -C-CONH 2 peptides was measured, the average size of the bilayer was found to be ∼2.92 nm with a SD of 0.3 nm for 80 pairs. We could see a significant difference (p ≤ 0.05, Student's unpaired t-test) between the sizes of all bis(Ac-FLIVIGSII)-K-K 4 -C-CONH 2 and bis(Ac-FLIVI)-K-K 4 -C-CONH 2 -only peptide bilayers, as expected on the basis of data from prior studies, providing further proof of peptide binding to gold nanoparticles.…”

“…33,38 On the basis of our modeling studies, these two residues should come in close proximity to one another when the bilayer forms. 35 The bis(Ac-F CN LIVIGSII)-K-K 4 -C-CONH 2 peptide used in the previous titration experiment was used for this FRET experiment.…”