NOESY NMR Crosspeaks between Lipid Headgroups and Hydrocarbon Chains: Spin Diffusion or Molecular Disorder?

NOESY NMR Crosspeaks between Lipid Headgroups and Hydrocarbon Chains:  Spin Diffusion or Molecular Disorder

Probing the Role of Ceramide Headgroup Polarity in Short-Chain Model Skin Barrier Lipid Mixtures by ²H Solid-State NMR Spectroscopy

The thermoptropic phase behaviors of two <i>stratum corneum</i> model lipid mixtures composed of equimolar contributions of either Cer­[NS18] or Cer­[NP18] with stearic acid and cholesterol were compared. Each component of the mixture was specifically deuterated such that the temperature-dependent ²H NMR spectra allowed disentanglement of the complicated phase polymorphism of these lipid mixtures. While Cer­[NS] is based on the sphingosine backbone, Cer­[NP] features a phytosphingosine, which introduces an additional hydroxyl group into the headgroup of the ceramide and abolishes the double bond. From the NMR spectra, the individual contributions of all lipids to the respective phases could be determined. The comparison of the two lipid mixtures reveals that Cer­[NP] containing mixtures have a tendency to form more fluid phases. It is concluded that the additional hydroxyl group of the phytosphingosine-containing ceramide Cer­[NP18] in mixture with chain-matched stearic acid and cholesterol creates a packing defect that destabilizes the orthorhombic phase state of canonical SC mixtures. This steric clash favors the gel phase and promotes formation of fluid phases of Cer­[NP] containing lipid mixtures at lower temperature compared to those containing Cer­[NS18]

Conformation of Pyroglutamated Amyloid β (3–40) and (11–40) Fibrils – Extended or Hairpin?

Amyloid β (Aβ) is a hallmark protein of Alzheimer‘s disease. One physiologically important Aβ variant is formed by initial N-terminal truncation at a glutamic acid position (either E3 or E11), which is subsequently cyclized to a pyroglutamate (either pE3 or pE11). Both forms have been found in high concentrations in the core of amyloid plaques and are likely of high importance in the pathology of Alzheimer’s disease. However, the molecular structure of the fibrils of these variants is not entirely clear. Solid-state NMR spectroscopy studies have reported a molecular contact between Gly25 and Ile31, which would disagree with the conventional hairpin model of wildtype (WT-)Aβ1–40 fibrils, most often described in the literature. We investigated the conformation of the monomeric unit of pE3-Aβ3–40 and pE11-Aβ11–40 (and for comparison also wildtype (WT)-Aβ1–40) fibrils to find out whether the hairpin or a newly suggested extended structure dominates the structure of the Aβ monomers in these fibrils. To this end, solid-state NMR spectroscopy was applied probing the inter-residual contacts between Phe19/Leu34, Ala21/Leu34, and especially Gly25/Ile31 using suitable isotopic labeling schemes. In the second part, the flexible turn of the Aβ40 peptides was replaced by a (3-(3-aminomethyl)phenylazo)phenylacetic acid (AMPP)-based photoswitch, which can predefine the peptide conformation to either an extended (trans) or hairpin (cis) conformation. This enables simultaneous spectroscopic assessment of the conformation of the AMPP-photoswitch, allowing in situ structural investigations during fibrillation in contrast to structural techniques such as NMR spectroscopy or cryo-EM, which can only be applied to stable conformers. Both methods confirm an extended structure for the peptidic monomers in fibrils of all investigated Aβ variants. Especially the Gly25/Ile31 contact is a decisive indicator for the extended structure along with the characteristic absorption spectra of trans-AMPP-Aβ

Pseudopeptides Designed to Form Supramolecular Helixes: The Role of the Stereogenic Centers

The two epimers Boc-l-Phe-d-Oxd-(S)-β3-hPhg-OBn (1) and Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn (2) have been prepared by standard methods in solution, and their conformation was analyzed both in solution and in the solid state. While in solution 1 shows a random coil structure, 2 tends to assume a γ-turn conformation that is nearly retained in the solid state. On the other hand, in the solid state molecules of 1 associate generating a helix that involves the formation of elongated crystals with hexagonal cross-section. This effect is not observed in the crystals formed by Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn 2

Transmembrane Helix Induces Membrane Fusion through Lipid Binding and Splay

The fusion of biological membranes may require splayed lipids whose tails transiently visit the headgroup region of the bilayer, a scenario suggested by molecular dynamics simulations. Here, we examined the lipid splay hypothesis experimentally by relating liposome fusion and lipid splay induced by model transmembrane domains (TMDs). Our results reveal that a conformationally flexible transmembrane helix promotes outer leaflet mixing and lipid splay more strongly than a conformationally rigid one. The lipid dependence of basal as well as of TMD-driven lipid mixing and splay suggests that the cone-shaped phosphatidylethanolamine stimulates basal fusion via enhancing lipid splay and that the negatively charged phosphatidylserine inhibits fusion via electrostatic repulsion. Phosphatidylserine also strongly differentiates basal and helix-driven fusion, which is related to its preferred interaction with the conformationally more flexible transmembrane helix. Thus, the contribution of a transmembrane helix to membrane fusion appears to depend on lipid binding, which results in lipid splay

Transmembrane Helix Induces Membrane Fusion through Lipid Binding and Splay

The fusion of biological membranes may require splayed lipids whose tails transiently visit the headgroup region of the bilayer, a scenario suggested by molecular dynamics simulations. Here, we examined the lipid splay hypothesis experimentally by relating liposome fusion and lipid splay induced by model transmembrane domains (TMDs). Our results reveal that a conformationally flexible transmembrane helix promotes outer leaflet mixing and lipid splay more strongly than a conformationally rigid one. The lipid dependence of basal as well as of TMD-driven lipid mixing and splay suggests that the cone-shaped phosphatidylethanolamine stimulates basal fusion via enhancing lipid splay and that the negatively charged phosphatidylserine inhibits fusion via electrostatic repulsion. Phosphatidylserine also strongly differentiates basal and helix-driven fusion, which is related to its preferred interaction with the conformationally more flexible transmembrane helix. Thus, the contribution of a transmembrane helix to membrane fusion appears to depend on lipid binding, which results in lipid splay

Pseudopeptides Designed to Form Supramolecular Helixes: The Role of the Stereogenic Centers

The two epimers Boc-l-Phe-d-Oxd-(S)-β3-hPhg-OBn (1) and Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn (2) have been prepared by standard methods in solution, and their conformation was analyzed both in solution and in the solid state. While in solution 1 shows a random coil structure, 2 tends to assume a γ-turn conformation that is nearly retained in the solid state. On the other hand, in the solid state molecules of 1 associate generating a helix that involves the formation of elongated crystals with hexagonal cross-section. This effect is not observed in the crystals formed by Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn 2

Pseudopeptides Designed to Form Supramolecular Helixes: The Role of the Stereogenic Centers

The two epimers Boc-l-Phe-d-Oxd-(S)-β3-hPhg-OBn (1) and Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn (2) have been prepared by standard methods in solution, and their conformation was analyzed both in solution and in the solid state. While in solution 1 shows a random coil structure, 2 tends to assume a γ-turn conformation that is nearly retained in the solid state. On the other hand, in the solid state molecules of 1 associate generating a helix that involves the formation of elongated crystals with hexagonal cross-section. This effect is not observed in the crystals formed by Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn 2