Incorporation of antimicrobial peptides into membranes: a combined liquid-state NMR and molecular dynamics study of alamethicin in DMPC/DHPC bicelles.

Detailed insight into the interplay between antimicrobial peptides and biological membranes is fundamental to our understanding of the mechanism of bacterial ion channels and the action of these in biological host-defense systems. To explore this interplay, we have studied the incorporation, membrane-bound structure, and conformation of the antimicrobial peptide alamethicin in lipid bilayers using a combination of 1H liquid-state NMR spectroscopy and molecular dynamics (MD) simulations. On the basis of experimental NMR data, we evaluate simple in-plane and transmembrane incorporation models as well as pore formation for alamethicin in DMPC/DHPC (1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine/1,2-dihexanoyl-sn-glycero-3-phosphatidylcholine) bicelles. Peptide-lipid nuclear Overhauser effect (NOE) and paramagnetic relaxation enhancement (PRE) data support a transmembrane configuration of the peptide in the bilayers, but they also reveal that the system cannot be described by a single simple conformational model because there is a very high degree of dynamics and heterogeneity in the three-component system. To explore the origin of this heterogeneity and dynamics, we have compared the NOE and PRE data with MD simulations of an ensemble of alamethicin peptides in a DMPC bilayer. From all-atom MD simulations, the contacts between peptide, lipid, and water protons are quantified over a time interval up to 95 ns. The MD simulations provide a statistical base that reflects our NMR data and even can explain some initially surprising NMR results concerning specific interactions between alamethicin and the lipids.

Unique identification of supramolecular structures in amyloid fibrils by solid-state NMR spectroscopy.

The fibril structure formed by the amyloidogenic fragment SNNFGAILSS of the human islet amyloid polypeptide (hIAPP) is determined with 0.52 A resolution. Symmetry information contained in the easily obtainable resonance assignments from solid-state NMR spectra (see picture), along with long-range constraints, can be applied to uniquely identify the supramolecular organization of fibrils.

An automatic solid-phase synthesis of peptaibols.

An automated approach to peptaibols using microwave-assisted solid-phase peptide synthesis is demonstrated with a combination of HBTU and acid fluoride mediated couplings for normal and alpha,alpha-dialkylated amino acids, respectively. The method is utilized for the automated synthesis of several full-length peptaibols, including alamethicin, tylopeptin, ampullosporin, bergofungin, cervinin, trikoningin, trichogin, and peptaibolin, reducing both synthesis time and costs significantly as compared to other approaches. Furthermore, the use of noncommercially available reagents is minimized.

Resolution enhancement in solid-state NMR of oriented membrane proteins by anisotropic differential linebroadening.

We demonstrate that a significant improvement in the spectral resolution may be achieved in solid-state NMR experiments of proteins in inhomogeneously disordered oriented lipid bilayers. Using 1H homonuclear decoupling instead of standard 1H heteronuclear decoupling, the 15N line widths may be reduced by up to seven times for such samples. For large oriented membrane proteins, such resolution enhancements may be crucial for assignment and structural interpretation.

Membrane-bound conformation of peptaibols with methyl-deuterated alpha-amino isobutyric acids by 2H magic angle spinning solid-state NMR spectroscopy.

We present the use of 2H magic-angle spinning (MAS) NMR on methyl-deuterated alpha-amino isobutyric acid (Aib) as a new method to obtain fast and accurate structural constraints on peptaibols in membrane-bound environments. Using nonoriented vesicle-reconstituted samples we avoid the delicate preparation of oriented samples, and the use of MAS ensures high sensitivity and thereby very fast acquisition of experimental spectra. Furthermore, given the high content ( approximately 40%) of Aib in peptaibols and the fact that the amino acid Aib may be synthesized from cheap starting materials, even in the case of 2H, 13C, or 15N labeling, this method is ideally suited for studies of the membrane-bound conformation of peptaibols.

2D separated-local-field spectra from projections of 1D experiments.

A novel procedure for reconstruction of 2D separated-local-field (SLF) NMR spectra from projections of 1D NMR data is presented. The technique, dubbed SLF projection reconstruction from one-dimensional spectra (SLF-PRODI), is particularly useful for uniaxially oriented membrane protein samples and represents a fast and robust alternative to the popular PISEMA experiment which correlates (1)H-(15)N dipole-dipole couplings with (15)N chemical shifts. The different 1D projections in the SLF-PRODI experiment are obtained from 1D spectra recorded under influence of homonuclear decoupling sequences with different scaling factors for the heteronuclear dipolar couplings. We demonstrate experimentally and numerically that as few as 2-4 1D projections will normally be sufficient to reconstruct a 2D SLF-PRODI spectrum with a quality resembling typical PISEMA spectra, leading to significant reduction of the acquisition time.

Synthesis of ellipticine: a radical cascade protocol to aryl- and heteroaryl-annulated[b]carbazoles.

[reaction: see text] Imidoyl selanides, synthesized from amides, have been used as radical precursors of imidoyl radicals in cascade reactions. The novel radical cascade has been developed for the simple synthesis of the medicinally important aryl-annulated[b]carbazoles. The protocol has been exemplified with the high-yielding total synthesis of the anticancer alkaloid ellipticine.