Channel properties of template assembled alamethicin tetramers

Total synthesis of the peptaibols hypomurocin A3 and hypomurocin A5, and their conformation analysis

The total syntheses of hypomurocin A3 and hypomuricin A5 (HM A3 and HM A5, resp.) in solution phase are described. These syntheses have been successfully achieved by applying the 'azirine/oxazolone method' to introduce the two Aib-Pro units into the backbone of these undecapeptaibols in one step with methyl 2,2-dimethyl-2H-azirine-3-prolinate as the 'Aib-Pro synthon'. The coupling of Z-protected (Z=(benzyloxy)carbonyl) amino acids or peptide acids with amino acid tert-butyl esters and of peptide segments was carried out according to the TBTU (=O-(benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium tetrafluoroborate) and HOBt (=1-hydroxybenzotriazole) protocol. Purification by reversed-phase HPLC gave the peptides in pure form. The products were characterized by optical rotation, NMR and IR spectroscopy, mass spectrometry, and elemental analysis. The crystal structures of HM A3 and of an octapeptide fragment of HM A5 could be obtained. An NMR analysis was also carried out with HM A3 and HM A5 to determine their conformations in solution. A global structural comparison between the three sequences of HM A1, HM A3, and HM A5 was performed, as well as the HPLC correlation of the natural HM A family and the synthetic samples.

Cyclodextrin-scaffolded alamethicin with remarkably efficient membrane permeabilizing properties and membrane current conductance

Bacterial resistance to classical antibiotics is a serious medical problem, which continues to grow. Small antimicrobial peptides represent a potential solution and are increasingly being developed as novel therapeutic agents. Many of these peptides owe their antibacterial activity to the formation of trans-membrane ion-channels resulting in cell lysis. However, to further develop the field of peptide antibiotics, a thorough understanding of their mechanism of action is needed. Alamethicin belongs to a class of peptides called peptaibols and represents one of these antimicrobial peptides. To examine the dynamics of assembly and to facilitate a thorough structural evaluation of the alamethicin ion-channels, we have applied click chemistry for the synthesis of templated alamethicin multimers covalently attached to cyclodextrin-scaffolds. Using oriented circular dichroism, calcein release assays, and single-channel current measurements, the α-helices of the templated multimers were demonstrated to insert into lipid bilayers forming highly efficient and remarkably stable ion-channels.

Modulation of the conductance of a 2,2'-bipyridine-functionalized peptidic ion channel by Ni2+

An α-helical amphipathic peptide with the sequence H₂N-(LSSLLSL)₃-CONH₂ was obtained by solid phase synthesis and a 2,2′-bipyridine was coupled to its N-terminus, which allows complexation of Ni²⁺. Complexation of the 2,2′-bipyridine residues was proven by UV/Vis spectroscopy. The peptide helices were inserted into lipid bilayers (nano black lipid membranes, nano-BLMs) that suspend the pores of porous alumina substrates with a pore diameter of 60 nm by applying a potential difference. From single channel recordings, we were able to distinguish four distinct conductance states, which we attribute to an increasing number of peptide helices participating in the conducting helix bundle. Addition of Ni²⁺ in micromolar concentrations altered the conductance behaviour of the formed ion channels in nano-BLMs considerably. The first two conductance states appear much more prominent demonstrating that the complexation of bipyridine by Ni²⁺ results in a considerable confinement of the observed multiple conductance states. However, the conductance levels were independent of the presence of Ni²⁺. Moreover, from a detailed analysis of the open lifetimes of the channels, we conclude that the complexation of Ni²⁺ diminishes the frequency of channel events with larger open times.

Ligand-induced extramembrane conformation switch controlling alamethicin assembly and the channel current

In this review, we describe our approach to creating artificial receptor-channel proteins or sensor systems, using an extramembrane segment conformationally switchable by external stimuli. Alamethicin is known to self-assemble in membranes to form ion channels with various open states. Employment of an alpha-helical leucine-zipper segment resulted in the effective modulation of the association states of alamethicin to produce a single predominant channel-open state. A decrease in the helical content of the extramembrane segments was found to induce a channel-current increase. Therefore, conformational changes in the extramembrane segments induced by the interaction with ligands can be reflected in the current levels.

Channel-Forming Activity of Alamethicin: Effects of Covalent Tethering

In this short review article, the effects of covalent tethering of alamethicin molecules on channel-forming behavior are described. Broadly speaking, these chemical modifications have provided insight into all three aspects of channel behavior: the structure of the conducting state, the ion-selectivity and ion-permeation properties, and the voltage dependence. Each of these aspects are discussed in turn.

Oral secretions from herbivorous lepidopteran larvae exhibit ion channel-forming activities

Feeding insects introduce oral secretions (OS) into the wounded tissue of the attacked plant. Various OS-derived molecules must be involved in subsequent processes including the induction of plant defence reactions. Using the planar lipid bilayer membrane technique, isolated OS were analyzed with respect to their membrane activities. Transmembrane ion fluxes were generated by OS of eight different lepidopteran larvae, all of which form comparable ion channels in artificial membranes. Currents were characterized by long lasting open times and conductivities from 250pS up to 1100pS. Channels formed by Spodoptera exigua secretions showed a preference for cations over anions. OS also induced a transient increase of the cytosolic calcium concentration in soybean cells, determined by the aequorin technique. Known compounds of the OS, fatty-acid-glutamine conjugates, also interfered with the membrane but were unable to form stable channels. Since ion fluxes and depolarization are early responses upon insect feeding, OS-derived components may be involved in the elicitation process by direct interaction with the plant membranes.

The first total synthesis of the peptaibol hypomurocin A1 and its conformation analysis: an application of the 'azirine/oxazolone method'

The first total synthesis of Hypomurocin A1 (HM A1) in solution phase is described. As members of the peptaibol family, hypomurocins are constituted by two groups of peptides: six undecapeptides (undecamers) in the HM A group and six octadecapeptides (18-mers) in the HM B group. The synthesis presented has been successfully achieved by the 'azirine/oxazolone method' to introduce the two Aib-Pro sequences included in this undecapeptaibol in one step with methyl 2,2-dimethyl-2H-azirine-3-prolinate as the building block. The coupling reactions of the Z-protected amino acids or peptide acids involved the use of N,N,N',N'-tetramethyluronium tetrafluoroborate (TBTU) and 1-hydroxybenzotriazole (HOBt), and led to the peptides in good-to-very-good yields. The peptides were purified by reverse-phase HPLC and characterized by NMR spectroscopy (1H, 13C, COSY, TOCSY, HSQC, HMBC, ROESY), ESI-MS, IR, elemental analysis, optical rotation, and X-ray crystallography. An NMR analysis of HM A1 was also carried out in deuterated micelles to perform a structural comparison of the helix in solution and in membranes.

Synthetic Peptide Templates for Molecular Recognition: Recent Advances and Applications

The creation of molecular systems that can mimic some of the properties of natural macromolecules is one of the major endeavors in contemporary protein chemistry. However, the construction of artificial proteins with predetermined structure and function is difficult on account of complex folding pathways. The use of topological peptide templates has been suggested to induce and stabilize defined secondary and tertiary structures. This is because the recent advances in the chemistry of coupling reagents, protecting groups, and solid-phase synthesis have made the chemical synthesis of peptides with conformationally controlled and complex structures feasible. Besides their use as structure-inducing devices, these peptide templates can also be utilized to construct novel structures with tailor-made functions. Herein, we present recent advances in the field of peptide-template-based approaches with particular emphasis on the demonstrated utility of this approach in molecular recognition, along with related applications.

Total synthesis in solution of alamethicin F50/5 by an easily tunable segment condensation approach

A total synthesis in solution of the 19-mer peptide component F50/5 of alamethicin, the most extensively investigated among the channel-former peptaibol antibiotics, is reported. Three peptide segments (A, B, C) were prepared and assembled, followed by incorporation of the acetylated N-terminal amino acid. The synthetic modules B and C are characterized by three Glu(OMe) residues (at positions 7, 18, and 19) that, after completion of the synthesis, were reacted with ammonia to provide alamethicin F50/5. By use of this general strategy, we also prepared the [Gln7, Glu(OMe)18,19] alamethicin F50/5 analogue. The purity and conformation of the final products were assessed by chromatographic, spectrometric, and spectroscopic techniques. This tunable segment condensation approach will pave the way for an easy synthesis of alamethicin analogues bearing amino acid residues with desired side-chain probes even at the N-terminus and in internal positions of the sequence.

Helical kink and channel behaviour: a comparative study with the peptaibols alamethicin, trichotoxin and antiamoebin

Kinks or bends introduced in peptides and proteins by "helical distorter" residues such as proline, other imino acids and glycine, especially when these are in close proximity in the sequence, are increasingly recognized as playing an essential role in the gating of channel-forming peptides as well as of physiological ion channels. Peptaibols are useful simple models for the much more complex biological ion channels, especially voltage-gated ones. In this short review, we compare the monomeric structures of three selected peptaibols (alamethicin, trichotoxin and antiamoebin) that widely differ with regards their near-central kink angles and dipolar moment orientations. These structural features are then shown to be correlated to the different patterns of channel activity, both at the macroscopic and single-channel levels of investigation.