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Several ion channel forming cyclic peptides have been reported over the past two decades and various ion conducting mechanisms have been proposed. In this article, we report on amino acid substitut...

The hydrophobic value and hydrophobic moment value of helical peptides correlate with membrane–peptide interactions such as ion channel forming or membrane perturbation. In this paper, two ...

2-Aminoisobutyric acid (Aib,1 2-amino-2-methylpropionic acid) is an unusual amino acid, which is present in some antimicrobial peptides. To elucidate the impact of Aib residues on the conformation and biological activity of ion channel forming peptides, Aib residues in Ac-(Aib-Lys-Aib-Ala)5-NH2 were partially and fully replaced by Ala residues. A total of three analogs were thus synthesized. Two of these analogs, Ac-(Aib-Lys-Ala-Ala)5-NH2 (BKAA-20) and Ac-(Ala-Lys-Aib-Ala)5-NH2 (AKBA-20), had similar amino acid composition and were not hemolytic. BKAA-20 exhibited comparable antimicrobial activity to gramicidin S against Gram-positive bacteria (minimum inhibitory concentration: ∼3.13 µg ml-1), whereas AKBA-20 was not active. Single-channel measurements showed that there was a clear correlation between the antimicrobial activity and the ion channel activity in these analogs. The third analog, Ac-(Ala-Lys-Ala-Ala)5-NH2, was biologically inactive. Circular dichroism spectroscopy revealed that the helix formi...

The synthesis and conformational analysis of cyclo(Pro-Leu-Aib)2 by means of NMR technique are reported. The cyclohexapeptide was synthesized in the classical high-dilution solution method. Its yield in the cyclization reaction from linear hexapeptide active ester was lower than that of the analogous cyclo(Pro-Leu-Gly)2 previously reported. Two sets of amide proton signals for Leu and Aib in NMR spectra and temperature coefficients indicated an asymmetric conformation which contains both cis and trans Aib-Pro bonds. Here Pro amino acids occupy the positions i+2 of β-turns. The CD spectrum gave two negative bands at 202 and 230 nm, which is considered to be the sum of type II′ (when Aib-Pro is trans) and type VI (when Aib-Pro is cis) β-turns.

Two cyclic peptides containing the C-terminal tripeptide sequence of oxytocin, cyclo(Pro–Leu–Gly)2 and cyclo(Pro–Leu–Gly)4, have been synthesized from the corresponding linear hexapeptide active ester under high dilution conditions. Cyclo(Pro–Leu–Gly)4 serves as an example of the synthesis of cyclic peptides from linear peptides containing an even number of amino acid residues through dimerization. These two compounds are characterized by TLC, HPLC, amino acid analysis, NMR, and mass spectra. Cyclo(Pro–Leu–Gly)2 has also been synthesized from the linear tripeptide precursor independently, and its properties have been shown to be identical to that from linear hexapeptide. Circular dichroism indicated that cyclo(Pro–Leu–Gly)2 contains β-turns as expected; in contrast, cyclo(Pro–Leu–Gly)4 exhibited a random structure pattern. The NMR spectra demonstrated that cyclo(Pro–Leu–Gly)2 is fixed in only one conformer in DMSO, while in a CDCl3 solution, several conformations with a rapid interconversion are present. ...

Based on increased affinity and selectivity of dimeric enkephalin analogs for opioid receptors, divalent interactions have been demonstrated between opioid peptides and receptors. To explore the receptor multivalency suggested by fluorescent microscopic studies, a novel type of tetrameric enkephalin analog was designed and synthesized. The tetrameric enkephalin comprises four [D-Ala2, Leu5]enkephalin molecules coupled with the amino groups of the gem-diamino derivative of cystine, which was derived from cysteinamide by a Hofmann-type rearrangement, causing an amide-to-amine conversion. By the reduction of the gem-cystine disulfide bond in the tetramer, two dimeric analogs with a free or p-methoxybenzyl-protected mercapto group were also prepared. The receptor binding characteristics of these tetramer and dimers for the δ and μ receptors were analyzed in an NG108-15 cell and rat-brain membranes. The affinity of the tetrameric analog for the δ receptors was about equal to that of the dimers, but 2—4-fold lo...

In order to inspect the structural elements of Ser-1 in receptor activation by SFLLRNP (one-letter amino acid code), a ligand peptide tethered to the thrombin receptor, a series of analogs with such replacements as D-Ser, Ala, Thr, and Ac-Ser have been synthesized. These analogs were evaluated for their ability to hydrolyze the phosphoinositide in human neuroblastoma SH-EP cells. It was found that the α-amino group and L-configuration of Ser-1 are very important in the activation of receptors. N-Acetylation or deletion of Ser-1 completely eliminated the activity of SFLLRNP (a half-maximal effective concentration, EC50 = 0.89 μM (1 M = 1 mol dm−3)), and these modifications induced no antagonist activity. Incorporation of D-Ser also drastically diminished the activity, but retained about 50% activity of the maximal response by 100 μM SFLLRNP. The Ser/Ala substitution sustained 30% of the activity of SFLLRNP to elicit a full stimulation. The Ser/Thr substitution, however, enhanced the activity (20%) in spite...

NMR spectroscopy is a versatile method for the conformational analysis of peptides and proteins. The hydrogen–chlorine exchange of amide NH protons is detected by 1H NMR and used as a method to distinguish between intramolecularly hydrogen-bonded and solvent-exposed NH moieties. The method has been applied to hydrogen bond detection in naturally occurring antibiotic peptides, such as gramicidin S, and CH3CONH–X (X = alkyl- or aryl-) derivatives. The deuterium exchange method was compared with this method in parallel experiments. In the case of chlorine exchange, in contrast to deuterium exchange, the hydrogen-bonded amide protons are replaced much faster than their solvent-exposed counterparts and the duration of the experiments is considerably less. It is highly possible that the hydrogen–chlorine exchange reaction under the present experimental conditions, in the dark and at room temperature, proceeds through an electrophilic polar mechanism.

p-Nitrophenylalanine (Phe(p-NO2)) was incorporated in place of Phe4 of [D-Ala2, Leu(CH2S-Npys)5]enkephalin. In the rat brain receptor binding assay, Npys-containing Phe(p-NO2)4- and Phe4-enkephalins exhibited almost unchanged affinities for μ receptors (IC50 = 16—21 nM). However, when the membrane was incubated with each analog, Phe(p-NO2)4-enkephalin (EC50 = 6.90 nM) could label the μ receptors about three times more effectively than Phe4-enkephalin.

The resolution of racemic 2,3-methanopyroglutamic acid (∇Glp) was accomplished using L- and D-Leu-NH2 and both isomers were hydrolyzed to give optically active (Z)-2,3-methanoglutamic acids1) (∇ZGlu), respectively. The optical rotations of these samples were compared with ∇ZGlu obtained by hydrolysis of ∇Glp prepared by oxidation of (2S,3R)- or (2R,3S)-2,3-methanoproline1) (∇Pro), the absolute configurations of which were determined by X-ray diffraction earlier. These experiments showed (−)- and (+)-∇Glp to have the (2S,3S)- and (2R,3R)-configurations, respectively.

In order to investigate the relationship between the structure of a peptide backbone and the formation of an intramolecular disulfide bond (S–S) in the peptide, a linear analog (1) of gramicidin S and a heterodetic cyclic peptide containing an S–S bond (2) were synthesized by the conventional solution method. Since a disulfide bonded compound (2) was easily formed by a treatment of the acetamidomethyl(Acm)-protected linear peptide (1) with iodine in good yield (70%), it is suggested that the mutual position between Cys1 and Cys8 residues becomes approximate, owing to the contribution of –D-Phe–Pro– part of the sequence. A heterodetic cyclic peptide (2) showed about 1/16–1/8 fold activity compared to that of GS; 1 as inactive. By the construction of an S–S bridge on the peptide backbone (1), an inactive peptide derivative was effectively converted to an active analog (2). The CD spectrum pattern also suggests that the heterodetic cyclic peptide (2) has a GS-like structure.

A convenient procedure to prepare L-homoglutamic acid (L-2-aminoadipic acid; L-Aad) is described. Chlorination of e-amino group in acetyl-L-lysine ethyl ester with t-butyl hypochlorite (t-BuOCl) followed by dehydrochlorination with pyridine gave ethyl 5-cyanovalerate which on acid hydrolysis afforded L-Aad in good yield.