Dedicated to Professor Teruaki Mukaiyuma, a dear friend and revered colleague on the occasion of his 70th birthday(1 3. VIII. 96)Structural prerequisites for the stability of the 3, helix of p-peptides can be defined from inspection of models (Figs. 1 and 2 ) : lateral non-Hi-substituents in 2-and 3-position on the 3-amino-acid residues of the helix are allowed, axial ones are forbidden. To be able to test this prediction, we synthesized a series of hepta- (1) and of the p-heptapeptide H-P-HVal-P-HAla-p-HLeu-(S,S)-~-HAla(a Me)-p-HVal-p-HAla-p-HLeu-OH (22), with a central (2S,3S)-3-amino-2-methylbutanoic-acid residue, confirm the helical structure of such 8-peptides (previously discovered in pyridine solution) (Fig. 3 and Tables 1-5). The CD spectra of helicalp-peptides, the residues of which were prepared by (retentive) Arndt-Eistert homologation of the (S)-or ~-a-amino acids, show a trough at 215 nm. Thus, this characteristic pattern of the CD spectra was taken as an indicator for the presence of a helix in methanol solutions of compounds 1 S 2 2 and 25 (including partially and fully deprotected forms) (Figs. 4 4 ) . The results fully confirm predicted structural effects: incorporation of a single 'wrong' residue ((R)-P-HAla, p-HAib, (R,S)-B-HAla(aMe), or N-Me-P-HAla) in the central positioh of thep-heptapeptide derivatives A (see 17, 18,20, or 21, resp.) causes the CD minimum to disappear. Also, the P-heptadepsipetide 25 (missing H-bond) and the /I-heptapeptide analogs with a single a-amino-acid moiety in the middle (13 and 14) are not helical, according to this analysis. An interesting case is the heptapeptide 15 with the central achiral, unsubstituted 3-aminopropanoicacid moiety: helical conformation appears to depend upon the presence or absence of terminal protection and upon the solvent (MeOH vs. MeOH/H20).

Enantiomerically pure a-amino-acid derivatives with the side chains of A h , Val, and Leu in the 2-or 3-position (8'-and P3-amino acids, resp.), as well as with substituents in both the 2-and 3-positions (Pz.3-amino acids, of like-configuration) have been prepared (compounds 8-17) and incorporated (by stepwise synthesis and fragment coupling, intermediates 24-34) into P-hexa-, P-hepta-, and j-dodecapeptides (1 -17). The new and some of the previously prepared P-peptides (35-39) showed NHiND exchange rates (in MeOH at room temperature) with T~,~ values of up to 60 days, unrivalled by short chain a-peptides. All P-peptides 1-7 were designed to be able to attain the previously described 3,-helical structure (Figs. I and 2). C D Measurements (Fig. 4), indicating a new secondary structure of certain P-peptides constructed of P'-and p3-arnino acids, were confirmed by detailed NMR solution-structure analyses: a P*-heptapeptide (2c) and a Pz.3-hexapeptide (7c) have the 3,-helical structure (Figs. 6 and 7), while to a P2/P3-hexapeptide (4) with alternating substitution pattern H-(P2-Xaa-P3-Xaa),-OH a novel, unusual helical structure (in (DJpyridine, Fig. 8; and in CD30H, Figs. 9 and f0) was assigned, with a central ten-membered and two terminal twelve-membered H-bonded rings, and with C=O and N-H bonds pointing alternatively up and down along the axis of the helix (Fig. 11). Thus, for the first time, two types of fi-peptide turns have been identified in solution. Hydrophobic interactions ofand hindrance to solvent accessibility by the aliphatic side chains are discussed as possible factors influencing the relative stability of the two types of helices.

The 8-hexapeptide (H-8-HVal-8-HAla-8-HLeu)2-OH (2) was prepared from the component L-8-amino acids by conventional peptide synthesis, including fragment coupling. A cyclo-8-tri-and a cyclo-8-hexapeptide were also prepared. The 8-amino acids were obtained from a-amino acids by Amdf-Eisfert homologation. All reactions leading to the 8-peptides occur smoothly and in high yields. The 8-peptides were characterized by their CD and NMR spectra (COSY, ROESY, TOCSY, and NOE-restricted modelling), and by an X-ray crystal-structure analysis. 8-Sheet-type structures (in the solid state) and a compact, left-handed or ( M ) 3 , helix of 5-A pitch (in solution) were discovered. Comparison with the analogous secondary structures of a-peptides shows fundamental differences, the most surprising one at this point being the greater stability of$peptide helices. There are structural relationships of 8-peptides with oligomers of 8-hydroxyalkanoic acids, and dissimilarities between the two classes of compounds are a demonstration of the power of H-bonding. Thep-hexapeptide 2 is stable to cleavage by pepsin at pH 2 in H 2 0 for at least 60 h at 37", while the corresponding a-peptide H-(Val-Ala-Leu),-OH is cleaved instantaneously under these conditions. The implication of the described results are discussed.Peptides and proteins are molecules central to life on our planet. When proteinogenic or ribosomal, they consist of a-amino acids linked together by amide bonds. We have recently embarked on a project aimed at the synthesis of oligomers from /?-amino acids (P-peptides), in order to be able to compare the structures and the properties of these unnatural peptide analogs with those of the natural products. Another point which attracted our interest in the /?-oligopeptides is their resemblance with poly(/?-hydroxyalkanoates) (PHA), an ubiquitous class of biopolymers which have been the subject of research in our group for many years [ 11.

We identified the thiomuracins, a novel family of thiopeptides produced by a rare-actinomycete bacterium typed as a Nonomuraea species, via a screen for inhibition of growth of the bacterial pathogen Staphylococcus aureus. Thiopeptides are a class of macrocyclic, highly modified peptides that are decorated by thiazoles and defined by a central six-membered heterocyclic ring system. Mining the genomes of thiopeptide-producing strains revealed the elusive biosynthetic route for this class of antibiotics. The thiopeptides are chromosomally encoded, ribosomally synthesized proteins, and isolation of gene clusters for production of thiomuracin and the related thiopeptide GE2270A revealed the post-translational machinery required for maturation. The target of the thiomuracins was identified as bacterial Elongation Factor Tu (EF-Tu). In addition to potently inhibiting a target that is unexploited by marketed human therapeutics, the thiomuracins have a low propensity for selecting for antibiotic resistance and confer no measurable cross-resistance to antibiotics in clinical use.

Careful regulation of mRNA half-lives is a fundamental mechanism allowing cells to quickly respond to changing environmental conditions. The mRNA-binding Hu proteins are important for stabilization of short-lived mRNAs. Here we describe the identification and mechanistic characterization of the first low-molecular-weight inhibitors for Hu protein R (HuR) from microbial broths (Actinomyces sp.): dehydromutactin (1), MS-444 (2) and okicenone (3). These compounds interfere with HuR RNA binding, HuR trafficking, cytokine expression and T-cell activation. A mathematical and experimental analysis of the compounds' mode of action suggests that HuR homodimerizes before RNA binding and that the compounds interfere with the formation of HuR dimers. Our results demonstrate the chemical drugability of HuR; to our knowledge HuR is the first example of a drugable protein within the Hu family. MS-444, dehydromutactin and okicenone may become valuable tools for studying HuR function. An assessment of HuR inhibition as a central node in malignant processes might open up new conceptual routes toward combatting cancer.