Application of t-butyldimethylsilyl ethers of serine, threonine and tyrosine in peptide syntheshsis

Aquimarins, Peptide Antibiotics with Amino-Modified C-Termini from a Sponge-Derived Aquimarina sp. Bacterium

Genome mining and bioactivity studies suggested the sponge-derived bacterium Aquimarina sp. Aq135 as a producer of new antibiotics. Activity-guided isolation identified antibacterial peptides, named aquimarins, featuring a new scaffold with an unusual C-terminal amino group and chlorine moieties. Responsible for the halogenation is the Fe^II /α-ketoglutarate-dependent chlorinase AqmA that halogenates up to two isoleucine residues in a carrier protein-dependent fashion. Total syntheses of two natural aquimarins and eight non-natural variants were developed. Structure-activity relationship (SAR) studies with these compounds showed that the synthetically more laborious chlorinations are not required for antibacterial activity but enhance cytotoxicity. In contrast, variants lacking the C-terminal amine were virtually inactive, suggesting diamines similar to the terminal aquimarin residue as candidate building blocks for new peptidomimetic antibiotics.

Modulation of receptor binding to collagen by glycosylated 5-hydroxylysine: Chemical biology approaches made feasible by Carpino's Fmoc group

The creation of the 9-fluorenylmethoxycarbonyl (Fmoc) group by the Carpino laboratory facilitated the synthesis of peptides containing acid-sensitive groups, such as O-linked glycosides. To fully investigative collagen biochemistry, one needs to assemble peptides that possess glycosylated 5-hydroxylysine (Hyl). A convenient method for the synthesis of Fmoc-Hyl(ε-tert-butyloxycarbonyl (Boc),O-tert-butyldimethylsilyl (TBDMS)) and efficient methods for the synthesis of Fmoc-Hyl[ε-Boc,O-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)] have been developed. Glycosylated Fmoc-Hyl derivatives were used to construct a series of types I-IV collagen-model triple-helical peptides (THPs) that incorporated known or proposed receptor binding sites. Glycosylation of Hyl was found to strongly down-regulate the binding of CD44 and the α3β1 integrin to collagen, while the impact on α2β1 integrin binding was more modest. Molecular modeling of integrin binding indicated that Hyl glycosylation directly impacted the association between the α3β1 integrin metal ion-dependent adhesion site (MIDAS) and the receptor binding site within type IV collagen. The Fmoc solid-phase strategy ultimately allowed for chemical biology approaches to be utilized to study tumor cell interactions with glycosylated collagen sequences and document the modulation of receptor interactions by Hyl posttranslational modification.

Total Synthesis of Alloviroidin

Alloviroidin is a cyclic heptapeptide, produced by several species of Amanita mushrooms, that demonstrates high affinity for F-actin as is characteristic of virotoxins and phallotoxins. Alloviroidin was synthesized via a [3 + 4] fragment condensation of Fmoc-d-Thr(OTBS)-d-Ser(OTBS)-(2 S,3 R,4 R)-DHPro(OTBS)₂-OH and H-Ala-Trp(2-SO₂Me)-(2 S,4 S)-DHLeu(5-OTBS)-Val-OMe to form bond A. The linear heptapeptide favored a turn conformation, facilitating cyclization between Val¹ and d-Thr² (position B). Global deprotection and HPLC purification afforded alloviroidin with NMR spectra in excellent agreement with the natural product.

Tetrahydropyranyl: A Non-aromatic, Mild-Acid-Labile Group for Hydroxyl Protection in Solid-Phase Peptide Synthesis

The use of the tetrahydropyranyl (Thp) group for the protection of serine and threonine side-chain hydroxyl groups in solid-phase peptide synthesis has not been widely investigated. Ser/Thr side-chain hydroxyl protection with this acid-labile and non-aromatic moiety is presented here. Although Thp reacts with free carboxylic acids, it can be concluded that to introduce Thp ethers at the hydroxyl groups of N-protected Ser and Thr, protection of the C-terminal carboxyl group is unnecessary due to the lability of Thp esters. Thp-protected Ser/Thr-containing tripeptides are synthesized and the removal of Thp studied in low concentrations of trifluoroacetic acid in the presence of cation scavengers. Given its general stability to most non-acidic reagents, improved solubility of its conjugates and ease with which it can be removed, Thp emerges as an effective protecting group for the hydroxyl groups of Ser and Thr in solid-phase peptide synthesis.

Electroselective α-bromination of acetophenone using in situ bromonium ions from ammonium bromide

A greener and expeditious method for the side chain bromination of acetophenone using in situ generated bromonium ions from NH₄Br in a H₂O:CH₃CN medium at ambient temperature has been developed in an undivided cell equipped with a Pt/Pt electrode.

Monoclonal antibody-based screening assay for factor inhibiting hypoxia-inducible factor inhibitors

The factor-inhibiting hypoxia-inducible factor (FIH) hydroxylates the asparagine 803 (Asn803) residue of the hypoxia-inducible factor 1alpha (HIF-1alpha), and the modification abrogates the transcriptional activity of HIF-1alpha. Because FIH is more active on HIF-1alpha than prolyl hydroxylase domain proteins under hypoxic conditions, its inhibitors have potential to be developed as anti-ischemic drugs targeting normal cells stressed by hypoxia. In this study, the authors developed the first monoclonal antibody, SHN-HIF1alpha, specifically to Asn803 hydroxylated HIF-1alpha and a sensitive assay system for FIH inhibitors using the monoclonal antibody (Mab). SHN-HIF1alpha showed 740 times higher affinity to the Asn803 hydroxylated HIF-1alpha peptide than the unmodified one. An enzyme-linked immunosorbent assay-based system using SHN-HIF1alpha displayed at least 30 times more sensitivity than previous methods for screening FIH inhibitors and was easily applicable to develop a high-throughput screening system. SHN-HIF1alpha also showed an Asn803 hydroxylation-dependent specificity to HIF-1alpha in cells. Taken together, the results suggest that it may be used to analyze the in vivo and in vitro activities of FIH inhibitors.

Chemical synthesis of picornaviral protein primers of RNA replication

Naturally occurring nucleopeptidic replication primers (VPg-pUpU) of poliovirus and coxsackie virus were chemically synthesized. The synthesis was accomplished via block-coupling of two minimally protected fragments of the target structures: a short RNA-nucleopeptide and a longer peptide segment containing diverse side-chain functionalities. The synthetic VPg-pUpU of coxsackie virus was characterized by NMR spectroscopy.

Long-distance rotational echo double resonance measurements for the determination of secondary structure and conformational heterogeneity in peptides

The utility of rotational echo double resonance (REDOR) NMR spectroscopy for determining the conformations of linear peptides has been examined critically using a series of crystalline and amorphous samples. The focus of the present work was the evaluation of long-distance (> 5 A) interactions using 13C-15N dephasing. Detailed studies of specifically labeled melanostatin and synthetic analogs of the alpha-factor yeast mating hormone show that nitrogen-dephased, carbon-observe REDOR measurements are reliable for distances up to 6.0 A, and that dipolar interactions can be detected for distances up to 7 A. By contrast, nitrogen-observe REDOR gives reliable results only for distances shorter than 5.0 A. To measure distances accurately, REDOR data must be corrected for the effects of natural-abundance spins. These corrections are particularly important for measuring long distances, which are of the greatest value for determining peptide secondary structure. We have developed a spherical shell model for calculating the effect of these background spins. The REDOR studies also indicate that in a lyophilized powder, the tridecapeptide alpha-factor mating pheromone from Saccharomyces cerevisiae (WHWLQLKPGQPMY) probably exists as a distribution of different turn structures around the KPGQ region. This finding revises previous solid-state NMR studies on this peptide, which concluded alpha-factor assumes a distorted type-I beta-turn in the Pro-Gly central region of the molecule [J.R. Garbow, M. Breslav, O. Antohi, F. Naider, Biochemistry, 33 (1994) 10094].