Masked side-chain aldehyde amino acids for solid-phase synthesis and ligation

Synthesis and conformational analysis of an expanded cyclic ketoxime-hexapeptide

In this work the synthesis of a linear hexapeptide with a hydroxylamine functionality at the N-terminus and a ketone instead of the carboxylic acid at the C-terminus is described. Cyclization by ketoxime formation yields the 19-membered ring-expanded cyclic hexapeptide cyclo[Goly-Val-Ala-Pro-Leu-Kly] which adopts a main conformer with two intramolecular hydrogen bonds. The hydrolytic stability of a ketoxime lies between the inert amide and the labile imine. The substitution of an amide bond for an iminium bond transforms the irreversible macrocyclization into a reversible process, but macrocyclic imines are difficult to isolate because they are prone to hydrolysis. The enhanced chemical stability of the ketoxime justifies its application in ligation protocols. The detailed NMR analysis of a ketoxime linkage presented here identifies its local conformational preferences in a constrained peptide environment.

2-Oxazoline formation for selective chemical labeling of 5-hydroxylysine

Hydroxylation of lysine, one of posttranslational modifications of proteins, generates 5-hydroxylysine (Koh) and plays a crucial role in regulating protein functions in cellular activity. We have developed a chemical labeling method of Koh. The 1,2-aminoalcohol moiety of Koh in synthetic peptide sequences was trapped by an alkyne-containing benzimidate to form a 2-oxazoline ring. An additional ammonia treatment process removed the undesirable amidine residue formed between benzimidate and lysine. During the ammonia treatment, the oxazoline residue formed at Koh mainly remained intact, and the ring opening to the amide form was observed for only part of oxazoline, indicating that the chemical labeling is amino acid selective. Azide-substituted biotin or fluorescent dye was attached to the peptide through Huisgen cycloaddition at Koh and converted into an alkyne-labeled oxazoline form. The Koh-labeling assay could provide a platform to enhance proteomic research of lysine hydroxylation.

From pico to nano: biofunctionalization of cube-octameric silsesquioxanes by peptides and miniproteins

Polyhedral silsesquioxanes are considered valuable conjugation scaffolds. Nevertheless, only a few examples of silsesquioxane-assembled peptide oligomers have been reported to date. We developed a new bioorthogonal cube-octameric silsesquioxane (COSS) scaffold bearing eight aminooxy coupling sites allowing for the conjugation of diverse peptides via oxime ligation. We found that the coupling efficacy depends on the ligand in view of steric hindrance and electrostatic repulsion. For the first time scaffold-based conjugation of cystine-knot miniproteins having a backbone of about thirty amino acids was successfully accomplished without loss of bioactivity. Atomic force microscopy (AFM) provided further knowledge on the size of COSS verifying them as picoscaffolds growing upon bioconjugation to nano-dimension.

Concise synthesis of an enantiopure bicyclic pyrazinone as constrained peptidomimetic building block

A concise synthetic route has been developed for the preparation of a constrained peptidomimetic pyrazinone building block. From hydroxy-L-lysine, the desired pyrazinone is obtained in 43% overall yield (6 steps) via an efficient deprotection-double cyclization sequence.

The 2-oxoglutarate-dependent oxygenase JMJD6 catalyses oxidation of lysine residues to give 5S-hydroxylysine residues

Amino acid analyses reveal that JMJD6-catalysed hydroxylation of RNA-splicing regulatory protein fragments occurs to give hydroxylysine products with 5S stereochemistry. This contrasts with collagen lysyl hydroxylases, which give 5R-hydroxylated products. The work suggests that more than one subfamily of lysyl hydroxylases has evolved and illustrates the importance of stereochemical assignments in proteomic analyses.

Development of aggregation inhibitors for amyloid-? peptides and their evaluation by quartz-crystal microbalance

A series of amyloid-beta aggregation inhibitors composed of a molecular recognition element (KLVFF) and an aggregation-disrupting part (having an electrostatic and hydrophilic nature) based on amino acid analogs have been synthesized. A quartz-crystal microbalance (QCM) method was applied and found to be very successful in evaluating the inhibitory activity of the Abeta aggregation, which was observed when the frequency was increased. The QCM can detect a mass change with differences in frequency that correspond to a 1 Hz frequency decrease per 30 pg mass increase on a 4.9 mm(2) electrode. Furthermore, bioassay results showed no toxicity of the inhibitor itself against IMR-32 neuroblastoma cells, and remarkably reduced cytotoxicities of both Abeta1-40 and Abeta1-42 were exhibited in the presence of these inhibitors. The KLVFF-(EEX)3 derivative was the most efficient Abeta aggregation among the inhibitors examined here.

Efficient synthesis of C-terminal modified peptide ketones for chemical ligations

Synthesis of a C-terminal modified peptide with an alpha-amido methylketone was efficiently carried out using a backbone-amide-type linker loading with a monofunctionalized diamine, provided that no base such as piperidine or diisopropylethylamine or a reducing agent such as triisopopylsilane was used for the synthetic pathway. The ketoxime-forming chemoselective ligation between a methylketone and an aminooxy was quantitative in 5h at pH 2.

Synthesis of Branched Oxime-Linked Peptide Mimetics of the MUC1 Containing a Universal T-Helper Epitope

Our goal was to develop mimics of MUC1, highly immunogenic to induce an efficient immune response against the tumor-associated form of MUC1, and sufficiently different from the natural antigen to bypass the tolerance barrier in humans. With the aim of obtaining a well-defined peptide construct as a means of evoking the precise immune responses required in immunotherapy, we synthesized artificial mimics of the MUC1 protein composed of two MUC1 repeat units of inverse orientation and a universal T-helper epitope. To synthesize these heteromeric peptide constructs, we followed a convergent approach using chemoselective ligation based on oxime chemistry. A stem peptide was first synthesized bearing two orthogonally masked aldehydes. After successive deprotection, two oxime bonds can be specifically generated. The proposed strategy proved to be concise and robust, and allowed the synthesis of the tri-branched protein in a very satisfactory yield. The different constructs were tested for their ability to generate antibodies able to recognize the MUC1 protein.