Over the last years, several articles about aminooxy-amino acid derivatives were published. Within the following, we are aiming at giving a brief review about various applications that are possible with aminooxy-derivatives offered by Iris Biotech.

Replacing the primary amino group of an amino acid by an aminooxy moiety leads to an increase in nucleophilicity. Thus, after completion of the peptide synthesis and deprotection of the aminooxy-function, chemoselective reactions with carbonyl compounds under formation of a kinetically stable oxime bond can be performed. Compared to imines, oximes display much higher stability toward hydrolysis. This increased nucleophilicity and stability is explained by the α-effect provided by the heteroatom adjacent to the sp² nitrogen.

Mechanism of aldoximine/ketoxime formation upon reaction of an aminooxy derivative with an aldehyde or ketone, respectively.

This simple reaction can be utilized for the synthesis of large proteins by fragment assembly (A) or for the preparation of cyclic peptides (B, C). In the first case, successive deprotection of aminooxy moieties allows for the controlled and sequential ligation of peptide fragments. The latter one, peptide cyclization, imposes significant conformational constraints on peptide folding that is essential for biological activity. As exemplified in nature,  this typically occurs via disulfide bridges between thiol-containing residues. Using aminooxy derivatives, peptide cyclization can either be achieved by reacting an aminooxy and aldehyde side chain within one peptide sequence, or via bridging of two aminooxy side chains and a dialdehyde linker.

Illustration of exemplary applications of aminooxy derivatives: (A) Fragment condensation via oxime ligation. (B) and (C) Cyclization via oxime formation.

Besides oxime formation, the aminooxy moiety allows for chemoselective side-chain modification, e.g. for the formation of glycoconjugates. The reaction of a reducing carbohydrate with an aminooxy group leads to unnatural, N-glycosylated peptides named neo-glycopeptides. Those constructs are valuable tools for the ease of investigation of the influence of glycosylation patterns on structure and function of peptides and proteins. One key advantage of this approach is that a single peptide, synthesized by SPPS, can easily be reacted with a variety of sugars providing a whole library for structure-activity studies.

Furthermore, the aminooxy moiety can act as a donor group for chelation. As an example, our hydroxamate-ornithine derivative N-delta-hydroxy-N-delta-acetyl-ornithine (FAA4220) is a bidentate ligand that is found in many siderophores. Siderophores are known for the formation of stable hexadentate octahedral complexes with Fe³⁺ and other metal ions. They are taken up by specific microbial receptors and transport systems, a property that can be exploited to deliver antibiotics via Siderophore-Drug-Conjugates. Since it bypasses the bacterium’s defences, this method is aptly termed the “Trojan Horse” strategy. Another example were chelation plays a crucial role, are Histone deacetylase (HDAC) inhibitors. HDACs are pivotal enzymes in the regulation of gene expression and can be efficiently inhibited by substrate peptidomimetic inhibitors (SPIs), as shown by Jamieson et al. The inhibiting properties of SPIs stem from the presence of non-natural amino acids able to coordinate the HDAC active site zinc(II).

This multitude of examples highlights the potential and versatility of aminooxy amino acid derivatives. Iris Biotech offers corresponding building blocks compatible with standard SPPS protocols.

References:

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• Site-specific cross-linking of proteins to DNA via a new biorthogonal approach employing oxime ligation; S. S. Pujari, Y. Zhang, S. Ji, M. D. Distefano, N. Y. Tretyakova; Chem. Commun. 2018; 54: 6296-6299. https://doi.org/10.1039/C8CC01300D. https://doi.org/10.1016/j.bmcl.2015.03.069.

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