Design, synthesis, and applications of ring-functionalized histidines in peptide-based medicinal chemistry and drug discovery

Modified and synthetic α-amino acids are known to show diverse applications. Histidine, which possesses numerous applications when subjected to synthetic modifications, is one such amino acid. The utility of modified histidines varies widely from remarkable biological activities to catalysis, and from nanotechnology to polymer chemistry. This renders histidine residue an important place in scientific research. Histidine is a well-studied scaffold and constitutes the active site of various enzymes catalyzing important reactions in the biological systems. A rational modification in histidine structure with a distinctly developed protocol extensively changes its physical and chemical properties. The utilization of modified histidines in search of potent, target selective and proteostable scaffolds is vital in the development of bioactive peptides with enhanced drug-likeliness. This review is a compilation and analysis of reported side-chain ring modifications at histidine followed by applications of ring-modified histidines in the synthesis of various categories of bioactive peptides and peptidomimetics.

Translational incorporation of modified phenylalanines and tyrosines during cell-free protein synthesis

Inherent promiscuity of bacterial translation is demonstrated by mass spectrometric quantification of the translational incorporation of ring-substituted phenylalanines and tyrosines bearing fluoro-, hydroxyl-, methyl-, chloro- and nitro-groups in an E. coli-derived cell-free system. Competitive studies using the cell-free system show that the aminoacyl-tRNA synthetases (aaRS) have at least two orders of magnitude higher specificity for the native substrate over these structural analogues, which correlates with studies on the purified synthetase.

E. coli wild-type translational machinery utilizes a range of nonproteinogenic amino acids for protein synthesis with incorporation levels greater than 95%.

Selective Synthesis of Fluorine-Containing Cyclic β-Amino Acid Scaffolds

Fluorine-containing organic molecules have generated increasing impact in drug research over the past decade. Their preparation and development of novel synthetic methods towards new types of fluorinated molecules among them of β-amino acid derivatives has received large interest. Our research group have designed various highly selective and stereocontrolled methods for the construction of fluorine-containing cyclic β-amino acid derivatives. The synthetic approaches developed for the synthesis of various pharmacologically interesting cyclic β-amino acid derivatives as monomers with multiple stereogenic centers might be valuable protocols for the access of other classes of organic compounds.

Peptide Synthesis through Cell-Free Expression of Fusion Proteins Incorporating Modified Amino Acids as Latent Cleavage Sites for Peptide Release

Chlorinated analogues of Leu and Ile are incorporated during cell-free expression of peptides fused to protein, by exploiting the promiscuity of the natural biosynthetic machinery. They then act as sites for clean and efficient release of the peptides simply by brief heat treatment. Dehydro analogues of Leu and Ile are similarly incorporated as latent sites for peptide release through treatment with iodine under cold conditions. These protocols complement enzyme-catalyzed methods and have been used to prepare calcitonin, gastrin-releasing peptide, cholecystokinin-7, and prolactin-releasing peptide prohormones, as well as analogues substituted with unusual amino acids, thus illustrating their practical utility as alternatives to more traditional chemical peptide synthesis.