Synthesis of six-membered carbocyclic ring ?,?-disubstituted amino acids and arginine-rich peptides to investigate the effect of ring size on the properties of the peptide

Cell-penetrating peptides (CPPs) have been attracting attention as tools for intracellular delivery of membraneimpermeant functional molecules. Among the variety of CPPs that have been developed, many are composed of both natural and unnatural amino acids. We previously synthesized ?,?-disubstituted ?-amino acids (dAAs) containing a five-membered carbocyclic ring in its side chain and revealed the utility of dAAs for the development of novel CPPs. In the present study, we designed a six-membered carbocyclic ring dAA with an amino group on the ring and introduced it into arginine (Arg)-rich peptides to further investigate the value of dAAs for developing CPPs. We also assessed the effects of the size of the dAA carbocyclic ring on cellular uptake of dAAcontaining peptides. The stability of the peptide?s secondary structure and its membrane permeability were both greater in dAA-containing peptides than in an Arg nonapeptide. However, the number of carbon atoms in the dAA side chain ring had little effect. Nevertheless, these results show the utility of cyclic dAAs in the design of novel CPPs containing unnatural amino acids.

Automated summary

The introduction of dAAs containing both natural and unnatural amino acids can enhance the cellular uptake and membrane permeability of novel CPPs. The size of the dAA ring has a significant impact on cellular uptake, while the number of carbon atoms in the side chain ring has minimal effect.