Cell-penetrating peptides (CPPs) based on an oligoproline scaffold - design, synthesis and biological evaluation and conformational analysis of functionalized proline derivatives

Abstract: The effective delivery of drugs and other biomolecules into cells is challenging, since poor translocation across the plasma membrane is a major limitation. Cell-penetrating peptides (CPPs) are a promising tool to address this issue. Despite the progress that has been made in recent years, there is still a need for the development of new CPPs with superior physicochemical properties, enhanced uptake efficiency, intracellular targeting of specific organelles and proteolytic stability. Herein we present a novel type of oligoproline-based CPPs that shows a number of advantages over existing systems. The approach exploits the conformationally well-defined and functionalizable polyproline type II (PPII) helix of Azp-containing oligoprolines as a scaffold. Peptides bearing either (4R)- or (4S)-configured amino or guanidino proline residues with different geometrical designs along the helical axis as well as different chain lengths were prepared and their cellular uptake was evaluated. All of these oligoproline-based CPPs penetrate into different human cancer cells under physiological conditions. Members of these novel CPPs have higher levels of internalization than the established reference peptides Tat(48-60), octaarginine, and penetratin. Confocal microscopy studies demonstrated that the oligoproline-based CPPs localize in the nucleus of Hela cells. The proteolytic stability of the CPPs proved to be high and it was shown that all L- and D-oligoproline-based CPPs are stable towards trypsin and in human blood serum for at least 48 h. In addition, the peptides have only a low cytotoxicity towards Hela cells and no haemolytic activity against human erythrocytes. A preliminary siRNA delivery experiment showed a promising delivery potential into Hela cells, compared to the reference peptides Tat(48-60) and octaarginine. These results demonstrate the potential of the novel oligoproline-based CPPs for future biomedical applications.
A second goal of this work was to investigate structure directing properties of electron-withdrawing groups (EWGs) derived from an azide at the C(4) position of proline. In the course of the studies C(4)-endo puckered amino proline derivatives favoring a trans conformation around the tertiary prolyl amide bond were developed. Such conformers with an endo ring pucker favoring a trans amide bond had not been established before. In these derivatives a hydrogen bond between the substituent at C(4) and the carbonyl group of the amide backbone stabilizes the endo ring pucker and favors the trans amide conformer. Furthermore we demonstrated that these derivatives are valuable tools to tune the cis/trans amide bond ratio within peptides.