Helical Template Peptides Containing Cyclic α,α-Disubstituted Amino Acids: Ring-Size Effects on Conformation and...

Area of Science:

• Medicinal Chemistry
• Structural Biology
• Biotechnology

Background:

• Cyclic α,α-disubstituted amino acids (dAAs) are crucial for stabilizing peptide secondary structures, particularly helices.
• The influence of the cyclic dAAs' ring size on peptide helical conformation and stability is not fully understood.
• Understanding these effects is vital for designing novel peptide-based therapeutics and delivery systems.

Purpose of the Study:

• To investigate the impact of different cyclic dAAs (Ac3c, Ac4c, Ac5c) on peptide helix stabilization.
• To elucidate the structural basis of ring-size-dependent helix stabilization in peptides.
• To evaluate the functional consequences of incorporating these cyclic dAAs into cell-penetrating peptide conjugates for siRNA delivery.

Main Methods:

• Design and synthesis of template peptides containing 1-aminocyclopropane-1-carboxylic acid (Ac3c), 1-aminocyclobutane-1-carboxylic acid (Ac4c), and 1-aminocyclopentane-1-carboxylic acid (Ac5c).
• Circular dichroism (CD) and infrared (IR) spectroscopy to analyze helical structures in solution.
• X-ray crystallography to determine the solid-state structures of Ac3c- and Ac4c-containing peptides.
• Incorporation of modified peptides into cell-penetrating peptide conjugates (Block3 derivatives) for functional assays.

Main Results:

• Oligomers with Ac3c, Ac4c, and Ac5c residues demonstrated stable helical structures in solution via CD and IR spectroscopy.
• X-ray crystallography revealed well-defined right-handed helices stabilized by specific hydrogen bonds (i → i+3 and i → i+4) in Ac3c- and Ac4c-containing peptides.
• Ring strain in Ac3c reduced its helicogenic effect compared to Ac4c and Ac5c, influencing backbone torsion angles and hydrogen bonding.
• Ac4c- and Ac5c-modified Block3 derivatives maintained helicity and facilitated siRNA internalization, whereas the Ac3c analogue lost helical structure and activity.

Conclusions:

• Cyclic dAAs are effective modules for inducing and stabilizing peptide helices, with ring size being a critical determinant.
• The study provides novel structural insights into how ring size affects peptide backbone conformation and helical stability.
• These findings offer design principles for creating peptide foldamers and optimizing peptide-based drug delivery systems, particularly highlighting the functional trade-offs associated with ring strain.