6‐Strand to Stable 10/12 Helix Conformational Switch by Incorporating Flexible β‐hGly in the Homooligomers of Camphor Derived β‐Amino Acid: NMR and X‐Ray Crystallographic Evidence

• Published in: Angewandte Chemie International Edition Vol. 63; no. 22; pp. e202403321 - n/a
• Main Authors: Sukumar, Genji, Rahul, Nayani, Kiranmai, Mainkar, Prathama S., Prashanth, Jupally, Sridhar, Balasubramanian, Sarma, Akella V. S., Bharatam, Jagadeesh, Chandrasekhar, Srivari
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 27-05-2024
• Edition: International ed. in English
• Subjects: 10/12-mixed helix; Amino acids; Amino Acids - chemistry; Camphor; Camphor - chemistry; Camphor-derived β-amino acid; Catalysis; Chirality; Crystallography; Crystallography, X-Ray; DFT; Magnetic Resonance Spectroscopy; Models, Molecular; NMR; NMR spectroscopy; Nuclear magnetic resonance; X-ray crystallography; Camphor-derived β-amino acid; X-ray crystallography; DFT; NMR spectroscopy; 10/12-mixed helix
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202403321

Rational design of unnatural amino acid building blocks capable of stabilizing predictable secondary structures similar to protein fragments is pivotal for foldamer chemistry/catalysis. Here, we introduce novel β‐amino acid building blocks: [1S,2R,4R]exoCDA and [1S,2S,4R]endoCDA, derived from the abundantly available R(+)‐camphor, which is traditionally known for its medicinal value. Further, we demonstrate that the homooligomers of exoCDA adopt 6‐strand conformation, which switches to a robust 10/12‐helix simply by inserting flexible β‐hGly spacer at alternate positions (1 : 1 β‐hGly/exoCDA heterooligomers), as evident by DFT‐calculations, solution‐state NMR spectroscopy and X‐ray crystallography. To the best of our knowledge, this is the first example of crystalline‐state structure of left‐handed 10/12‐mixed helix, that is free from the conventional approach of employing β‐amino acids of either alternate chirality or alternate β2/β3 substitutions, to access the 10/12‐helix. The results also show that the homooligomers of heterochiral exoCDA don′t adopt helical fold, instead exhibit banana‐shaped strands, whereas the homodimers of the other diastereomer endoCDA, nucleate 8‐membered turns. Furthermore, the homo‐exoCDA and hetero‐[β‐hGly‐exoCDA] oligomers are found to exhibit self‐association properties with distinct morphological features. Overall, the results offer new possibilties of constructing discrete stable secondary and tertiary structures based on CDAs, which can accommodate flexible residues with desired side‐chain substitutions.
Novel β‐amino acid building blocks (CDA) derived from naturally occuring camphor are introduced as new design tools to construct diverse secondary structures. Specifically, the conformational switching to a robust 10/12‐helix, by simply inserting flexible achiral β‐hGly spacers in a 6‐strand stabilizing CDA‐homooligomers, exemplifies the potential of the CDAs in backbone regulation and in developing functional foldamers.