Three-dimensional organization of helices: design principles for nucleobase-functionalized beta-peptides.

The construction and molecular recognition of various three-dimensional biomimetic structures is based on the predictable de novo design of artificial molecules. In this regard beta-peptides are especially interesting, since stable secondary structures are obtained already with short sequences; one of them is the 14-helix in which every third residue has the same orientation. The covalent functionalization of every third 14-helix side chain with nucleobases was used for a reversible organization of two helices based on nucleobase pairing. A series of beta-peptides with various nucleobase sequences was synthesized and the stability of double strand formation was investigated. As few as four nucleobases are sufficient for considerable duplex stability. The stability of base pairing was examined by temperature-dependent UV spectroscopy and the formation of the 14-helix was confirmed by circular dichroism (CD) spectroscopy. The preferred strand orientation of complementary-nucleobase-modified beta-peptide helices was investigated as well as the influence of helix content on the duplex stability. The preorganization of a 14-helix in regard to double-strand recognition was tuned by the sequential order of polar beta-amino acids or by the amount of 2-aminocyclohexanecarboxylic acid units incorporated, which are known to facilitate 14-helix formation, respectively.