β‐Loop, γ‐loop, and helical peptide conformations in cyclopeptides containing a steroidal pseudo‐amino acid

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Abstract
The steroidal pseudo‐amino acids 3ã‐amino‐5β‐cholan‐24‐oic acid (2a), 12ã‐acetoxy‐3ã‐ammonia‐5β‐cholan‐24‐oic acid (2b), and 7ã,12ã‐diacetoxy‐3ã‐amino‐5β‐cholan‐24‐oic acid (2c) are used as rigid spacers in the backbone of the cyclic peptides cyclo(–2a–Phe‐Phe–)2 (1a), cyclo(–2b–Phe‐Phe–)2 (1b), and cyclo(–2c‐Phe‐Phe–)2 (1c). A homogeneous β‐loop conformation is found in the peptide chains of 1a and 1b, while 1c exists as a mixture of ã‐helical and γ‐loop conformations. The structure and homogeneity of the conformations are established by several NMR techniques and are supported by molecular‐dynamics calculations. The peptide conformations depend on the distance and attraction of the two large and lipophilic steroidal parts of the cyclic molecules.
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