Antiparallel β‐Sheet Conformation in Cyclopeptides Containing a Pseudo‐amino Acid with a biphenyl moiety

The biphenyl‐containing pseudo‐amino acids 2′‐(aminomethyl)biphenyl‐2‐carboxylic acid (Abc; 1) and 2′‐(aminomethyl)biphenyl‐2‐acetic acid (Aba; 2) are used as rigid spacers in the backbone of the cyclic peptides cyclo (‐Abc‐Ala‐Phe‐Gly‐)2 (5), cyclo(‐Abc‐Ala‐Val‐Gly‐)2 (6), cyclo(‐Aba‐Gly‐Phe‐Ala‐)2 (7), and cyclo(‐Aba‐Ala‐Phe‐Gly‐)2(8). Three different interconverting diastereoisomers are found in solutions of each of these cyclopeptides due to the atropisomerism of the biphenyl units. NMR Techniques and molecular‐dynamics calculations allow to conclude that the major diastereoisomer of 5 (and 6) in (D6)DMSO adopts a β‐sheet conformation. It is proposed that the pseudo‐amino acid 1 of (R)‐chirality forms, with attached L‐amino acids, a H‐bonding pattern comparable to a β‐turn (see D in Fig. 4 and F).