Synthetic oligomers of b-amino acids, that is, b-peptides, [1] are amongst the most studied class of molecules in foldamer chemistry. Changes in their substitution pattern generate a variety of interesting structural features in these molecules, [2][3][4][5] which have thus begun to prove immensely useful in bioactive peptide mimicry. [6] Amongst the various peptide secondary structures, mixed helices are unique to b-peptides. These structures contain intertwined 12 and 10-membered Hbonded rings and are unprecedented in a-proteins. Seebach et al. [4a] were the first to demonstrate the existence of righthanded 12/10 mixed helical structures in b-peptides with alternating b 2 and b 3 residues. Kessler and co-workers [4b] have reported such structures in mixed peptides containing constrained f-sugar amino acid and b-hGly repeats. We recently reported [7] the formation of both 10/12 and 12/10 right-handed helices by b-peptides derived from C-linked carbo-b 3 -amino acids (Caa) with "alternating chirality." Although a variety of helical structures have been discovered in b-peptides, no reports have been published in this active area of research that describe left-handed mixed helices. Several natural proteins [8] have been found to contain small fragments of left-handed helices, most of which occur in or near an active or binding site and are thus likely to be of functional importance. Herein, we report the design, synthesis, and structural study of "mixed b-peptides" with right-handed and novel left-handed mixed (10/12 and 12/10) helical structures. These peptides are derived from alternating epimeric Clinked carbo-b 3 -amino acids [9] (1 or 2; Scheme 1) and b-hGly units.
Our previous study revealed that Caa 1 occupies an energetically unfavorable position in the carbo-b-peptides 3 and 4, [7] although robust mixed helical structures were formed by these peptides. We envisaged a new design intended to provide more conformational freedom and relieve steric strain [10,11] through the inclusion of alternating Caa (1 or 2) and b-hGly residues. Mixed b-peptides 5-12 (5/6 and 7/8 with Caa 2 and 1 at the N terminus, respectively; 9/10 and 11/12 with b-hGly at the N terminus) were prepared by conventional procedures (1-ethyl-3(3-dimethylaminopropyl)carbodiimide, 1-hydroxy-1H-benzotriazole) and differences in the conformational behavior of the various peptides were studied. We anticipated that Caas 1 and 2 would define the conformational behavior of b-hGly in 5-12 (Scheme 1).
Structural studies on these peptides were carried out by using NMR and CD spectroscopic techniques. The 1 H NMR spectra of 5 and 6 in CDCl 3 show well-resolved backbone proton signals, as well as low-field NH resonances (NH(2)-NH(3) for 5; NH(2)-NH(5) for 6), which indicate the involvement of these NH groups in H bonding. This bonding was further confirmed by solvent titration studies. [12] The coupling constants 3 J CaH/CbH of the peptides (> 10 Hz or < 5 Hz) clearly demonstrate the predominance of a single conformation about the CaÀCb bond (single q value), except in the N-terminal residue. The 3 J CaH/CbH values, along with various CaH/CbH and strong sequential NOE correlations (between the NH group of b-hGly or Caa 2 and the preceding CaH (pro-R) or CaH (pro-S) atom, respectively), confirm a q value of around 608 for these peptides. The ROESY spectrum of 5 revealed the presence of long-range NOE correlations (CbH(1)/NH(3), CbH(1)/CaH (pro-R) (3)) unique to the signature of a 12-membered ring connected by an H bond involving tert-butoxycarbonyl (Boc)-CO and NH(3). A weak NH(2)/NH(3) NOE was observed, which supports the existence of a 10-membered ring with an H bond between NH(2) and CO(3), and confirms that 5 has a right-handed 12/ 10 helical structure. Similarly, the strong long-range backbone NOE interactions CbH(1)/NH(3), CbH(1)/CaH (pro-R) (3), CbH(3)/NH(5), and CbH(3)/CaH (pro-R) (5), and weak NH(2)/ NH(3) and NH(4)/NH(5) NOE correlations observed in the ROSEY spectrum of 6 (Figure 1 A) provide ample evidence of a propagated right-handed 12/10/12/10 helical structure for this peptide. The coupling constants of both 5 and 6 ( 3 J NH/ CbH > 8.9 Hz for Caa 2, 3 J NH/CbH > 8.9 Hz and < 3.3 Hz for b-hGly except in the C-terminal residue) correspond to j f j % 1008, which is consistent with a mixed helical structure. The first Caa residue has a 3 J CbH/C4H value of 6.8 Hz, which indicates averaging of the conformation about CbHÀCbÀ C4 À C4H (c 1 ). The other Caa residues have coupling constants 3 J CbH/C4H > 9.5 Hz; this value implies the preponderance of a structure with c 1 % 1808. The presence of several conforma-[*] Dr.