Solid‐Phase Synthesis of a β‐Dodecapeptide with Seven Functionalized Side Chains and CD‐Spectroscopic Evidence for a Dramatic Structural Switch When Going from Water to Methanol Solution

An all‐β³‐dodecapeptide with a protected N‐terminal thiol‐anchoring group and with seven side chains has been synthesized in multi‐mg amounts by the manual solid‐phase technique, applying Fmoc methodology and the Wang resin. The sequence is β‐HLys‐β‐HPhe‐β‐HTyr‐β‐HLeu‐β‐HLys‐β‐HSer‐β‐HLys‐β‐HPhe‐β‐HSer‐β‐HVal‐β‐HLys‐β‐HAla‐OH (from N‐ to C‐terminus; see 1 ). The functional groups in the side chains of the building blocks were Boc (β‐HLys) or t‐Bu ether (β‐HSer, β‐HTyr) protected to allow for simultaneous deprotection and detachment from the resin with trifluoroacetic acid. All coupling steps were achieved with HBTU (=O‐(1H‐benzotriazol‐1‐yl)‐1,1,3,3‐tetramethyl uronium hexafluorophosphate)/HOBt (=1‐hydroxy‐1H‐benzotriazole) in DMF. For Fmoc (=(9H‐fluoren‐9‐yl)methoxycarbonyl) deprotection, a protocol was developed to surmount the previously reported problems arising in solid‐phase synthesis of β‐peptides when the chain length exceeds seven or eight amino‐acid moieties: for up to seven amino acids, a 20% solution of piperidine in DMF was used for removal of Fmoc; for the subsequent five amino acids, DBU and piperidine were employed for complete deprotection. The crude product was purified by preparative reversed‐phase HPLC, and the yield of pure β‐dodecapeptide derivative ( 1 ) was 23%. As the compound is well‐soluble in H₂O, it was characterized by ¹H‐NMR (in MeOH and H₂O), ¹³C‐NMR (in MeOH), and CD spectroscopy (in MeOH and in H₂O at pH values ranging from 3.5 to 11), and its molecular weight and composition were confirmed by high‐resolution mass spectrometry (Figs. 1 – 4). In MeOH solution, the β‐dodecapeptide exhibits the expected CD pattern typical of an (M)‐3₁₄‐helical secondary structure. In H₂O, however, the characteristic trough near 215 nm is missing in the CD spectrum, only a strong positive Cotton effect at 202 nm was observed, indicating the presence of β‐peptidic secondary structures, containing ten‐membered H‐bonded rings, such as the 12/10 helix (Fig. 4, right) or the hairpin. Only a detailed NMR solution‐structure analysis will provide the clues necessary for understanding the effects leading to the observed dramatic structural change of the highly functionalized β‐dodecapeptide described.