Multicyclic Polypeptide Model Compounds. 2. Synthesis and Conformational Properties of a Highly α-Helical Uncosapeptide Constrained by Three Side-Chain to Side-Chain Lactam Bridges

Further to our earlier report of the synthesis and conformational properties of cyclo(3-7,10-14,17-21)-H-[Lys-LeuLysGluLeuLysGlu]₃-OH (1-1-1) (Ösapay and Taylor, J. Am. Chem. Soc. 1990, 112, 6046-6051), two new amphiphilic α-helical peptides, cyclo(3-7,10-14,17-21)-H-[LysLeuLysGluLeuLysAsp]₃-OH (2-2-2) and its linear homologue H-[Lys-LeuLys(Ac)GluLeuLysLeuGln]₃-OH (3-3-3), have been synthesized in order to assess the relative helix stabilizing properties of multiple lactam bridges linking Lysⁱ,Gluⁱ⁺⁴ and Lysⁱ, Aspⁱ⁺⁴ residue pairs. These peptides were assembled using a combination of solid-phase peptide synthesis on the Kaiser-oxime resin and solution-phase segment condensations. During the preparation of the protected 7-residue building unit (7) for 2-2-2, peptide cyclization with concomitant cleavage from the oxime resin yielded 54% product. Circular dichroism spectropolarimetry indicated that model peptide 2-2-2 was highly helical in aqueous buffer, pH 7.0, at 25 °C ([θ]₂₂₂ = -23 800 deg-cm² dmol^-1). This bridged helical structure was resistant to thermal and chemical denaturation, being incompletely unfolded at 90 °C and, based on [θ]₂₂₂. only 50% unfolded in 7.30 M guanidinium hydrochloride at 25 °C. In contrast, peptide 1-1-1 and the acyclic peptide 3-3-3 both displayed very little helical character in the aqueous buffer and were readily denaturated by guanidinium hydrochloride. However, in 50% trifluoroethanol or bound to hydrophobic coated quartz slides, the multicyclic peptides 1-1-1 and 2-2-2 gave almost identical CD spectra indicative of a highly α-helical conformation ([θ]₂₂₂ = - 31 000 deg-cm² dmol^-1), whereas the linear peptide 3-3-3 was significantly less helical. From these results, we conclude that multiple lactam bridges linking the side chains of Lysⁱ,Aspⁱ⁺⁴ residue pairs are strongly helix stabilizing, and those linking Lysⁱ,Gluⁱ⁺⁴ residue pairs are weakly helix stabilizing.