MULTICYCLIC POLYPEPTIDE MODEL COMPOUNDS .2. SYNTHESIS AND CONFORMATIONAL PROPERTIES OF A HIGHLY ALPHA-HELICAL UNCOSAPEPTIDE CONSTRAINED BY 3 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-[LysLeuLysGluLeuLysGlu]3-OH (1-1-1) (Osapay and Taylor, J. Am. Chem. Soc. 1990,112,6046-6051), two new amphiphilic alpha-helical peptides, cyclo(3-7,10-14,17-21)-H-[LysLeuLysGluLeuLysAsp]3-OH (2-2-2) and its linear homologue H-[LysLeuLys(Ac)GluLeuLysLeuGln]3-OH (3-3-3), have been synthesized in order to assess the relative helix stabilizing properties of multiple lactam bridges linking Lys(i),Glu(i+4) and Lys(i),Asp(i+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-degrees-C ([theta]222 = -23 800 deg.cm2 dmol-1). This bridged helical structure was resistant to thermal and chemical denaturation, being incompletely unfolded at 90-degrees-C and, based on [theta]222, only 50% unfolded in 7.30 M guanidinium hydrochloride at 25-degrees-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 a-helical conformation ([theta]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 Lys(i),Asp(1+4) residue pairs are strongly helix stabilizing, and those linking Lys(i),Glu(i+4) residue pairs are weakly helix stabilizing.