Helix Macrodipole Control of β³-Peptide 14-Helix Stability in Water

Abstract

beta-Peptides have attracted considerable attention by virtue of their ability to populate helical secondary structures in methanol, even in the absence of stabilizing tertiary interactions. Recent efforts in beta-peptide design have produced few beta3-peptides that form stable 14-helices in water; those that do require stabilizing intramolecular salt bridges on two of three helical faces and therefore possess limited utility as tools in biological research. Here we show that favorable interactions with the 14-helix macrodipole significantly stabilize the 14-helix in water, alleviating the need for multiple salt bridges on two of three helical faces. We also report the previously unrecognized stabilization of 14-helix structure by gamma-branched beta3-amino acids. The most structured molecules we describe are highly heterogeneous at the primary sequence level, containing seven different beta3-amino acids within an 11-residue sequence. These results represent the essential first step toward the design of well-folded 14-helices that explore the interactions between beta3-peptides and biological macromolecules in vitro and in vivo.

Relationship between Side Chain Structure and 14-Helix Stability of β3-Peptides in Water

“…26 Because of cross-correlation of molecular weight with baseline values, curve fits were insensitive to variations in this value. In fact, equally good curve fits could be obtained by assuming either monomer or dimer molecular weights.…”

“…35,36 The previously studied β-peptide 1 contains salt-bridges oriented to minimize the 14-helix macrodipole ( Figure 3A). 26 Initially, circular dichroism (CD) spectroscopy was used to characterize its structure. While CD data on β-peptides must be interpreted carefully, 37 it is reasonable to assume that, for β 3 -peptides in particular, changes in intensity of the 14-helical signature correlate to relative changes in overall mean 14-helical population.…”

Biophysical and Structural Characterization of a Robust Octameric β-Peptide Bundle

“…Structural features that promote stable 3 14 -helices include alternating cationic and anionic side chains arranged on one helical face to support salt bridge formation [13][14][15] and arranged to minimize the helix macrodipole. 12,16,17 Indeed, β 3 -peptides that embody these features tolerate extensive substitution of the remaining two helix faces 12,17,18 to generate highly protease-resistant 19 molecules that inhibit interactions between proteins both in vitro 18,20,21 and in cell-based assays. 22 Importantly, the inherent conformational flexibility of β 3 -peptides, when compared to cyclic analogues such as ACHC, 23 may be essential to the general success of this approach, as all β 3 -peptides of known structure that effectively inhibit protein interactions possess geometries that deviate from the ideal 3 14 -helix imposed by ACHC.…”

Co-assembly of helical β3-peptides: a self-assembled analogue of a statistical copolymer

“…Of particular interest are β 3 -peptides that fold into helices consisting 14-membered loops through hydrogen bonding between C=O of residues i and the N-H of the i-3 residue [16,17]. Accordingly, these so-called 14-helices exhibit a pitch of 3.0-3.1 amino acids per turn that is a substantial advantage over the α-helix that exhibits 3.6 residues per turn [16,18,19]. In the 14-helix the side chains align along the helix, providing a geometrically determined structure that is an ideal scaffold for a self-assembling building block [18,20].…”