Cationic Side Chain Identity Directs the Hydrophobically Driven Self-Assembly of Amphiphilic ß-Peptides in Aqueous Solution.

Hydrophobic interactions mediated by nonpolar molecular fragments in water are influenced by local chemical and physical contexts in ways that are not yet fully understood. Here, we use globally amphiphilic (GA) ß-peptides (GA-Lys and GA-Arg) with stable conformations to explore if replacement of ß3-homolysine (ßLys) with ß3-homoarginine (ßArg) influences the hydrophobically driven assembly of these peptides in bulk aqueous solution. The studies were conducted in 10 mM triethanolamine buffer at pH 7, where both ßLys (ammonium) and ßArg (guanidinium) side chains are substantially protonated. Comparisons of light scattering measurements and cryo-electron micrographs before and after the addition of 60 vol% MeOH indicate very different outcomes of the hydrophobically driven assembly of AcY-GA-Lys versus AcY-GA-Arg (AcY denotes an N-acetylated-ß3-homotyrosine (ßTyr) at each N-terminus). Nuclear magnetic resonance and analytical ultracentrifugation confirm that AcY-GA-Lys assembles into large aggregates in aqueous buffer, whereas AcY-GA-Arg at comparable concentrations forms only small oligomers. Titration of AcY-GA-Arg into aqueous solutions of AcY-GA-Lys reveals that the driving force for AcY-GA-Lys association is far stronger than that for AcY-GA-Arg association. We discuss these results in the light of past experimental observations involving single molecule force measurements with GA ß-peptides and hydrophobically driven dimerization of conventional peptides that form a GA α-helix upon dimerization (but do not display the Lys versus Arg trend predicted by extrapolating from the earlier AFM studies with ß-peptides). Overall, our results establish that the identity of proximal cationic groups, ammonium versus guanidinium, profoundly modulates the hydrophobically driven self-assembly of conformationally stable ß-peptides in bulk aqueous solution.

Retention of Coiled-Coil Dimer Formation in the Absence of Ion Pairing at Positions Flanking the Hydrophobic Core.

Hydrophobic interactions govern how proteins fold and interact with other molecules, but the impact of nearby polar functionality on the effective hydrophobicity of nonpolar surfaces remains unclear. Here we use a common protein quaternary structure motif, the parallel coiled-coil dimer, to ask whether the identity of basic residues (arginine vs lysine; guanidinium vs ammonium) arrayed along one side of the constituent α-helices influences the favorability of dimerization driven by burial of hydrophobic surface on the other side of each helix. Significant sequence redesign was necessary to achieve the desired juxtaposition of nonpolar and cationic functionality, because we needed to eliminate charged side chains from positions flanking the nonpolar helix surface. Natural and designed sequences that form coiled coils are almost universally rich in acidic and basic residues at these flanking positions. Our arginine coiled-coil dimer was moderately more stable than the lysine analogue, which contrasts with behavior previously observed with helical ß-amino acid oligomers bearing guanidinium versus ammonium groups. We attribute this backbone-dependent difference to variations in the extent to which the helical propensities of α- and ß-residues can be modulated by design. These findings highlight the challenge of identifying noncovalent forces that direct structure formed by a flexible backbone.

Lithium Enolates Derived from Pyroglutaminol: Aggregation, Solvation, and Atropisomerism.

Lithium enolates derived from protected pyroglutaminols were characterized by using (6)Li, (13)C, and (19)F NMR spectroscopies in conjunction with the method of continuous variations. Mixtures of tetrasolvated dimers and tetrasolvated tetramers in different proportions depend on the steric demands of the hemiaminal protecting group, tetrahydrofuran concentration, and the presence or absence of an α-fluoro moiety. The high steric demands of the substituted bicyclo[3.3.0] ring system promote dimers to an unusual extent and allow solvents and atropisomers in cubic tetramers to be observed in the slow-exchange limit. Pyridine used as a (6)Li chemical shift reagent proved useful in assigning solvation numbers.