Template Assembled Synthetic Proteins (TASPs). Are Template Size, Shape, and Directionality Important in Formation of Four-Helix Bundles?

Abstract

A series of rigid aromatic templates that vary in size, shape, and directionality have been investigated in template-assembled synthetic 4α-helix peptide bundles for their capacity to enhance the α-helicity of an amphiphilic peptide (DAATALANALKKL-[NHCH2CH2SH]). In aqueous phosphate buffer (10 mM, pH 7) the peptide has some innate helicity (∼30%) which is concentration-independent between 1 and 250 μM. Helicity is enhanced to 64−75% when 4 equiv of the peptide are connected to aromatic templates based on benzene, benzanilide, or a cyclic octapeptide. This effect is concentration-independent by circular dichroism spectroscopy (3−60 μM [TASP]), the TASPs are monomeric by sedimentation equilibrium experiments, and have comparable thermodynamic stabilities. Thus these templates induce intra- rather than inter- molecular peptide association and are equally effective despite variations in size, shape, and directionality. When the linker between the template and peptides is sufficently long, as in these cases, TASP formation is less sensitive to the dimensions of the template than to the communication between hydrophobic peptide side chains, which are the main determinants of helix separation, 4α-helix bundle size and stability. This greatly simplifies approaches to developing small molecule mimetics of interacting protein surfaces. However template size, shape, and directionality may still be important when the linker is short or when assembled peptide surfaces are isolated from one another and unable to communicate.