Poster Presentation 10th Australian Peptide Conference 2013

  The shortest known α-helix in water is a pentapeptide stabilized by cyclisation.¹ Here we closely compare the alpha helicity of several cyclic pentapeptides and then examine one as an alpha helix nucleator appended to the N- versus C-terminus of short unstructured peptides. The structures of cyclic pentapeptides Ac-(cyclo-1,5)-[KAAAD]-NH₂ (1), Ac-(cyclo-1,5)-[DAAAK]-NH₂ (2) and Ac-(cyclo-1,5)-[EAAAK]-NH₂ (3) were compared by CD and 2D proton NMR spectroscopy, with (1) being the most alpha helical in water.  However, detailed NMR studies revealed a small defect from idealised alpha helical geometry near the C-terminal Asp residue in (1), due to torsional strain imposed by the K_(i)¦D_(i+4) lactam bridge. The more water-soluble Ac-(cyclo-1,5)-[KARAD]-NH₂ (4), being identical to (1) in helicity and structure, was appended to the N- or C- or N- and C- termini of an unstructured palindromic peptide ARAARAARA (≤ 5% helicity), resulting in 65%, 90% and 100% alpha helicity respectively in water (10 mM phosphate buffer, pH 7, 25°C). Detailed analysis by 1D and 2D ¹H NMR spectra, including multiple solution structure determinations, Ramachandran plots and molecular dynamics simulations highlighted torsional effects imposed by the K_(i)¦D_(i+4) lactam bridge and explained why Ac-(cyclo-1,5)-[KARAD]-NH₂ is more effective as an alpha helical nucleator when appended to the C-terminus of a peptide sequence. Finally, the capacity of (4) to nucleate alpha helicity was investigated through attachment to extended 10-25 residue palindromic peptide sequences and it was found that 13 amino acids, or 4 helical turns, were maintained α-helical in water before the helix structure dissipated.

Figure 1. A model cyclic pentapeptide as N- and C-termini helical inducer caps. The dotted lines are represented H-bonds and atoms colored blue and red are possible H-bond donors and H-bond acceptors respectively.