Effects of charge and hydrophobicity on the oligomerization of peptides derived from IAPP.

Changes in pH resulting in modifications of charge can dramatically alter the folding and interaction of proteins. This article probes the effects of charge and hydrophobicity on the oligomerization of macrocyclic β-sheet peptides derived from residues 11-17 of IAPP (RLANFLV). Previous studies have shown that a macrocyclic β-sheet peptide containing this IAPP sequence (peptide 1Arg) does not form oligomers in aqueous solution at low millimolar concentrations. Replacing arginine with the uncharged isostere citrulline generates a homologue (peptide 1Cit) that forms a tetramer consisting of a sandwich of hydrogen-bonded dimers. The current study probes the role of charge and hydrophobicity by changing residue 11 to glutamic acid (peptide 1Glu) and leucine (peptide 1Leu). Diffusion-ordered spectroscopy (DOSY) studies show that peptides 1Glu and 1Leu form tetramers in solution. NOESY studies confirm that both peptides form the same sandwich-like tetramer as peptide 1Cit. 1H NMR spectroscopy at various concentrations reveals that peptide 1Leu has the highest propensity to form tetramers. The effects of pH and charge on oligomerization are further probed by incorporating histidine at position 11 (peptide 1His). DOSY studies show that peptide 1His forms a tetramer at high pH. At low pH, peptide 1His forms a new species that has not been previously observed by our research group-a dimer. These studies demonstrate the importance of charge and hydrophobicity in the oligomerization of IAPP-derived peptides.