Reversible Folding of a β-Hairpin Peptide by a Metal-Chelating Amino Acid.

5-(1-Hydroxy-pyridin-2(1H)-onyl)-l-alanine (Hop) is a N-hydroxy-1,2-pyridone functionalized α-amino acid with the desired metal-chelating properties of DOPA (3,4-dihydroxy phenylalanine) but without its unwanted redox activity. The Fmoc-protected amino acid Fmoc-l-Hop(tBu)-OH (11) was synthesized from glycine phosphonate followed by enzymatic hydrolysis of the methyl ester yielding the Hop l-isomer in 96 % ee. The amino acid 11 is used in automated peptide synthesis for the assembly of a 14mer β-hairpin peptide with the sequence [dsb1, 14 ]H-CHXETGKHGHKLVC-OH (X=W, l-Hop). While the 10 π electron containing indole side chain of l-Trp in peptide 14 completes the formation of a hydrophobic cluster and results in 90 % folding, the folded fraction is significantly decreased to approximately 30 % for the 6 π electron l-Hop side chain in peptide 16. Metal chelation of Ga3+ reconstitutes the folding of 16 to above 60 % due to the formation of the Ga(16)3 trimer. The chelation process of 16 is monitored by NMR spectroscopy and the subsequent release of Ga3+ by a competitive metal chelator exemplifies the reversible oligomerization of peptide epitopes by metal chelation, bearing the opportunity to synthesize protein-sized aggregates on the basis of reversible chemistry in water.