Lanthanide(III) Complexes with Two Hexapeptides Incorporating Unnatural Chelating Amino Acids: Secondary Structure and Stability
✍ Scribed by Federico Cisnetti; Christelle Gateau; Colette Lebrun; Pascale Delangle
- Publisher
- John Wiley and Sons
- Year
- 2009
- Tongue
- English
- Weight
- 561 KB
- Volume
- 15
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
Unnatural metal‐chelating amino acids bearing aminodiacetate side‐chains have been introduced into two hexapeptides to obtain efficient lanthanide‐binding peptides. The synthesis of the enantiopure Fmoc‐Ada~n~(__t__Bu)~2~‐OH synthons is described with overall yields of 32 and 50 % for n=2 and n=3 side‐chain carbon atoms, respectively. The two peptides AcWAda~n~PGAda~n~GNH~2~ (P^n^) were synthesized from the protected synthons by standard solid‐phase peptide synthesis. Studies of the lanthanide complexes of the two peptides P^n^ by luminescence titrations, mass spectrometry, circular dichroism, and solution NMR spectroscopy demonstrate that the Ada~n~ chain length has a dramatic effect on the complexation properties. Indeed, the flexible compound P^3^ forms a mononuclear complex of moderate stability (β~11~=10^9.9^), which tends to transform into a binuclear species in the presence of excess of the metal ion. Interestingly, the more compact peptide P^2^ provides stable Ln^3+^ complexes with the exclusive formation of the mononuclear LnP^2^ adduct. The stability constant of TbP^2^ is two orders of magnitude higher (β~11~=10^12.1^) than that measured for P^3^. The 800 MHz NMR spectrum of the La^3+^ complex of P^2^ evidences a well‐defined type II β‐turn as well as a hydrophobic Trp(indole)–Pro interaction. These interactions exemplify the non‐innocent character of the peptide spacer in the complex LaP^2^ as well as the role of a peptide secondary structure in the stabilization of metal complexes.