Synthesis of and conformational studies on proline-containing peptides : ion-binding by linear tetrapeptides

Abstract

The relationship between peptide backbone topology and ion-binding was investigated through proton and carbon-13 NMR, CD and IR spectroscopy. A literature review on calcium-binding proteins and peptides indicated that a structure characterized by two overlapping urns may be involved. To test this hypothesis two peptides which could potentially form the above structure were synthesized. -- The peptides NαtBocProDAlaAlaNHCH₃ and its glycine analogue NαtBocProGlyAlaNHCH₃ were characterized in the uncomplexed state by all of the above mentioned techniques in a variety of solvents. Proton NMR temperature-dependence studies in DMSO-d₆ and NH coupling constants, along with CD and IR spectroscopy indicated that both peptides in solution were made up of a Type II β-turn followed by an overlapping Type I β-turn. Carbon-13 NMR, IR and CD spectroscopy indicated that the DAla-peptide was, as expected, more stable than the Gly-peptide. -- The binding of metal ions to the peptides was monitored using CD spectroscopy. Both peptides were found to be ion-specific. While both were capable of binding calcium ion and incapable of binding sodium and lithium to any significant extent, they differed in their ability to bind magnesium. While the NαtBocProDAlaAlaNHCH₃ peptide bound magnesium weakly at a level comparable to its binding to the monovalent ions, the glycine analogue bound magnesium to a significant extent. The Gly-peptide also required a lower concentration of calcium ion to reach binding saturation. This was attributed to the greater flexibilty of glycine and hence, its enhanced ability to "fit" the metal ion. -- The conformational change of the peptides upon calcium ion titration was followed by proton and carbon-13 NMR. The changes in chemical shifts of the carbonyl resonances and the amide proton resonances indicated that all four peptide carbonyls coordinated to the calcium ion resulting in a breaking of the intramolecular hydrogen bonds of the uncomplexed species. Analyses of the CD binding-curves indicated that at low concentrations of calcium to peptide a 2:1 peptide:ion complex was formed, while at higher concentrations a 1:1 complex was predominant. The 2:1, peptide:ion complex is able to fill all eight calcium coordination sites with the peptide carbonyls while the 1:1 complex requires either the perchlorate anion or water molecule to fill the remaining coordination sites.