Preferred Conformations of Peptides Containing tert-Leucine, a Sterically Demanding, Lipophilic alpha-Amino Acid with a Quaternary Side-Chain C-beta-Atom.

Terminally protected homopeptides of tert-leucine, from the dimer to the hexamer, co-oligopeptides of tert-leucine in combination with a-aminoisobutyric acid or glycine residues up to the hexamer level, and simple dipeptides representing known scaffolds for catalysts in asymmetric organic reactions were prepared by solution methods and fully characterized. The results of conformation analysis, performed by use of FF-IR absorption, NMR, CD, and X-ray diffraction techniques, indicate that this hydrophobic alpha-amino acid with tetrasubstitution at the C-beta atom is structurally versatile. We show that it prefers extended or semi-extended conformations, but can also be accommodated in folded structures, provided that these are biased by the presence of helicogenic residues. The current large-scale production of Tle, combined with its conformational preferences unravelled in this work, should make this bulky, hydrophobic, C-alpha-trisubstituted alpha-amino acid a regular building block of any strategy seeking to tailor peptides with improved catalytic and pharmacological properties.