Groups on the Intramolecular Interaction Behavior of Dipeptide and Tripeptide Derivatives
The intramolecular interaction of protected dipeptides and tripeptides containing the amino acid units Ala, Phe, and Val was studied by means of ir spectroscopy. The NH and CO regions of the compounds dissolved in carbon tetrachloride clearly show the existence ofdifferent intramolecular hydrogen bonds. Using solvents with higher polarity such as chloroform and methylene chloride, the association bands disappear. Investigating the substances with the same amino acid sequence but opposite chirality of the central C atom in the peptide chain, we observed diflerent band shapes in the CO and NH regions. Large effects were found when the chirality of the Phe unit in the second position was changed. This is probably due to the steric hindrance originated by the rotation of the aromatic ring in the side chain.
The protecting groups, Z (benzyloxycarbonyl) or Boc (tert-butyloxycarbonyl) residues at the N-terminal group and methyl-or tert-butyl esters at the C-terminal group, influence the solubility ofthe substances in nonpolar solvent, as well as the NH and CO association band profiles in the methylene chloride solutions.
The consequences of changing the sequence of the amino acids are discussed for the tripeptide derivatives.
Besides a qualitative discussion, some quantitative considerations concerning the intramolecular interaction are also given to illustrate the diferent stabilities of the associates. 0 I996