Experimental and theoretical investigation of the aromatic-aromatic interaction in isolated capped dipeptides.

Among the forces responsible I'm shaping proteins, interactions between side chains of aromatic residues play all Important role I,, they are involved in the secondary and the tertiary structures of proteins contributing to the formation of hydrophobic domains. The purpose of this paper is to document this interaction in two capped dipeptides modeling a segment of a protein chain having two consecutive Phe residues, Ac-Phe-Phe-NH2 and Ac-Phe-D-Phe-NH2 These two molecules have been investigated in the gas phase by IR/UV double resonance Spectroscopy, and the assignment of the observed conformers has been done by comparison with quantum chemistry calculations. Both peptides ire found to adopt it beta-turn type I conformation stabilized by interaction between the two aromatic rings. Comparison with other dipeptides in the literature demonstrates the Impact of fill.,, aromatic-aromatic interaction oil the shape adopted by the peptide chain, and its role among the Other shaping forces (H-bonds, NH-pi interactions) is discussed As an illustration, the H-bond strength IS found to be Significantly lower in the beta-turn type I conformer, in which the two rings interact. as compared to (lie Similar conformer where such all interaction does not exist this structural feature clue to the backbone distortion induced by the interaction between the aromatic rings makes this system a good test for evaluating the ability Of computational methods to correctly account for the competition between these force. MP2, SCS-MP2. DFT, and DFT-D methods have been assessed in this respect. Comparison between geometries, energies, and frequency calculations illustrate their respective limitations in describing conformations resulting from a subtle equilibrium between the several interactions at play.