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

Among the forces responsible for shaping proteins, interactions between side chains of aromatic residues play an important role as 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-NH(2) and Ac-Phe-D-Phe-NH(2). 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 are found to adopt a beta-turn type I conformation stabilized by an edge-to-face interaction between the two aromatic rings. Comparison with other dipeptides in the literature demonstrates the impact of this aromatic-aromatic interaction on 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 the similar conformer where such an interaction does not exist. This structural feature due 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 forces. 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.