Internal proton transfer leading to stable zwitterionic structures in a neutral isolated peptide.

Decades of gas-phase spectroscopy of small biomolecules have enabled some of the intrinsic physical and chemical properties of the building blocks of life to be unraveled. Bridging the gap towards an understanding of the biological function of these biomolecules has become an essential issue. For the processes that take place in the active sites of functional proteins at the molecular level to be understood, two critical aspects must be taken into account: 1) interactions with the biological environment (protons, electrons, metal ions, water molecules) and 2) the specific organization of a few significant amino acid residues nested in the welldefined local environment shaped by the entire protein. In this context, it is important to pursue a bottom-up approach, whereby elements of the environment can be introduced stepby-step in a controlled fashion until the biological function emerges. A crucial discrepancy between the gas-phase structure of isolated amino acids and peptides and their biologically relevant counterparts is the transition from the canonical to the zwitterionic form. Whereas neutral, isolated amino acids and peptides have always been found in their canonical form, studies on ionic complexes have shown that zwitterionic forms may be stabilized by the addition of a proton, an electron, a metal cation, or a metal dication or by microsolvation. In the case of overall-neutral complexes, the canonical-to-zwitterionic transition was observed upon the stepwise addition of solvent molecules. We report herein the first observation of an “autozwitterion” formed by intramolecular proton transfer between nearby residues in a neutral, isolated peptide. We specifically designed the pentapeptide Ac-Glu-Ala-Phe-Ala-Arg-NHMe (EAFAR; Scheme 1) with an appropriate structure for