Secondary structure effects on internal proton transfer in poly-peptides

A pump-probe approach was designed to determine the internal proton transfer (PT) rate in a series of poly-peptide radical cations containing both histidine and tryptophan. The proton transfer is driven by the gas-phase basicity difference between residues. The fragmentation scheme indicates that the gas-phase basicity of histidine is lower than that of radical tryptophan so that histidine is always pulling the proton away from tryptophan. However, the proton transfer requires the two basic sites to be in close proximity, which is rate limited by the peptide conformational dynamics. PT rate measurements were used to probe and explore the peptide conformational dynamics in several poly-glycines/prolines/alanines. For small and unstructured peptides, the PT rate decreases with the size, as expected from a statistical point of view in a flat conformational space. Conversely, if structured conformations are accessible, the structural flexibility of the peptide is decreased. This slows down the occurrence of conformations favorable to proton transfer. A dramatic decrease in the PT rates was observed for peptides HA_nW, when n changes from 5 to 6. This is attributed to the onset of a stable helix for n = 6. No such discontinuity is observed for poly-glycines or poly-prolines. In HA_nW, the gas-phase basicity and helix propensity compete for the position of the charge. Interestingly, in this competition between PT and helix formation in HA₆W, the energy gain associated with helix formation is large enough to slow down the PT beyond experimental time but does not ultimately prevail over the proton preference for histidine.