Laser Photodissocation, Action Spectroscopy and Mass Spectrometry Unite to Detect and Separate Isomers

The separation and detection of isomers remains a challenge for many areas of mass spectrometry. This article highlights laser photodissociation and ion mobility strategies that have been deployed to tackle...

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 HAnW, 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 HAnW, 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.

Electron-Proton Transfer Mechanism of Excited-State Hydrogen Transfer in Phenol-(NH3 )n (n=3 and 5)

Excited-state hydrogen transfer (ESHT) is responsible for various photochemical processes of aromatics, including photoprotection of nuclear basis. Its mechanism is explained by internal conversion from the aromatic ππ* to πσ* states via conical intersection. This means that the electron is transferred to a diffuse Rydberg-like σ* orbital apart from proton migration. This picture means the electron and the proton do not move together and the dynamics are different in principle. Here, we have applied picosecond time-resolved near-infrared (NIR) and infrared (IR) spectroscopy to the phenol-(NH₃ )₅ cluster, the benchmark system of ESHT, and monitored the electron transfer and proton motion independently. The electron transfer monitored by the NIR transition rises within 3 ps, while the overall H transfer detected by the IR absorption of NH vibration appears with a lifetime of about 20 ps. This clearly proves that the electron motion and proton migration are decoupled. Such a difference of the time-evolutions between the NIR absorption and the IR transition has not been detected in a cluster with three ammonia molecules. We will report our full observation together with theoretical calculations of the potential energy surfaces of the ππ* and πσ* states, and will discuss the ESHT mechanism and its cluster size-dependence between n=3 and 5. It is suggested that the presence and absence of a barrier in the proton transfer coordinate cause the different dynamics.

Vibronic spectra of protonated hydroxypyridines: contributions of prefulvenic and planar structures

The UV photofragmentation spectra of cold protonated hydroxypyridines display well resolved vibrational structures, interpreted with calculations at the CC2 level.