Electron-proton transfer mechanism of excited-state hydrogen transfer in phenol−(NH3) (n = 5) studied by delayed ionization detected femtosecond time-resolved NIR spectroscopy

Tracking local and global structural changes in a protein by cold ion spectroscopy

Characterization of native structures of proteins in the gas phase remains challenging due to the unpredictable conformational changes the molecules undergo during desolvation and ionization. We spectroscopically studied cryogenically cooled protonated protein ubiquitin and its microhydrated complexes prepared in the gas phase in a range of charge states under different ionization conditions. The UV spectra appear vibrationally resolved for the unfolded protein, but become redshifted and smooth for the native-like structures of ubiquitin. This spectroscopic change results from the H-bonding of the hydroxyl of Tyr to the amide group of Glu-51 in the compact structures; the minimum length of this bond was estimated to be ∼1.7 Å. IR spectroscopy reflects the global structural change by observing redshifts of free NH/OH-stretch vibrational transitions. Evaporative cooling of microhydrated complexes of ubiquitin keeps the protein chilly during ionization, enabling native-like conformers with up to eight protons to survive in the gas phase.

Revealing the role of excited state proton transfer (ESPT) in excited state hydrogen transfer (ESHT): systematic study in phenol-(NH3) n clusters

Excited State Hydrogen Transfer (ESHT), proposed at the end of the 20th century by the corresponding authors, has been observed in many neutral or protonated molecules and become a new paradigm to understand excited state dynamics/photochemistry of aromatic molecules. For example, a significant number of photoinduced proton-transfer reactions from X–H bonds have been re-defined as ESHT, including those of phenol, indole, tryptophan, aromatic amino acid cations and so on. Photo-protection mechanisms of biomolecules, such as isolated nucleic acids of DNA, are also discussed in terms of ESHT. Therefore, a systematic and up-to-date description of ESHT mechanism is important for researchers in chemistry, biology and related fields. In this review, we will present a general model of ESHT which unifies the excited state proton transfer (ESPT) and the ESHT mechanisms and reveals the hidden role of ESPT in controlling the reaction rate of ESHT. For this purpose, we give an overview of experimental and theoretical work on the excited state dynamics of phenol–(NH₃)_n clusters and related molecular systems. The dynamics has a significant dependence on the number of solvent molecules in the molecular cluster. Three-color picosecond time-resolved IR/near IR spectroscopy has revealed that ESHT becomes an electron transfer followed by a proton transfer in highly solvated clusters. The systematic change from ESHT to decoupled electron/proton transfer according to the number of solvent molecules is rationalized by a general model of ESHT including the role of ESPT.

A general model of excited state hydrogen transfer (ESHT) which unifies ESHT and the excited state proton transfer (ESPT) is presented from experimental and theoretical works on phenol–(NH₃)_n. The hidden role of ESPT is revealed.

Excited state hydrogen transfer dynamics in phenol-(NH3)2 studied by picosecond UV-near IR-UV time-resolved spectroscopy

Time-evolutions of excited state hydrogen transfer (ESHT) in phenol (PhOH)-(NH₃)₂ clusters have been measured by three-color picosecond (ps) ultraviolet (UV)-near infrared (NIR)-UV pump-probe ion dip spectroscopy. The formation of a reaction product, ˙NH₄NH₃, is detected by its NIR absorption due to a 3p-3s Rydberg transition. The ESHT reactions from all of the vibronic levels show biexponential time-evolutions, even from the S₁ origin. Based on the biexponential time-evolution, it is suggested that there is a second reaction path via the triplet πσ* state, which gives the slow component. The fast time-evolution of the ESHT reaction from the S₁ origin is measured to be 268 ps, which is 10-times slower than that in PhOH-(NH₃)₃, and a higher barrier between the ππ* and reactive πσ* states is suggested. The size dependence of the ESHT reaction rates is discussed based on a potential distortion due to the proton transferred state in the ππ* potential surface.

Identification of an Emitting Metastable State of p-Fluorophenol-Ammonia 1:2 Complex by Laser-Induced Fluorescence Spectroscopy

We have demonstrated here, for the first time to our knowledge, the formation of an emitting metastable species upon lowest electronic excitation (S₁) of a hydrogen-bonded 1:2 complex of para-fluorophenol (pFP) with ammonia (NH₃), which is known to be one of the smallest reactive complexes to undergo excited state H-atom transfer (HAT) reaction to produce ^•NH₄(NH₃) radical fragment. The emission spectrum of the species is characterized to be red-shifted, broad, and structureless. From the viewpoint of energy balance, an excited state proton transfer (ESPT) is unfavorable, but according to predicted electronic structure parameters, the metastable state species could be stabilized by charge transfer (CT) interaction at the hydrogen-bonded geometry of the complex. We propose that this species could act as an intermediate to the HAT process in the excited state. The observation of such a state could be valuable to understand the complex dynamics of similar events in biologically relevant systems.