Characterization of neutral fragments issued from the photodissociation of protonated tryptophane

Characterization of Insulin Dimers by Top-Down Mass Spectrometry

High-molecular weight products (HMWP) are an important critical quality attribute in research and development of insulin biopharmaceuticals. We here demonstrate on two case studies of covalent insulin dimers, induced by Fe²⁺ incubation or ultraviolet (UV) light stress, that de novo characterization in top-down mass spectrometry (MS) workflows can identify cross-link types and sites. On the MS² level, electron-transfer/higher-energy collision dissociation (EThcD) efficiently cleaved the interchain disulfide bonds in the dimers to reveal cross-link connectivities between chains. The combined utilization of EThcD and 213 nm ultraviolet photodissociation (UVPD) facilitated identification of the chemical composition of the cross-links. Identification of cross-link sites between chains at residue level was achievable for both dimers with MS³ analysis of MS² fragments cleaved at the cross-link or additionally the interchain disulfide bonds. UVPD provided identification of cross-link sites in the Fe²⁺-induced dimer without MS³, while cross-link site identification with MS² was not possible for the UV light-induced dimer. Thus, using varied multistage approaches, it was discovered that in the UV light-induced dimer, Tyr14 of the A-chain participated in an -O-S- cross-link in which the sulfur was derived either from Cys7 or Cys19 of the B-chain. In the Fe²⁺-induced dimer, Phe1 from both B-chains were cross-linked through a -CH₂-. The UV chromophoric side chain of Phe1 was indicated in the cross-link, explaining why UVPD-MS² was effective in fragmenting the cross-link and nearby backbone bonds. Our results demonstrated that higher-energy collisional dissociation (HCD), EThcD, and UVPD combined with MS³ were powerful tools for direct de novo characterization of cross-linked insulin dimers.

Excited State Deactivation Mechanism in Protonated Uracil: New Insights from Theoretical Studies

We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the ¹ππ* state of enol-enol form (EE⁺) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S₁ lifetime of this species. For the second most stable isomer (EK⁺), however, a significant barrier predicted in the CASPT2 S₁ potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally.

Influence of the N atom and its position on electron photodetachment of deprotonated indole and azaindole

Dipole bound state and its vibrational structure observed in deprotonated 7-azaindole by recording the signal of 7-azaindolyl stable neutral radical.

Chiral Differentiation of Non-Covalent Diastereomers Based on Multichannel Dissociation Induced by 213-nm Ultraviolet Photodissociation

Here we present the implementation of 213-nm ultraviolet photodissociation (UVPD) in a FT-ICR mass spectrometer for chiral differentiation in the gas phase. The L/D amino acid-substituted serine octamer ions were selected as examples of diastereoisomers for chiral analysis. Several kinds of fragment ions were observed in these experiments, including fragment ions that are similar to the ones observed in corresponding collision-activated dissociation (CAD) experiments, fragment ions generated with different protonation sites by only destroying non-covalent bonds, and unique non-covalent cluster radical ions. The latter two kinds of fragment ions are found to be more sensitive to the chirality of the substituted units. Further experiments suggest that the formation of radical ions is mainly affected by chromophores on side chains of the substituted units and micro surroundings of the characterized non-covalent diastereoisomers. A comparing experiment performed by only changing the wavelength of UV laser to 266 nm shows that the 213-nm UV laser has the priority in the diversity of fragmentation pathways and potential of further application in chiral differentiation experiments.

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.

Classical Dynamics Simulations of Dissociation of Protonated Tryptophan in the Gas Phase

Gas phase decomposition of protonated amino acids are of great interest due to their role in understanding protein and peptide chemistry. Several experimental and theoretical studies have been reported in the literature on this subject. In the present work, decomposition of the aromatic amino acid protonated tryptophan was studied by on-the-fly classical chemical dynamics simulations using density functional theory. Mass spectrometry and electronic structure theory studies have shown multiple dissociation pathways for this biologically relevant molecule. Unlike aliphatic amino acids, protonated tryptophan dissociates via NH₃ elimination rather than the usual iminium ion formation by combined removal of H₂O and CO molecules. Also, a major fragmentation pathway in the present work involves C_α-C_β bond fission. Results of the chemical dynamics simulations reported here are in overall agreement with experiments, and detailed atomic level mechanisms are presented.

Photofragmentation mechanisms in protonated chiral cinchona alkaloids

Photo-fragmentation of protonated alkaloids results in C₈–C₉ cleavage accompanied or not by hydrogen migration, with a stereochemistry-dependent branching ratio.

A comprehensive study of cold protonated tyramine: UV photodissociation experiments and ab initio calculations

Excited state properties of cold protonated ions are revealed by a combination of laser spectroscopy and ab initio calculations.

Excited states of proton-bound DNA/RNA base homodimers: pyrimidines

We are presenting the electronic photofragment spectra of the protonated pyrimidine DNA base homodimers. Only the thymine dimer exhibits a well structured vibrational progression, while the protonated monomer shows broad vibrational bands. This shows that proton bonding can block some nonradiative processes present in the monomer.

Development of Ultraviolet-Ultraviolet Hole-Burning Spectroscopy for Cold Gas-Phase Ions

A new ultraviolet-ultraviolet hole-burning (UV-UV HB) spectroscopic scheme has been developed for cold gas-phase ions in a quadrupole ion trap (QIT) connected with a time-of-flight (TOF) mass spectrometer. In this method, a pump UV laser generates a population hole for the ions trapped in the cold QIT, and a second UV laser (probe) monitors the population hole for the ions extracted to the field-free region of the TOF mass spectrometer. Here, the neutral fragments generated by the UV dissociation of the ions with the second laser are detected. This UV-UV HB spectroscopy was applied to protonated dibenzylamine and to protonated uracil. Protonated uracil exhibits two strong electronic transitions; one has a band origin at 31760 cm(-1) and the other at 39000 cm(-1). From the UV-UV HB measurement and quantum chemical calculations, the lower-energy transition is assigned to the enol-keto tautomer and the higher-energy one to the enol-enol tautomer.