High-energy collision-activated and electron-transfer dissociation of gas-phase complexes of tryptophan with Na

The structure and reactivity of gas-phase complexes of tryptophan (Trp) with Na+, K+, and Ca2+ were examined by high-energy collision-activated dissociation (CAD) and electron transfer dissociation (ETD) using alkali metal targets. In the CAD spectra of M+Trp (M = Na and K), neutral Trp loss was the primary dissociation pathway, and the product ion of collision-induced intracomplex electron transfer from the indole π ring of Trp to the alkali metal ion was observed, indicating a charge-solvated structure in which Trp is non-zwitterionic. The NH3 loss observed in the CAD spectrum of Ca2+Trp2 is ascribed to a CZ (mixed charge-solvated/zwitterionic)-type structure, in which one Trp is non-zwitterionic and the other Trp adopts a zwitterionic structure with an NH\hbox{$_{3}^{+}$} moiety. The H atom and NH3 losses observed in the ETD spectrum of Ca2+Trp2 indicate the formation of a hypervalent radical in the complex, R–NH3, via electron transfer from the alkali metal target to the NH\hbox{$_{3}^{+}$} group of the CZ-type structure. Ca2+ attachment to Trp cluster induces the zwitterionic structure of Trp in the gas phase, and an electron transfer to the zwitterionic Trp forms the hypervalent radical as a reaction intermediate

INFRARED SPECTROSCOPY OF 7-AZAINDOLE TAUTOMERIC DIMER: OBSERVATION OF THE ND STRETCH

Author Institution: Department of Chemistry, Graduate School of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan7-azaindole (7-AI) dimer is a very attractive species as a model system of nucleic-acid base pair. The 7-AI dimer is known to exhibit the excited-state double proton transfer (DPT) reaction. The tautomeric dimer produced in the DPT reaction goes back to normal form in the electronic ground state, in solution. In general, the proton-transfer reaction is a fundamental and an important elementary reaction in various chemical and biological systems. However, the ground-state reverse DPT reaction is not thoroughly studied, so far. Thus, we carry out infrared (IR) spectroscopy of the jet-cooled 7-AI tautomeric dimer. In our previous study, we measured IR spectra of the tautomeric dimer and its deuterated species in the NH stretch region and discussed the vibrational dynamic based on the band profiles. In order to obtain more precise information about the deuteration effect, we have observed the ND stretch bands of the deuterated dimers in the present study.\par The deuteration of the NH hydrogen provides three deuterated species, such as the NH-NH, NH-ND, and ND-ND dimers. The NH stretch band of the NH-NH dimer appears at 2680 cm$^{-1}$. It exhibits a less-structured and broad profile whose width is $\sim$245 cm$^{-1}$. On the contrary, the NH-ND dimer exhibits a narrower NH stretch band width. This difference is attributed to a change in the vibrational energy flow between the two monomer units in the dimer. In the present study, we have succeeded in measuring the ND stretch bands of the NH-ND and the ND-ND dimers. The ND stretch band of the ND-ND dimer appears at 2120 cm$^{-1}$ and its width is found to be $\sim$90 cm$^{-1}$, whereas that of the NH-ND dimer is red-shifted and exhibits rather narrow width. Based on these observations, the single-deuteration effect on the vibrational dynamics and its relation to the DPT reaction is discussed in the paper. \pa

Chiral and Molecular Recognition through Protonation between Aromatic Amino Acids and Tripeptides Probed by Collision-Activated Dissociation in the Gas Phase

Chiral and molecular recognition through protonation was investigated through the collision-activated dissociation (CAD) of protonated noncovalent complexes of aromatic amino acid enantiomers with l-alanine- and l-serine-containing tripeptides using a linear ion trap mass spectrometer. In the case of l-alanine-tripeptide (AAA), NH3 loss was observed in the CAD of heterochiral H+(d-Trp)AAA, while H2O loss was the main dissociation pathways for l-Trp, d-Phe, and l-Phe. The protonation site of heterochiral H+(d-Trp)AAA was the amino group of d-Trp, and the NH3 loss occurred from H+(d-Trp). The H2O loss indicated that the proton was attached to the l-alanine tripeptide in the noncovalent complexes. With the substitution of a central residue of l-alanine tripeptide to l-Ser, ASA recognized l-Phe by protonation to the amino group of l-Phe in homochiral H+(l-Phe)ASA. For the protonated noncovalent complexes of His enantiomers with tripeptides (AAA, SAA, ASA, and AAS), protonated His was observed in the spectra, except for those of heterochiral H+(d-His)SAA and H+(d-His)AAS, indicating that d-His did not accept protons from the SAA and AAS in the noncovalent complexes. The amino-acid sequences of the tripeptides required for the recognition of aromatic amino acids were determined by analyses of the CAD spectra

PHOTODISSOCIATION SPECTROSCOPY OF Ca+^+-H2_2O IN THE TEMPERATURE-VARIABLE ION TRAP

Author Institution: Department of Chemistry, Graduate School of Science, Kobe University, Nada-ku, Kobe 657-8501, JapanIn the last two decades, developments of infrared spectroscopy and theoretical calculations on gas-phase molecular clusters have revealed detailed solvation structures of various systems, especially of hydrogen-bonded systems. One of the remained problems in studies on microscopic solvation or hydration is a temperature dependence of solvation structures. Lisy and coworkers succeeded in interpreting the hydration structures of alkali metal ions by taking temperature- or entropic effect \textbf{130}, 15393 (2008).}. They utilized Ar vaporization to cool down the temperature of clusters. \par Another method for controlling temperature of cluster ions is a buffer gas cooling in an ion trap. In the present study, we have measured photodissociation spectra of Ca$^+$-H$_2$O in our temperature-variable ion trap \textit{J.~Phys.~Chem.~A} \textbf{112}, 1457 (2008); A.~Fujihara, \textit{et al.} \textit{J.~Phys.~Chem.~A} \textbf{113}, 8169 (2009).}. In the present study, we examined the temperature of the Ca$^+$-H$_2$O in the trap by simulating the rotational profile of the 0-0 band of the $^2B_1 - ^2A_1$ transition. The observed rotational profile is similar to that reported by Duncan and coworkers \textbf{104}, 4591 (1996).}. By changing the trap period from 10 ms to 40 ms, it was confirmed that the trap period of 10 ms is sufficient to get temperature equilibrium in our experimental condition. Details of the experimental results will be presented in the paper

INFRARED SPECTROSCOPY OF 7-AZAINDOLE TAUTOMERIC DIMER AND ITS ISOTOPOMERS

Author Institution: Department of Chemistry, Graduate School of Science, Kobe University, Nada-ku, Kobe 657-8501, Japan7-azaindole (7-AI) dimer is a very attractive species as a model system of neucleic-acid base pair. It is well-known that it exhibits the excited-state double proton transfer (DPT) reaction. In solution, the tautomeric dimer generated in the DPT reaction goes back to normal form in the electronic ground state. This ground-state inverse DPT reaction is not thoroughly studied, so far. Thus, we carry out infrared spectroscopy of the jet-cooled 7-AI tautomeric dimer and examine the possibility of the vibrational-excitation promoted inverse DPT reaction. In the IR spectrum of the 7-AI tautomeric dimer, a very strong and broad band appears at 2680 cm$^{-1}$ and is assigned as the anti-symmetric NH stretch mode. On the contrary, sharp bands around 3100 cm$^{-1}$ are assigned as the CH stretch modes. Such a large difference in the band profiles among these bands is related to the difference in the vibrational anharmonicity of these modes. To discuss the vibrational anharmonicity of the NH stretch mode and the relation to the DPT reaction, we have also recorded IR spectra of several deuterated dimers. Comparison among the IR spectra of isotopomers will be discussed in the paper. \pa