1A07
Gas phase spectroscopy of capped tyrosine by laser desorption supersonic jet technique: Conformational dependence of excited state dynamics
Woon Yong Sohn a, Yoko Shimozonoa, Pierre Ç arçabalb, Shun-ichi Ishiuchia, Masaaki FujiiaaChemical Resources Laboratory, Tokyo Tech.

bInstitut des Sciences Moléculaires d'Orsay, CNRS, Université Paris Sud XI.

Introduction Electronically excited state dynamics of biomolecule sometimes changes between the bulk andin vivo. For instance, a chromophore of green fluorescence protein, which is a modified residue of tyrosine (Tyr),emits green fluorescence in the literature, however, does not in aqueous solution. This is explained byconformational change in the protein, i.e., the chromophore forms a stressful structure, which inhibits the nonradiative process, by encaged in the protein. 1 Another instance is DNA base pairs. In gas phase, not only theWatson-Crick’s (WC) structures but also non-WC structures are observed. While the latters show sharp
electronic spectra, the formers give broad ones, which indicates the existence of rapid relaxation process in theelectronic excited state. The facts that only the wild type structures, i.e. WC structures, show rapid relaxation
process and DNA actually has low quantum yield of photo-chemical reaction imply that the living things mayutilize that rapid relaxation process to avoid photo damages.
As these example, the living things may utilize excited state dynamics
conveniently by controlling the conformations. So in this work, we took noticeof one of aromatic amino-acids, Tyr, and peptides containing it. Recently
Ç arçabal and co-workers reported a resonance enhanced multiphoton ionization(REMPI) spectrum of capped Tyr (cTyr), in which N and C-termini are protectedby acetyl group and methylamide group (Fig.1), respectively.2 The REMPI
spectrum shows a broad absorption with several sharp bands. They also
Fig. 1 Capped tyrosine
measured UV-UV hole burning (HB) spectra and found that two conformers,
one of which gives the broad absorption and another gives the sharp bands, contribute to the REMPI spectrum.
This result indicates the excited state dynamics is different with each conformer. However, it is difficult toconclude whether other conformers exist or not because of low spectral quality. In this work, we re-measured
REMPI and HB spectra of the cTyr and found tht it has at least 5 conformers. So we re-considered the structuresof each conformer by IR spectra and quantum chemical calculations.
Experiment In the HB spectroscopy, two tunable UV lasers are employed (Fig. 2).
The first UV laser, the probe laser, is fixed to a certain band observed in the REMPI
spectrum, and ion current due to the probe laser is monitored, which is proportional
to the population of a certain isomer at zero-vibrational level in S0. Before irradiationof the probe laser, the second UV laser, the burn laser, is introduced and scanned. If
the burn laser is resonant to any electronic transitions of the certain isomer, which
will be electronically excited, the population of the isomer in S0 decreases, and thus
the monitored ion current also decreases. Such depletions of ion current take place
only when the specified isomer by the probe laser is excited by the burn laser.
Therefore the isomer-selected electronic spectra can be measured as depletions of ioncurrent. In addition, if a tunable IR laser is employed as the second UV laser, isomerselected IR spectra can be measured (IR dip spectroscopy).
Results and Discussion REMPI and HB spectra ofcTyr is presented in Fig. 3. The burn laser was fixed ateach transition indicated by dotted lines. In this work, we
successfully measured 5 different HB spectra, not 2,thanks to high quality REMPI spectrum. Based on thepatterns of vibronic bands, it is possible to make threegroups, 1 and 2, 4 and 5, and 3, respectively. It implies
that each group has similar structure exept OH direction.
Of course, it is highly supposed for conformer 3 to have apair including the different OH direction. However, at
this moment, it has been difficult to find it due to thesmall intensity of the peak. The most important feature ofFig. 2 The schematicfor HB spectroscopy

Fig. 3 REMPI and HB spectra of cTyr

REMPI spectrum is the broad absorption around 35450 cm-1. According to HB spectra, the absorption arose fromboth conformer 1 and 2.
In order to figure the structures of each conformer out, IR dip spectroscopy was applied and the spectra arepresented in Fig. 4 with simulated spectra which were calculated by CAM-B3LYP/cc-pVDZ. All spectra showa sharp peak around 3660 cm-1 which is assigned to a free phenolic OH stretch. Other bands are assigned to NH
stretchs. Based on the spectral features of the NH stretchs, they could be classified to 3 groups, 1 and 2, 4 and 5,and 3, respectively, which corresponds to those by HB spectra. The first group shows two sharp bands at 3441,
3466 cm-1 for conformer 1 and 3436, 3468 cm-1 for conformer 2. On the other hand, in the second group, a sharpband at 3438 cm-1 for conformer 4, 3439 cm-1 for conformer 5 and a broad band around 3350 cm-1 are observed.
In the third group, conformer 3, a slightly broad band is observed at 3438 cm-1. According to the previous study,3
we can assign what kinds of hydrogen bond each NH group forms based on the frequency of the NH stretch
vibration. The broad band observed around 3350 cm-1 is assigned to a NH stretch which forms C7-type
intramolecular hydrogen bonds, in which the number of atoms between amine hydrogen and carbonyl oxigen
getting involved in the hydrogen bond is seven. Similarly, NH stretch observed at 3436 ~ 3441 cm-1 is assigned
to that forming C5-type intramolecular hydrogen bond. Thus, we tentatively assigned that the conformers 1 and2 form C5-type hydrogen bond and the conformers 4 and 5 form C7-type.
In order to assigne the structures more precisely, we compared the observed IR spectra with calculated ones.
In the spectra of the first group, the IR transitions located in 3466 cm-1 for conformer 1 and 3468 cm-1 forconformer 2 correspond to N1H stretch indicated in Fig. 1 and the observed bands at 3441 cm-1 for conformer 1and 3436 cm-1 correspond to N2H stretch. In the spectra of the second group, the IR transitions located in 3438
cm-1 for the conformer 4 and 3439 cm-1 for the conformer 5 correspond to N2H stretch and the broad IR
transitions located around 3350 cm-1 correspond to hydrogen bonded N1H stretch. In the spectrum of conformer
3, two NH stretching vibrational modes are overlapped in 3438 cm-1. According to the result about the relative
energy among the conformers, the most stable one is the conformer including C7-type. It is more stable thanconformers including C5-type, around 1 kcal/mol. It energetically supports the the intensity pettern of REMPI
spectrum of cTyr, in which the strongest peak arose from the conformer including C7-type.
As mentioned above, conformers 1 and 2, which form the C5-type hydrogen bond, have broad electronic
absorption, which may indicate a existence of a rapid relaxation process in the S 1 state. The relation between therelaxation process in S1 state and conformational structure will be discussed in the presentation.

Fig. 4 IR dip spectra of each conformer with simulated spectra by CAM-B3LYP/cc-pVDZ
References(1) Niwa, H.; Inouye, S.; Hirano, T.; Matsuno, T.; Kojima, S.; Kubota, M.; Ohashi, M.; Tsuji, F. I. Proc. Natl.
Acad. Sci. USA 1996, 93, 13617.
(2) Stanca-Kaposta, E. C.; Carcabal, P.; Cocinero, E. J.; Hurtado, P.; Simons, J. P. J. Phys. Chem. B 2013, 117,8135.
(3) Chin, W.; Piuzzi, F.; Dimicoli, I.; Mons, M. Phys. Chem. Chem. Phys. 2006, 8, 1033.