Conformer- and Mode-Specific Excited State Lifetimes of Cold

Letter pubs.acs.org/JPCL

Conformer- and Mode-Specific Excited State Lifetimes of Cold Protonated Tyrosine Ions Satchin Soorkia,† Michel Broquier,†,‡ and Gilles Grégoire*,§ †

CNRS, Université Paris Sud, Institut des Sciences Moléculaires d’Orsay (ISMO) UMR 8624, 91405 Orsay Cedex, France Université Paris Sud, CLUPS (Centre Laser de l’Université Paris Sud) LUMAT FR 2764, 91405 Orsay Cedex, France § CNRS, Université Paris 13, Sorbonne Paris Cité, Laboratoire de Physique des Lasers, UMR 7538, F-93430 Villetaneuse, France ‡

ABSTRACT: The excited state lifetimes of conformer- and mode-selected cold protonated tyrosine have been measured for the first time through a picosecond pump−probe photodissociation scheme. Whereas the photofragmentation mechanism of protonated tyrosine ions strongly depends upon the interaction of the carboxylic acid group with the phenol ring, their excited state lifetimes are quite similar and decrease as the excess energy increases from 1.5 ns at the band origin to 900 ps and less than 500 ps for the 101 and 102 bands, respectively. Surprisingly, the excited state lifetime of the conformer with the anti orientation of the hydroxyl oxygen lone pair with respect to the ammonium group dramatically increases as compared to the others conformers up to 10 ns at the band origin and by more than a factor of 2 for the 101 band. The present experimental results clearly emphasize the subtle effect of the structural conformation on the excited state properties of molecular ions. SECTION: Spectroscopy, Photochemistry, and Excited States

T

of the hydroxyl oxygen lone pair with respect to the ammonium. Recently, we have revisited the photo fragmentation mechanisms of TyrH+ (m/z 182) and PheH+ (m/z 166) through a joint experimental and theoretical study.10 Several reactions have been enlightened, involving proton transfer from the ammonium group to the aromatic ring or the carbonyl group. The locally excited ππ* state, which bears the oscillator strength for UV excitation, decays through competitive nonradiative channels leading to different photo fragments. At the band origin, the most intense fragmentation channel corresponds to Cα−Cβ bond cleavage following a proton transfer reaction to the ring (m/z 108 for TyrH+). This fragment is specific to UV-PID and is not detected in lowenergy collision-induced dissociation (CID) experiment, suggesting that it is produced before internal conversion (IC) to the ground state. This reaction has, by far, the highest yield around the band origin. The minimum energy path (MEP) along the N−H stretch for the proton transfer reaction has been calculated at the CC2 level and exhibits a small barrier of 0.15 eV independently of the conformation. It however implies large nuclear displacements, with a lengthening of the N−H bond pointing to the ring along with a puckering of the aromatic plan of the chromophore to accept the proton. The most striking result of the UV-PID of TyrH+ is the abrupt change of the fragmentation branching ratio from the excitation of mode 1 in S1, i.e. only 800 cm−1 above the band origin. New

he photo physical processes in protonated aromatic amino acids have received much attention in very recent years. The coupling of laser spectroscopy with mass spectrometry techniques has greatly contributed to get a more and more detailed description of the fundamental mechanisms following electronic excitation in small building-blocks of peptides containing an UV chromophore as tryptophan, tyrosine, or phenylalanine. Ten years ago, the first UV photoinduced dissociation (UV-PID) mass spectra of protonated tryptophan were reported almost at the same time by several groups.1−3 These seminal works have paved the way for the use of UV-PID on larger and larger molecular systems as a complementary sequencing method in mass spectrometry. From the spectroscopic point of view, a key milestone has been reached in the group of T. Rizzo and O. Boyarkin with the first vibrationnally resolved photo dissociation spectrum of cold protonated tyrosine in a 22-pole trap.4 Since then, a complete set of laser spectroscopic techniques, including IR/UV5 and UV/ UV6−8 double resonance spectroscopy, have been employed to investigate the structural and electronic properties of single amino acids and peptide ions. The UV PID spectroscopy of TyrH+ has been reported by Stearn et al.9 and four low-lying energy conformers have been assigned by IR/UV photodissociation spectroscopy. They can be sorted in two main families, differing by a 2π/3 rotation along the Cα−Cβ bond of the amino acid group. In the stack conformer, both the ammonium and carboxylic acid group lie above the chromophore whereas in the rot conformation, the carboxylic group does not interact with the aromatic ring. For each family, two conformers coexist, with anti/syn orientation © XXXX American Chemical Society

Received: November 10, 2014 Accepted: December 4, 2014

4349

dx.doi.org/10.1021/jz502387q | J. Phys. Chem. Lett. 2014, 5, 4349−4355

The Journal of Physical Chemistry Letters

Letter

Figure 1. Picosecond excitation spectrum of TyrH+ in the range of 286 to 266 nm recorded on the m/z 108 fragmentation channel (bottom) and averaged on the other fragments (m/z 147, 136, 123, and 119). These photodissociation spectra are normalized by the parent signal and the laser power. The origin transitions of the three main conformers stack/syn (285.05 nm), rot/syn (284.80 nm) and rot/anti (283.90 nm) are labeled A, B, and D, respectively. The origin of the excess energy is defined by the origin transition of conformer stack/syn (A). The vibrational progression of the 1 mode (around 810 and 1610 cm−1) is shown for the three conformers.

protonated tryptophan,12,13 tyrosine,12 and small peptides14−16 have been investigated through a femtosecond photodissociation pump/probe scheme. In these experiments, ions were investigated at room temperature with pump (266 nm) and probe (800 nm) lasers at fixed wavelengths. Ultrafast excited state dynamics with bi-exponential decay were reported that could not be easily interpreted because of the lack of conformer selectivity and the large excess energy imparted by the fixed wavelength of the pump laser. In this Letter, we report the first excited state lifetime measurements on cold protonated aromatic amino acids. The nonradiative deactivation channels of TyrH+ have been tracked by picosecond time-resolved photodissociation spectroscopy. The picosecond UV-PID spectrum of TyrH+ exhibits wellresolved vibrational progression, which allows investigating the excited state lifetimes of the different conformers and as a function of the S1 vibrational modes. The picosecond time resolution of our laser is about 15 ps, which might be sufficient to investigate most of the deactivation mechanisms involved in molecules that exhibit well-resolved vibronic structure in the excited state. However, molecules that have unresolved electronic spectroscopy, such as protonated tryptophan, have expected excited state lifetimes in the 100 fs range and could not be studied with the picosecond lasers. The photodissociation spectrum of TyrH+ following picosecond excitation, monitored on the different fragmentation channels and recorded on a large spectral range from 286 to 266 nm, is reported in Figure 1. The main fragmentation channel around the band origin is m/z 108 (Figure 1, bottom) and the other ones at m/z 147, m/z 136, m/z 123, and m/z 119 (Figure 1, top) are 1 order of magnitude less intense. Higher in energy, the fragmentation yield of m/z 108 decreases, whereas the total photofragmentation yields recorded on the other ones gain in intensity. The spectral resolution of the picosecond laser (10 cm−1) is sufficient to resolve the vibronic bands in the excited state, which allows distinguishing the origin transition and the vibrational progression of three different conformers of

fragmentation channels appear, that is, m/z 147, 136, 123, and 119, which are the ones commonly observed in CID. Besides, those fragments become the only ones formed for the stack conformers, whereas for the rot conformers, both Cα−Cβ bond cleavage and CID-like fragments are detected. Ab initio calculations have pointed out the role of the ππCO* charge transfer state, which triggers a barrierless proton transfer reaction to the carbonyl group. The ππ*/ππCO* potential energy barrier, larger than the one calculated for the proton transfer to ring, is conformer dependent and lies at 0.4 and 0.6 eV for the stack and rot conformers, respectively. These calculations suggest that the coupling of the ππCO* state with the ππ* state increases with the excess energy and becomes the main nonradiative channel at the expense of the proton transfer to the ring. However, the steady-state spectroscopy does not provide information on the excited state dynamical processes. In particular, one can wonder whether the high conformer selectivity in the photofragmentation spectra of TyrH+ only depends on the energetics or involves drastic change in the excited state lifetime. The investigation of the excited state lifetime of protonated molecules through a photodissociation scheme implies special requirements, which differ from what is commonly used in photoionization. In both cases, the principle is to track the time evolution of the population created by a first laser (pump) in a given electronic excited state through the absorption of a second laser (probe) delayed in time. In photoionization, ions are detected as a result of the pump−probe scheme as long as the population evolving in a electronic state can be transferred in the ionic state of the system. It should be stressed that, in principle, none of the pump or probe pulses can ionize the system, although nonresonant multiphoton process can occur with femtosecond lasers. In a photodissociation experiment, the situation is quite different since the first laser (pump) already induces a photodissociation signal, so the probe laser must change the fragmentation branching ratio in order to reveal the excited state dynamics.11 The excited state lifetime of 4350



Letter

Figure 2. Difference photodissociation mass spectra (laser on−laser off) of TyrH+ (rot/syn) conformer with (a) pump excitation (101 band) only; (b) pump and probe (470 nm). The fragmentation yield of m/z 107 ion clearly increases with the probe laser, whereas the two-color signal on the other fragmentation channels is almost null. The “peak” seen around m/z 182 is noise due to saturation of the parent ion signal; (c) time evolution recorded on the m/z 107 ions as the function of the delay between the pump and probe lasers for two probe wavelengths. Within the S/N, the fitted excited state lifetime is independent of the probe wavelength.

protonated tyrosine previously assigned.9,10,17 The conformer stack/anti (noted C in ref 9), which is barely seen in the nanosecond spectrum cannot be assigned in this picosecond spectrum. As already reported, the rot conformer are mostly detected at the m/z 108 fragmentation channel, whereas both rot and stack conformations are seen on the other fragments (m/z 147, 136, 123, and 119) that have been averaged in Figure 1 (top). In overall, there is no effect of the temporal profile of the laser (picosecond vs nanosecond) on the photodissociation spectrum of protonated tyrosine. The intense transitions from 810 and 1610 cm−1 above the origin correspond to the excitation of the first two quanta of the vibrational mode 1 (ring breathing mode) according to the Wilson notation. The pump wavelength for the excited state lifetime measurements will be tuned on the transition of the rot and stack conformers of protonated tyrosine, which are clearly seen: for the rot/syn and rot/anti conformers noted B and D, respectively, and the stack/ syn conformer noted A (see Figure 1). The pump−probe excitation scheme used in this study allows monitoring the excited state lifetimes of protonated tyrosine for each conformer and as a function of the probed vibrational mode. In such an experiment, the pump laser is tuned to a resonant transition of the conformer that induces its photofragmentation spread along several fragmentation channels. In our case, the pump laser is set on the origin transition 000 of the locally excited ππ* and on the 101 and 102 bands of each conformer, which provide the highest fragmentation yields. The time evolution of the ππ* excited state lifetime can be measured through the absorption of a second laser (probe) delayed in time. In order to record the excited state lifetime, the probe laser must be absorbed while the system is still and only in the ππ* excited state. Besides, the probe laser must induce a change in the branching ratio of the different fragmentation channels, that is, an increase or decrease of the fragmentation yield on specific fragment ions. The variation of the branching

ratio with or without the probe laser is due to the fact that the fragmentation process can be different between the locally excited state (ππ*) and the final excited state reached by the probe laser. In our case, it turns out that the probe laser mainly produces a new fragment at m/z 107 following the direct Cα− Cβ bond cleavage from protonated tyrosine independently of the conformer or the probed mode. Because this fragmentation channel is not produced with the pump laser, the two-color signal-to-noise (S/N) is quite good although the absolute intensity recorded on the m/z 107 fragment is weak. The photodissociation mass spectra of TyrH+ induced by the pump and pump + probe excitations are reported in Figure 2a and 2b, respectively. They are difference mass spectra with and without laser excitation in which the parent signal at m/z 182 is saturated, causing a noise peak in the difference mass spectrum. As it can be seen in Figure 2c, the two-color signal of m/z 107 rises within the cross correlation of the pump and probe laser without time delay or rising time constant. Besides, the probe wavelength has been scanned from 450 to 600 nm without notable change in the two-color signal. The transient recorded with the probe wavelength at 600 nm is similar, within the experimental uncertainty, to the one recorded at shorter probe wavelength (470 nm). This indicates that, at least within this spectral range, the probe laser wavelength has no or little effect on the pump−probe signal of TyrH+, which insures consistency of the measured excited state lifetimes. It is noteworthy that the probe laser does not induce any fragmentation without the pump laser or at negative time delays (probe laser before the pump laser). On average, the fragmentation yield of the photofragments produced by the pump excitation slightly decreases with the absorption of the probe laser but the two-color S/N are too weak to be firmly analyzed. By definition, in any pump−probe experiment, the probe laser should specifically be absorbed from the excited state 4351



Letter

Figure 3. Excited state lifetime (picoseconds) of the three different conformers of TyrH+ (A, B, and D) as a function of the excitation wavelength recorded on the m/z 107 fragmentation yield. The fitted curve (red color line) is a monoexponential decay function.

range of the bluest probe wavelength used in our case. An interesting point is that the radical fragments exclusively through the H-loss channel leading to the formation of m/z 107 ion, with a fragmentation time smaller than the crosscorrelation function of the nanosecond laser (20 ns). Therefore, one might wonder if the picosecond transient of TyrH+ recorded on the m/z 107 ion signal could be affected by the rapid formation of m/z 108 radical ions that further fragments into m/z 107 after the probe laser excitation. In that case, the pump−probe signal recorded on the m/z 107 ion should be a complex function that could not be fitted by single monoexponential decay. In particular, a constant signal at long time delay, related to the absorption of the probe laser by the nascent and stable m/z 108 fragment, should be observed. The scans recorded at the band origin are too short to get the tail of the exponential but the pump−probe signals recorded for the 101 and 102 bands clearly show the absence of constant signal at long time delay. Finally, one should keep in mind that the m/z 108 fragmentation channel almost closes up 1500 cm−1 above the band origin. Therefore, the dynamics recorded for the 102 band could not be affected by the m/z 108 ion. Although we cannot completely rule out this possibility for the origin and 101 bands, the similar pump−probe signals recorded with the probe at 470 and 600 nm and the absence of rising time or a constant signal at long time delay in the m/z 107 ion signal indicate that this process is negligible. Finally, none of the other fragments produced by the pump excitation (CO + H2O loss at m/z, 136 for instance) are known to fragment through Cα−Cβ bond cleavage through electronic excitation.3 Therefore, we can confidently assert that the m/z 107 fragment is produced from the excited state of TyrH+ through absorption of the probe laser.

populated by the pump laser in order to measure any excited state lifetime. Because the spectral resolution of the picosecond laser is good enough to resolve the vibrational progression of the conformers of TyrH+, the time evolution recorded in our case should follow a monoexponential decay function with a time constant τ corresponding to the excited state lifetime of the probed vibrational mode. The time evolution of the fragmentation yield of m/z 107 has thus been fitted by a mono exponential decay function (starting from the zero time delay t0) convoluted by a Gaussian function to account for the temporal width of the laser (cross-correlation l0 of 15 ps) that leads to eq 1 S(t ) =

⎛ t − t 0 ⎞⎤ ⎛ t − t0 ⎞ 1⎡ ⎟ ⎢1 + erf⎜ ⎟⎥exp −⎜ ⎝ τ ⎠ 2 ⎢⎣ ⎝ l0 ⎠⎥⎦

(1)

The fragmentation channel at m/z 107 is thought to be related to the excitation from the ππ* state to a higher electronic excited state that triggers the Cα−Cβ bond cleavage. This ion could also be formed as a secondary fragment of the tyrosine cation produced after H-loss reaction from protonated tyrosine. This reaction has been observed in the photofragmentation spectrum of TyrH+ with an onset at 1 eV above the band origin.18 However, in our pump−probe experiment, Tyr+ cation at m/z 181 has not been detected, so the m/z 107 fragment is more likely produced directly from protonated tyrosine ions. It should be stressed that Dedonder et al.19 very recently reported the photofragmentation spectrum of the radical m/z 108 issued from the Cα−Cβ bond breaking of protonated tyrosine. The radical, stored in the cryogenically cooled ion trap (30 K), exhibits a well-resolved vibronic progression with a band origin at 436 nm. The onset of the fragmentation is red-shifted for the hot m/z 108 radical in the 4352



Letter

excitation of the parent molecule is less selective due to a large vibrational congestion, as it can be seen in Figure 1. It is therefore not surprising to get similar excited state lifetime for the three different conformers of protonated tyrosine at high excess energy. The difference in excited state lifetime of the anti (D) and syn (B) conformers was not suspected because both conformers show exactly the same photofragmentation behavior. It seems however that the orientation of the oxygen lone pair of the hydroxyl group plays a major role in the excited state dynamics of TyrH+ and represents a challenge for theoretical predictions. Experimentally, we are planning to investigate in details the excited state lifetimes of protonated tyramine, which might provide a simpler system because of the absence of the carboxylic acid group.20 Very recently, A. V. Zabuga et al.21 reported a detailed study of the UV photofragmentation mechanism in Ac−FA5K−H+ and Ac−YA5K−H+ peptides. They concluded that intersystem crossing (ISC) plays a key role in the fragmentation of these peptides, as evidenced by the large enhancement of the Cα−Cβ bond cleavage fragmentation channel by IR light up to tens of millisecond after the UV excitation. Tseng et al.22 also invoked ISC to the triplet state for the fragmentation process in neutral phenylalanine analogs. In our pump/probe scheme, if the system evolves to the triplet, one might observe a long time component in the picosecond pump/probe signal. For the 101 and 102 bands, the pump/probe signals recorded for each conformer do no exhibit such constant signal at long time delays. According to the present results, we can conclude that ISC to the triplet is not occurring in TyrH+ starting from 800 cm−1 of excess energy. For the band origin, as already stated, the recorded dynamics is too long and we cannot assert whether or not a constant signal exists. However, there is no obvious reason to think that the probability to go to the triplet should be totally different within such a small energy range. Besides, all the transients have been fitted with the same mono

In Figure 3, we have reported the time evolution of the fragmentation yield on the m/z 107 channel as the function of the pump and probe delay up to 1500 ps recorded for three different excitation energies of conformer stack/syn (A) and rot/syn (B) and rot/anti (D), that is, at the origin transition and at the 101 and 102 bands at +810 and +1610 cm−1 of excess energy, respectively. The time evolution recorded for the three conformers of TyrH+ follows the same trend, with a rapid decrease of the excited state lifetime as a function of the excess energy. For the two syn conformers (A and B), the lifetime is in the order of 1.6−1.7 ns at the band origin and drops almost linearly by a factor of 2 and 4 to 800−900 ps and 450−500 ps for the 101 and 102 bands, respectively (see Table 1). Although Table 1. Conformer- and Mode-Selected Excited State Lifetimesa of TyrH+

a

conformer

band origin

101 band

102 band

stack/syn (A) rot/syn (B) rot/anti (D)

1550 ± 100 1700 ± 100 >10 000

900 ± 100 860 ± 100 2300 ± 100

450 ± 50 490 ± 50 460 ± 50

In picoseconds.

these two stack and rot conformers do not have the same behavior regarding their fragmentation channels, their excited state lifetimes are quite similar. The most striking result is found for the anti conformer (rot conformer D), which shows a much longer excited state lifetime around the band origin and even for the 101 band. At the band origin, the excited state lifetime is obviously too long (more than 10 ns) to be firmly determined but is about 1 order of magnitude longer than for the other conformers. For the 101 band, its lifetime is more than twice longer, whereas being in the same range as the other conformers for the 102 band at +1610 cm−1 of excess energy. It should be stressed that at such excess energy in the S1 state, the

Figure 4. (a) Excited state lifetime (ps) of TyrH+ excited at 266 nm recorded on the m/z 107 fragmentation yield. The fitted curve (red color line) is a monoexponential decay function. (b) Linear evolution (log y scale) of the excited state lifetime of TyrH+ (rot/syn) as a function of the excess energy in S1. The last point at 22 ps corresponds to the previous measurement of hot TyrH+ with femtosecond pump/probe scheme.12 4353



■

Letter

EXPERIMENTAL METHOD The experimental setup used to generate and photofragment protonated species has been described previously.20,25 Tyrosine (Sigma-Aldrich, 95% purity) solution is prepared by dissolving the product in a 1:1 mixture of methanol and water at a concentration of 50 μM. Protonated molecules are produced continuously in an electrospray source26 operating in positive ion mode. The ions are transferred from the exit of the capillary to an octopole through a skimmer and trapped for 100 ms by applying a positive voltage at the exit electrode slightly higher than the bias of the octopole. A bunch of ions is ejected at 10 Hz by applying a negative pulsed voltage (−20 V) on the exit electrode, further accelerated to 200 V and mass-selected by a pulsed mass-gate during their flight to the quadrupole ion trap (Jordan ToF Inc.) located 50 cm downstream in a second vacuum chamber. The trap is biased at voltage that matches the kinetic energy of the incoming ions to avoid collision-induced dissociation of the protonated molecules with the He buffer gas injected through a pulsed valve (general valve) a few milliseconds before. The quadrupole ion trap is mounted on the cold head (CH-204S, Sumitomo) of a cryostat connected to a water-cooled He compressor. The temperature of the trap is controlled by two temperature diode sensors located on the cold head and on the top of the copper box housing the trap, with typical values of 10 and 14 K. The overall trapping time is set to 80 ms, the lasers being triggered after 30 ms to ensure an efficient cooling of the parent ions. The parent and photofragment ions are ejected from the trap in a homemade time-offlight mass spectrometer and detected on a set of microchannel plates (Z-Gap, Jordan Tof Inc.). Pump and probe laser beams are issued from the outputs of two OPA pumped by a mode-locked picosecond Nd:YAG laser (EKSPLA-SL300 LT-02300). The pump beam (290−260 nm) is generated by the second harmonic generation (SHG) system (EKSPLA-PG411) of the fundamental of the tunable visible OPA, whereas the probe beam is issued from the visible light (450−600 nm) generated by the second OPA system. Typical laser energies are 40 μJ/pulse and 200 μJ/pulse for the pump and probe laser, respectively. Both laser beams are focused in the center of the trap (1 mm2) with 1 m focal lenses. The crosscorrelation of the pulses have been estimated in previous pump−probe experiment to 15 ps fwhm,27 and the spectral resolution is 10 cm−1. The laser pulses are delayed between −100 and 1400 ps by a motorized optically delay line scanned in 6.6 ps steps, and the spectra are averages of 5 scans with 32 laser shots per scan step.

exponential decay function. The evolution of the excited state lifetimes as the function of the excess energy follows a linear trend on a log scale (Figure 4b). This gives confidence that there is no long time component attributed to the triplet at the band origin. It is also interesting to compare this picosecond excited state lifetime measurement on cold TyrH+ with the previous femtosecond pump−probe experiment performed at room temperature. The m/z 107 fragment was already used to record the ultra fast excited state lifetime of “hot” TyrH+ excited at 266 nm.12 In this previous experiment, the parent ions were continuously produced in the ESI source, transferred into vacuum through a heated capillary before being stored in the hexapole trap located in a second vacuum chamber. A bunch of ions were ejected from the hexapole, irradiated by 2 kHz femtosecond pump and probe laser pulses leading to the detection of the photofragments in a time-of-flight mass spectrometer. The excited state lifetime of TyrH+ was measured at 22 ps following 266 nm pump excitation but the internal energy of the protonated amino acids was undefined. We have recorded the excited state lifetime of cold TyrH+ at the same pump wavelength (Figure 4a) and found a much longer time constant of 230 ± 25 ps. In both experiment, such measurement represents the average of the excited state lifetimes of the different conformers because the UV spectrum does not exhibit resolved vibronic structure in this spectral region. Therefore, the different excited state lifetimes must be related to the initial internal energy imparted in the hot protonated molecule. At 266 nm, the excess energy in S1 is about 2500 cm−1. The evolution of the excited state lifetime as a function of the excess energy in S1 can be linearly extrapolated when plotted on a logarithm y scale (Figure 4b). Within this hypothesis, the 22 ps lifetime measured at 266 nm for the hot TyrH+ corresponds to roughly 5500 cm−1 of excess energy in S1, so with 3000 cm−1 of internal energy in the parent ions. Assuming that the protonated molecule can be treated as a thermodynamical set of vibrational oscillators at an effective temperature T, a vibrational energy E of 3000 cm−1 results in a temperature of 355 K (frequency calculations at the CC2/augcc-pVDZ), in agreement with the estimated temperature of the ions in this previous experiment. Therefore, the two excited state lifetime measurements at ambient and low temperatures are totally consistent. We have reported the first excited state lifetime measurements of three conformers of protonated tyrosine as a function of the excess energy in the ππ* state. For the syn conformers, though the rot and stack conformations of TyrH+ have drastic different photofragmentation properties with distinct fragmentation channels, their excited state lifetimes are similar within the experimental uncertainty and follow the same trends. At the band origin, the excited state lifetime is around 1.5 ns, similar to the one of phenol (2 ns)23 and slightly shorter than in neutral tyrosine (5−7 ns).24 The lifetime decreases as the excess energy increases down to 900 and 450 ps for the 101 and 102 bands, respectively. The excited state lifetime of the rot conformer with an anti orientation of the oxygen lone pair of the hydroxyl group of the phenol ring dramatically increases as compared to the other conformers. These experimental results represent a challenge for theory. The effect of the orientation of the hydroxyl lone pair will be further investigated in the simpler case of protonated tyramine, that is, in the absence of the carboxylic group.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been supported by the Université Paris Sud, by the ANR Research Grant (ANR 2010 BLANC 040501) and the RTRA ‘“Triangle de la Physique”’ (COMOVA and COMOVA II). We thank Christophe Jouvet (Université Aix-Marseille) for stimulating scientific discussions. 4354



■

Letter

Biochromophores; Nielsen, S. B., Wyer, J. A., Eds. Springer-Verlag and Co: Berlin/Heidelberg, 2013, 155−180. (19) Dedonder, C.; Feraud, G.; Jouvet, C. Identification of Daughter Ions Through Their Electronic Spectroscopy at Low Temperature. J. Chem. Phys. 2014, 141, 131101. (20) Feraud, G.; Broquier, M.; Dedonder, C.; Gregoire, G.; Soorkia, S.; Jouvet, C. Photofragmentation Spectroscopy of Cold Protonated Aromatic Amines in the Gas Phase. Phys. Chem. Chem. Phys. 2014, 16, 5250−5259. (21) Zabuga, A. V.; Kamrath, M. Z.; Boyarkin, O. V.; Rizzo, T. R. Fragmentation Mechanism of UV-excited Peptides in the Gas Phase. J. Chem. Phys. 2014, 141, 154309. (22) Tseng, C. M.; Lin, M. F.; Yang, Y. L.; Ho, Y. C.; Ni, C. K.; Chang, J. L. Photostability of Amino Acids: Photodissociation Dynamics of Phenylalanine Chromophores. Phys. Chem. Chem. Phys. 2010, 12, 4989−4995. (23) Lipert, R. J.; Bernudez, G.; Colson, S. D. Pathways of S1 Decay in Phenol, Indoles, and Water Complexes of Phenol and Indole in a Free Jet Expansion. J. Phys. Chem. 1988, 92, 3805−3809. (24) Inokuchi, Y.; Kobayashi, Y.; Ito, T.; Ebata, T. Conformation of L-Tyrosine Studied by Fluorescence-Detected UV-UV and IR-UV Double-Resonance Spectroscopy. J. Phys. Chem. A 2007, 111, 3209− 3215. (25) Alata, I.; Bert, J.; Broquier, M.; Dedonder, C.; Feraud, G.; Gregoire, G.; Soorkia, S.; Marceca, E.; Jouvet, C. Electronic Spectra of the Protonated Indole Chromophore in the Gas Phase. J. Phys. Chem. A 2013, 117, 4420−4427. (26) Andersen, L. H.; Lapierre, A.; Nielsen, S. B.; Nielsen, I. B.; Pedersen, S. U.; Pedersen, U. V.; Tomita, S. Chromophores of the Green Fluorescent Protein Studied in the Gas Phase. Euro. Phys. J. D 2002, 20, 597−600. (27) Pino, G. A.; Oldani, A. N.; Marceca, E.; Fujii, M.; Ishiuchi, S. I.; Miyazaki, M.; Broquier, M.; Dedonder, C.; Jouvet, C. Excited State Hydrogen Transfer Dynamics in Substituted Phenols and Their Complexes With Ammonia: ππ*-πσ* Energy Gap Propensity and Ortho-Substitution Effect. J. Chem. Phys. 2010, 133, 124313.

REFERENCES

(1) Kang, H.; Dedonder-Lardeux, C.; Jouvet, C.; Martrenchard, S.; Gregoire, G.; Desfrancois, C.; Schermann, J. P.; Barat, M.; Fayeton, J. A. Photo-Induced Dissociation of Protonated Tryptophan TrpH(+): A Direct Dissociation Channel in the Excited States Controls the Hydrogen Atom Loss. Phys. Chem. Chem. Phys. 2004, 6, 2628−2632. (2) Nolting, D.; Marian, C.; Weinkauf, R. Protonation Effect on the Electronic Spectrum of Tryptophan in the Gas Phase. Phys. Chem. Chem. Phys. 2004, 6, 2633−2640. (3) Talbot, F. O.; Tabarin, T.; Antoine, R.; Broyer, M.; Dugourd, P., Photodissociation Spectroscopy of Trapped Protonated Tryptophan. J. Chem. Phys. 2005122. (4) Boyarkin, O. V.; Mercier, S. R.; Kamariotis, A.; Rizzo, T. R. Electronic Spectroscopy of Cold, Protonated Tryptophan and Tyrosine. J. Am. Chem. Soc. 2006, 128, 2816−2817. (5) Rizzo, T. R.; Stearns, J. A.; Boyarkin, O. V. Spectroscopic Studies of Cold, Gas-Phase Biomolecular Ions. Int. Rev. Phys. Chem. 2009, 28, 481−515. (6) Choi, C. M.; Choi, D. H.; Heo, J.; Kim, N. J.; Kim, S. K. Ultraviolet-Ultraviolet Hole Burning Spectroscopy in a Quadrupole Ion Trap: Dibenzo 18 crown-6 Complexes with Alkali Metal Cations. Angew. Chem., Int. Ed. 2012, 51, 7297−7300. (7) Kang, H.; Feraud, G.; Dedonder-Lardeux, C.; Jouvet, C. New Method for Double-Resonance Spectroscopy in a Cold Quadrupole Ion Trap and Its Application to UV-UV Hole-Burning Spectroscopy of Protonated Adenine Dimer. J. Phys. Chem. Lett. 2014, 5, 2760−2764. (8) Feraud, G.; Dedonder, C.; Jouvet, C.; Inokuchi, Y.; Haino, T.; Sekiya, R.; Ebata, T. Development of Ultraviolet-Ultraviolet HoleBurning Spectroscopy for Cold Gas-Phase Ions. J. Phys. Chem. Lett. 2014, 5, 1236−1240. (9) Stearns, J. A.; Mercier, S.; Seaiby, C.; Guidi, M.; Boyarkin, O. V.; Rizzo, T. R. Conformation-Specific Spectroscopy and Photodissociation of Cold, Protonated Tyrosine and Phenylalanine. J. Am. Chem. Soc. 2007, 129, 11814−11820. (10) Feraud, G.; Broquier, M.; Dedonder-Lardeux, C.; Jouvet, C.; Gregoire, G.; Soorkia, S. Excited State Dynamics of Protonated Phenylalanine and Tyrosine: Photo-Induced Reactions Following Electronic Excitation. J. Phys. Chem. A 2014, DOI: 10.1021/ jp5065837. (11) Kang, H.; Jouvet, C.; Dedonder-Lardeux, C.; Martrenchard, S.; Charriere, C.; Gregoire, G.; Desfrancois, C.; Schermann, J. P.; Barat, M.; Fayeton, J. A. Photoinduced Processes in Protonated Tryptamine. J. Chem. Phys. 2005, 122, 084307. (12) Kang, H.; Jouvet, C.; Dedonder-Lardeux, C.; Martrenchard, S.; Gregoire, G.; Desfrancois, C.; Schermann, J. P.; Barat, M.; Fayeton, J. A. Ultrafast Deactivation Mechanisms of Protonated Aromatic Amino Acids Following UV Excitation. Phys. Chem. Chem. Phys. 2005, 7, 394− 398. (13) Kang, H.; Dedonder-Lardeux, C.; Jouvet, C.; Gregoire, G.; Desfrancois, C.; Schermann, J. P.; Barat, M.; Fayeton, J. A. Control of Bond-Cleaving Reactions of Free Protonated Tryptophan Ion by Femtosecond Laser Pulses. J. Phys. Chem. A 2005, 109, 2417−2420. (14) Nolting, D.; Schultz, T.; Hertel, I. V.; Weinkauf, R. Excited State Dynamics and Fragmentation Channels of the Protonated Dipeptide H2N-Leu-Trp-COOH. Phys. Chem. Chem. Phys. 2006, 8, 5247−5254. (15) Gregoire, G.; Kang, H.; Dedonder-Lardeux, C.; Jouvet, C.; Desfrancois, C.; Onidas, D.; Lepere, V.; Fayeton, J. A. Statistical vs. Non-Statistical Deactivation Pathways in the UV Photo-Fragmentation of Protonated Tryptophan-Leucine Dipeptide. Phys. Chem. Chem. Phys. 2006, 8, 122−128. (16) Gregoire, G.; Dedonder-Lardeux, C.; Jouvet, C.; Desfrancois, C.; Fayeton, J. A. Ultrafast Excited State Dynamics in Protonated GWG and GYG Tripeptides. Phys. Chem. Chem. Phys. 2007, 9, 78−82. (17) Redwine, J. G.; Davis, Z. A.; Burke, N. L.; Oglesbee, R. A.; McLuckey, S. A.; Zwier, T. S. A Novel Ion Trap Based Tandem Mass Spectrometer for the Spectroscopic Study of Cold Gas Phase Polyatomic Ions. Int. J. Mass Spectrom. 2013, 348, 9−14. (18) Dedonder, C.; Feraud, G.; Jouvet, C. Excited-State Dynamics of Protonated Aromatic Amino Acids. In Photophysics of Ionic 4355


Conformer- and Mode-Specific Excited State Lifetimes of Cold

Recommend Documents