Excited-state deactivation mechanisms of protonated and neutral phenylalanine: a theoretical study

Abstract

The potential energy (PE) profiles of neutral and protonated phenylalanine, as the simplest aromatic amino acid, at different electronic states have been investigated extensively using RI-MP2 and RI-CC2 methods. The PE profiles have been determined, considering the C_α–C_β and C_α–C_(COOH) bond stretching following proton transfer to the aromatic ring and CO group, respectively, as well as the hydrogen detachment reaction coordinate. The calculated results reveal that a low-barrier proton transfer process from ammonia to the aromatic chromophore, leading the excited system to C_α–C_β bond cleavage, plays the most prominent role in the deactivation mechanism of excited PheH⁺ at the origin of the S₁–S₀ electronic transition. On the contrary, for excited neutral phenylalanine at the band origin of the S₁–S₀ transition, a large barrier in the S₁ profile along the C_α–C_β bond-stretching hinders the excited system from approaching the dissociative part of PE curve. This barrier may explain the large lifetime of the S₁ excited phenylalanine (nanosecond range), while a low barrier in the S₁ PE profile of the protonated species along the PT process explains the short-range lifetime of the protonated species (in the picosecond range).

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1 Introduction

The molecular building blocks of organic systems readily absorb ultraviolet radiation. These molecules exhibit a large degree of photostability,¹ originating from the quenching of excited systems through ultrafast non-radiative relaxation processes. Despite their large UV absorption cross-sections, aromatic amino acids, like tryptophan, tyrosine and phenylalanine, show very small fluorescence quantum yields. This observation deals with the presence of fast radiationless processes, which efficiently quench fluorescence.^2,3 The radiationless process was assumed to be ultrafast internal conversion.^2,4 This so-called photostability protects these molecules from undesired photoreactions that are triggered by UV irradiation.^5,6

The spectroscopy of aromatic amino acids in molecular beams, either in their neutral state^7–12 or protonated forms,^13–15 has been extensively studied. Particularly, tryptophan is the most heavily studied owing to its large absorption cross section, high fluorescence intensity, and its strong sensitivity to the local protein environment.^16–18 Although tyrosine and phenylalanine have weaker UV absorptions than tryptophan, understanding their spectroscopy and photophysical behavior is important due to their crucial role in biological systems.^19–21

In addition, the fluorescence lifetime and intensity of aromatic amino acids are strongly dependent on the pH values of their environment and they drop dramatically at low pH. These results indicate a predominant non-radiative process, occurring after protonation.²² H. Shizuka and co-workers²¹ have shown that the fluorescence intensity of tryptophan markedly increases when it complexes with 18-crown-6. This observation suggests that the ammonium group plays a key role in the internal quenching of tryptophan. It has been stated that internal quenching is due to the hydrogen transfer from ammonia either to the ring or to the C=O group, which produce new conformers.^21,23 According to recent experimental and theoretical investigations, it has been well established that the C_α–C_β bond stretching plays a prominent role in the photophysics of neutral and protonated aromatic amino acids.^6,24 According to the experimental observations,^23–25 it has been evidenced that the ¹ππ* state undergoes a proton-transfer reaction from the amino group to the aromatic chromophore, which provides an efficient and fast pathway for internal conversion in protonated tryptophan and tyrosine, while a photo-fragmentation process occurring on the C_α–C_β bond has been reported as the most prominent relaxation channel in the case of protonated phenylalanine.²⁵ Also, in protonated tryptophan, a barrier-free H atom transfer to the carboxylic-acid oxygen through a small barrier has been predicted.²³ In addition, it is well known that the photophysical properties of aromatic amino acids depend strongly on the status of the amino and carboxyl groups (in neutral and protonated states).²⁶

The conformation of phenylalanine in a supersonic jet has been comprehensively studied by Martinez et al.⁹ They reported the laser induced fluorescence spectrum of the S₁–S₀ transition of jet-cooled phenylalanine and predicted the existence of multiple conformers using a laser saturation technique. Later, detailed investigation on the conformation of phenylalanine using IR-UV double resonance spectroscopy with ion detection and using ab initio calculations was presented by Snoek and co-workers.⁸

Regarding the excited state dynamics, in the case of protonated tryptophan (TrpH⁺), the S₁ excited-state lifetime was measured as 380 fs following 266 nm excitation, and calculations of the excited-state dynamics suggest it could be even shorter.^13,27 In contrast to protonated tryptophan, protonated tyrosine (TyrH⁺) shows a resolved vibronic spectrum and an excited-state lifetime of 22 picoseconds following excitation at 266 nm.²⁸ In the case of phenylalanine, a combination of conformer-specific infrared and ultraviolet hole-burning spectroscopy was employed to assign structures to the five conformers.^29,30 Lee et al.³⁰ have reported that the excited state lifetimes of several phenylalanine conformers lie between 20–120 nanosecond, while the excited state lifetime of protonated phenylalanine has been estimated to be in the picosecond range.²⁴ Nevertheless, little is known on the photophysics and photochemistry of amino acids; in particular, their relaxation dynamics are not well identified so far.

In the present paper we report our theoretical results on the deactivation mechanisms of protonated phenylalanine (PheH⁺) determined based on the RI-MP2/RI-CC2 methods. Thus, after summarizing the computational methods, we present ground- and excited-state optimized structures and excitation energies. We discuss and explain the potential energy curves, determined on the basis of several relaxation pathways of protonated phenylalanine, and compare our results with experimental observations on the fragmentation dynamics of protonated phenylalanine, taken from the literature. RI-CC2 is the method of choice because it gives reasonable results for medium-sized organic molecules, within a moderate computational time.^23,31–37

2 Computational details

The “ab initio” calculations have been carried out with the TURBOMOLE (V 6.3) program suit,^38,39 making use of the resolution-of-identity⁴⁰ (RI) approximation for the evaluation of the electron repulsion integrals.

The equilibrium geometry of the titled systems at the ground state has been determined at the RI-MP2 (Möller–Plesset second order perturbation theory) level.^41,42 The excitation energies and equilibrium geometry of the lowest excited singlet states have been determined at the RI-CC2 (the second-order approximate coupled-cluster method) level.^43–47

The Dunning’s correlation consistent split-valence double-ζ basis set with polarization functions on all atoms (cc-pVDZ)^48,49 and the augmented cc-pVDZ by diffuse functions on all atoms (aug-cc-pVDZ)⁵⁰ have been employed. All of the potential energy profiles have been determined at the RI-MP2 and RI-CC2 levels, using the aug-cc-pVDZ basis function. The vibrational frequencies in both the ground and the first excited states were calculated at the MP2 and CC2 levels with the def2-SVP and aug-cc-pVDZ basis sets, in order to confirm the stationary transition points on the PE profiles, and correcting the adiabatic S₁–S₀ transition energies with ΔZPE (the difference between zero point vibrational energy of the ground and S₁ excited states) respectively.

The abbreviations Phe and PheH⁺ will be used hereafter for neutral and protonated phenylalanine, respectively. In order to determine the potential energy profiles, the global minimum of protonated phenylalanine reported by Rizzo et al.¹⁴ has been considered as the initial structure of PheH⁺.

In addition, the validity of the RI-CC2 method for determination of the PE profiles of small organic systems has been established by Aquino and co-workers by comparison with accurate CASPT2 and MR-AQCC data.⁵¹ It has been shown that RI-CC2 predicts qualitatively reliable energy profiles of excited state proton transfer reactions. Hence, the RI-CC2 results are trustworthy for the qualitative determination of PE profiles.^52–59

3 Results and discussion

3-A Ground state equilibrium structures

The peptide backbone of the neutral and protonated aromatic amino acids is very flexible; thus, several stable conformers might be available for both forms of neutral and protonated Phe. Nevertheless, it has not been the goal of the present study to perform highly accurate exploration of conformers and geometric structures. We have rather been interested in the development of a clear qualitative picture of the basic mechanisms of non-radiative deactivation processes, especially in the global minimum of protonated phenylalanine as the simplest aromatic amino acid. Fortunately, comprehensive experimental and theoretical studies on the conformations of PheH⁺ have been carried out by Stearns and co-workers, where they introduced the global minimum conformer of PheH⁺.¹⁴ We have presented this structure in Fig. 1a. As shown, the structure is stabilized by two types of H-bond: the N-terminal hydrogen interacts with the π-system (N–H₁₅⋯π-ring) of the benzene ring, and another hydrogen atom (H₁₃), near to the carbonyl oxygen atom, makes a N–H₁₃⋯O=C hydrogen bond.

Fig. 1 Optimized geometries of (a) protonated phenylalanine (with its numbering pattern) and (b) the most stable conformer of neutral phenylalanine.

Similar to the protonated species, neutral phenylalanine (Phe) has several stable conformers in the gas phase. The electronic spectrum of phenylalanine in a supersonic jet, employing laser induced fluorescence (LIF) spectroscopy, has been recorded by Levy’s group.⁹ They also identified five different conformers, labeled A, B, C, D and E, stabilized in the low-temperature environment.⁹ Later, Z. Lin et al.,⁶⁰ by means of full conformational space exploration, performed a comprehensive theoretical exploration of phenylalanine, and introduced 37 stable gaseous conformers for this amino acid molecule.

It has been well established that the most stable structure of phenylalanine is conformer A (of Levy’s group⁹), corresponding to the optimized structure represented in Fig. 1b. This structure has been selected in our present work for determination of the transition energies and oscillator strength and potential energy profiles of neutral phenylalanine.

3-B Excitation energies and molecular orbitals

The calculated vertical transition energies of neutral and protonated phenylalanine (most stable conformers), of the four lowest singlet transitions (S₁–S₄) together with the adiabatic S₁–S₀ electronic transition, have been tabulated in Table 1. In addition to oscillator strengths, the first four singlet excited-states have been presented. The vertical transition energies for protonated and neutral phenylalanine have been calculated using the optimized geometry of the global minimum of protonated and neutral phenylalanine, shown in Fig. 1a and b. Within the accuracy of the calculation, the vertical S₁–S₀ transition energies (as well as the adiabatic) in protonated and neutral phenylalanine are quite similar, while there is a large discrepancy between the second electronic transition (S₂–S₀) in neutral and protonated species.

Table 1 Excited transition energies of neutral and protonated phenylalanine, computed at the RI-MP2/RI-CC2 levels with two different basis sets^a

State	Transition energy/eV		Oscillator strengths	Configurations
	aug-cc-pVDZ	cc-pVDZ
a The value in brackets represent the difference between the zero point vibrational energy of the S1 excited and ground states in eV. The experimental band origins of the S1–S0 electronic transition (^#ΔEexp.) for protonated and neutral phenylalanine have been adopted from ref. 25 and ref. 14, respectively.
Protonated phenylalanine (PheH^+)
S1 (^1ππ*)-vertical transition energy	5.07	5.17	0.0004	π2π1* (25%), π1π2* (25%), ππ1* (15%)
S1 (^1ππ*)-adiabatic transition energy [ΔZPE] ^#ΔEexp.	4.85 [−0.21] ^#4.65	4.96
S2 (ππCO*, πσ*)	6.08	6.28	0.0092	π3πCO* (18%), πσNH3* (14%), σπCO* (10%)
S3 (ππ*)	6.14	6.40	0.0160	π2π2* (28%), π1π1* (22%), π2π1* (21%)
S4 (πσ*, ππ*)	6.18	6.90	0.0170	π2σNH3* (55%), π2π2* (16%), π1σNH3* (8%)
Neutral phenylalanine (Phe)
S1 (^1ππ*)-vertical transition	5.08	5.18	0.0005	π1π1* (32%), π2π2* (30%), π1π2* (19%)
S1 (^1ππ*)-adiabatic transition energy ^#ΔEexp.	4.84 ^#4.65	4.99
S2 (nσ*, nπ*)	5.56	5.81	0.0009	nOσNH3* (28%), nOπCO* (20%)
S3 (ππ*)	6.04	6.51	0.0204	π2π2* (38%), π2π1* (23%), π1π1* (23%)
S4 (nπ*, ππ*)	6.25	7.10	0.0224	nOπ1* (33%), nOπ2* (31%), π2π1* (13%)

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In addition, the optical transitions to the lowest excited singlet states (S₁, S₂) of both neutral and protonated phenylalanine have weak oscillator strengths in the UV range. The frontier molecular orbitals (MOs) of protonated and neutral phenylalanine (the most stable conformers) are shown in Table 2. From the RI-CC2 calculations, it is found that the S₁–S₀ electronic transition of PheH⁺ gives rise to HOMO → LUMO + 1 (25%), HOMO − 1 → LUMO + 1 (15%) and HOMO − 1 → LUMO + 2 (25%) single electron transitions (HOMO and LUMO respectively indicate the highest occupied molecular orbital and the lowest unoccupied molecular orbital). As shown in Table 2, the HOMO and HOMO − 1 in PheH⁺ are of π nature, and LUMO + 1, LUMO + 2 are π*, located on the benzene ring. Thus, in PheH⁺, the first ¹ππ* is a local transition. According to Tables 1 and 2, the S₂–S₀ transitions in Phe and PheH⁺ have nσ*/nπ* and ππ* characters, respectively.

Table 2 Frontier MOs of protonated and neutral phenylalanine. Only the MOs making significant contributions to the S₁–S₄ transitions have been depicted. H and L indicate HOMO and LUMO, respectively

Protonated phenylalanine (PheH^+)
H − 4/π3	H − 2/σ2	H − 1/π2	H/π1	L/σNH3*	L + 1/π1*	L + 2/π2*	L + 3/πCO*
Neutral phenylalanine (Phe)
H − 2/nO	H − 1/π1	H/π2	L/π1*	L + 1/π2*	L + 2/σ1*	L + 3/πCO*

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For neutral phenylalanine, the S₁ state corresponds to the single electron transitions from HOMO → LUMO (32%), HOMO → LUMO + 1 (19%) and HOMO − 1 → LUMO + 1 (30%). As shown in Table 2, the HOMO and HOMO − 1 orbitals of Phe are different π orbitals, which are located on the benzene ring. Also, the LUMO, and LUMO + 1 are of π* nature, indicating that the S₁ state of phenylalanine has mostly ππ* nature.

In order to evaluate our method and basis set, we recalculated the adiabatic S₁–S₀ electronic transition energies of neutral and protonated phenylalanine, for which the jet cooled experimental data is available^9,14,61 (see Table 2). The S₁ band origin of neutral phenylalanine has been reported by Martinez et al., amounting to 37 537 cm⁻¹ (4.654 eV).⁹ For the protonated case (PheH⁺), it has been reported to be 37 529 cm⁻¹ (4.653 eV) by Stearns¹⁴ and Féraud.²⁵ As shown in Table 2, there is good agreement between the experimental band origins and our calculated values at the RI-CC2/aug-cc-pVDZ level of theory (ΔE_(S1–S0) = 4.85 eV and 4.84 eV, respectively, for protonated and neutral species). Nevertheless, considering the difference between the zero point vibrational energies of the ground and excited states (ΔZPE = −0.21 eV), the S₁–S₀ transition energy of the RI-CC2/aug-cc-pVDZ level (4.64 eV, ΔZPE corrected) is in the excellent agreement with the experimental band origin of PheH⁺ reported by Stearns¹⁴ and Féraud⁶¹ (see Table 1).

3-C. Photophysical behavior: potential energy profiles

3-C-A Protonated phenylalanine (PheH⁺).
I C_α–C_β bond stretching (before and after PT to aromatic ring). The photofragmentation spectrum of protonated phenylalanine (PheH⁺) has been recorded by Stearns et al.¹⁴ and most recently by Féraud and co-workers.²⁵ One of the most remarkable results of these experiments was the observation of specific fragmentation channels. Around the band origin of the S₁–S₀ electronic excitation, the protonated molecule mostly fragments through C_α–C_β bond rupture,^14,25 which is the only fragmentation leading to detection of m/z 75 and 92. The former fragment has been assigned to [HOOC–CH₂–NH₂]⁺, and the latter is related to [C₆H₅–CH₂]⁺ and [C₆H₅–CH₃]⁺. We have been motivated to determine the PE profiles relevant to the C_α–C_β bond rupture mechanism under two conditions: before and after proton transfer to the benzene ring.

In Fig. 2a, the potential energy profiles calculated along the minimum energy paths (MEPs) for the bond stretching of C_α–C_β at the S₀ and S₁ states are shown (full curves, designated by S₀^(S0) and S₁^(S1)). The coordinate-driven minimum-energy paths have been obtained by fixing the C_α–C_β bond distance and optimizing the lowest S₁ (¹ππ*) state with respect to all other coordinates. The geometry optimizations have been performed with the RI-CC2 method. The energies at the optimized geometries have been calculated at the RI-CC2/aug-cc-pVDZ level.

Fig. 2 Minimum energy paths (MEPs) of the S₀ state (circles) and the S₁ (ππ*) state (squares), determined at the RI-CC2/aug-cc-pVDZ level as a function of (a) C_α–C_β bond stretching, (b) N–H bond stretching, and (c) C_α–C_β bond stretching after PT in protonated phenylalanine. The solid lines (S₀^(s0) and S₁^(s1)) represent the energy profiles of the reaction paths determined in the same electronic state and the dashed lines (S₀^(s1) and S₁^(s0)) show the energy profiles determined in the complementary electronic states.

The PE profiles of the S₀ state, calculated with the ¹ππ*-optimized geometries (dashed lines with hollow circles), as well as the S₀-optimized geometries determined with the RI-MP2 method (solid lines with filled circles), are also shown. As shown, the S₁ PE profile along the C_α–C_β bond-stretching coordinate shows a barrier of 0.65 eV. The vibrational frequency analysis at R_Cα–Cβ = 2.0 Å shows an imaginary frequency, confirming the transition state (TS) nature of this point in the middle of the reaction coordinate (see ESI† file). Because of the large hindering effect of this barrier, the C_α–C_β bond stretching before a PT process is not suggested to play the role of a prominent relaxation-channel for the excited PheH⁺ at the origin of the S₀–S₁ transition.

However, according to the seminal work of C. Jouvet and co-workers,²³ it has been shown that proton transfer from the ammonium to the benzene ring is an important deactivation pathway in protonated tryptophan and tyrosine. Thus, we have investigated the minimum energy path (MEP) for PT from ammonia to the ring in PheH⁺ (Fig. 2b). The coordinate-driven minimum-energy paths for PT have been obtained by fixing the proton-transfer coordinates (N–H₁₅), containing the N–H₁₅⋯π hydrogen bond, and optimizing the lowest S₁ (¹ππ*) state geometry with respect to all other coordinates. As shown, the MEP exhibits a transition state at R_N–H15 = 1.5 Å, corresponding to the top of the barrier in the middle of reaction coordinate of the PT process (ΔE = 0.35 eV). Nevertheless, the barrier is not so large and can be overcome by a wave packet prepared at the origin of the S₁–S₀ transition. In addition, the tunneling effect of the hydrogen atom through the barrier, due to its slight mass, can be another possibility for passing the barrier and leading the excited system to the dissociation region of the PE profile. Under this condition, the excited system may evolve to C_α–C_β bond rupture.

In Fig. 2c, the MEP for C_α–C_β bond breaking, following the PT process to the aromatic ring, has been depicted. As shown, there is a small barrier in panel c, which is less pronounced than that in the previous panel. This barrier locates roughly under the S₁ band origin of global PheH⁺. Indeed, the C_α–C_β bond cleavage after the PT process under such conditions can be triggered by proton transfer from ammonia to the benzene ring. Moreover, from inspection of Fig. 2c, it is seen that a conical intersection appears at the end of the reaction coordinate (C_α–C_β ∼ 2.5 Å), where the C_α–C_β bond is almost broken. With such long lengths of the C_α–C_β bond, there is no sufficient force to turn the excited system back to the ground state, and thus photodissociation of the excited system after the CI must be more favored.

Additionally, vibrational frequency analysis has been carried out for selected points of Fig. 2a–c. The results have been presented in the ESI† file. All of the vibrational frequencies for the first points of panels a and b (Fig. 2a and b) are positive, confirming the stationary nature of these points on the PE surface. Nevertheless, in Fig. 2a–c there are three transition states, corresponding to the maximum points of each panel, having one imaginary frequency along the reaction coordinate.

II PT to C=O and hydrogen detachment reactions. According to the experimental results, by increasing the excitation photon energy, roughly above the band origin of the S₁–S₀ transition, a new fragmentation channel at m/z 120 appears. This fragmentation result has been assigned by Féraud et al.²⁵ to the bond breaking of C₈–C₉ following a proton transfer to the carbonyl group, leading to the formation of an iminium ion, CO and H₂O from PheH⁺. This fragmentation channel has been assigned to C_α–C_(COOH) bond breaking following the PT process from –NH₃ to carbonyl. Indeed, the carbonyl C=O group, in addition to –NH₂ and the benzene ring, is another electron rich center that attracts a proton. When the excess proton locates on the oxygen atom of the carbonyl group (C=O), it gives an isomer with an internal energy of ∼0.80 eV (77.2 kJ mol⁻¹) higher than that of global minimum. Nevertheless, we have determined the PE profiles of the PT reaction from –NH₃ to C=O at the S₁ and S₀ states (Fig. 3), in order to obtain more information on the PT process at the excited state. The PE profiles of Fig. 3a have been calculated along the N–H₁₃ bond, containing the (C=O⋯H₁₃–N) hydrogen bond, while in Fig. 3b, the C₈–C₉ bond has been selected as the reaction coordinate. Considering the MEP curves in Fig. 3a, in the S₁ (ππ*) state, there is a large barrier hindering the excited system from proceeding along the reaction coordinate. From Fig. 3a and b, it is seen that a wave packet prepared by excitation of PheH⁺ at the origin of the S₁ transition cannot simply overcome the barriers and proceed to the dissociative region of Fig. 3a and b. Thus, sufficient excess energy, more than the band origin of the S₁ state of PheH⁺, will be required for opening this dissociation channel.

Fig. 3 Minimum energy paths (MEPs) of the S₀ state (circles) and the S₁ (ππ*) state (squares), determined at the RI-CC2/aug-cc-pVDZ level as a function of (a) N–H₁₅, and (b) the C_α–C_β bond stretching and (c) the N–H₁₆ in the protonated phenylalanine.

Under such conditions, the excited system overcomes both barriers of Fig. 3a and b, and proceeds to the end of the reaction coordinate, which corresponds to the bond cleavage of C_α–C_(COOH). This bond cleavage can be responsible for photo-fragmentation of PheH⁺, producing CO + H₂O fragments, above the band origin of the S₁–S₀ transition.

It is noteworthy that the photo-fragment of m/z 120, corresponding to CO + H₂O fragments, has been observed 531 cm⁻¹ (0.065 eV) above the band origin of PheH⁺ by Féraud et al.²⁵ However, RI-CC2, being a single reference method, is unable to precisely determine the crucial points, such as conical intersections and potential barriers, and its results are only qualitatively reliable.^52–59 Additionally, the CO + H₂O fragments have been observed in the CID (Collision-Induced Dissociation) experiments too.⁶² Thus, there is another possibility that decreasing the S₁, S₀ energy gap of PheH⁺ after proton transfer from ammonia to C=O (Fig. 3, panel a) leads the excited system to the ground state via ultrafast internal conversions, and then the PheH⁺ system undergoes fragmentation from the S₀ PE profile.

The PE profiles for the hydrogen detachment channel have been investigated, as another suggestion for the deactivation of PheH⁺. The potential energy profiles calculated along the minimum energy paths (MEPs) for N–H₁₆ bond stretching at the S₀ and S₁ states are presented in Fig. 3c. As shown, there is a large barrier in the middle of the S₁ PE profile along the reaction coordinate. From Fig. 3c, it is seen that a large barrier of 0.90 eV height hinders the excited system of the S₁ (¹ππ*) state from proceeding along the reaction coordinate. Thus, the hydrogen detachment deactivation pathway has been predicted to be very unlikely. However, this deactivation mechanism can be accessed only when the excited system contains plenty of excess energy (0.9 eV more than the band origin of the S₁–S₀ transition).

Furthermore, the vibrational frequency analysis verifies the TS nature of the maximum points (R_N–H = 1.4 Å and R_Cα–Cβ = 1.8 Å, respectively, in Fig. 3a and b). However, the calculated vibrational frequencies for the maximum point of the S₁ PE profile of Fig. 3c (R_N–H16 = 1.4 Å) show more than one imaginary frequency, indicating a hilltop nature for this point. The excited system on the hilltop is extremely unstable and may undergo several reactions; among them, the N–H stretching leads the excited system to the end of the reaction coordinate, where the S₁/S₀ PE profiles cross with each other and produce a conical intersection in the multi dimensional picture. This CI could be responsible for the non-radiative deactivation of system via ultrafast internal conversions.

3-C-B Neutral phenylalanine. Although UV absorption and fluorescence spectra of phenylalanine and its derivatives have been studied by Wiczk’s group,⁶³ to our knowledge, there is no extensive theoretical report on the relaxation dynamics and deactivation pathways of this molecule. Nevertheless, the long lifetime of the S₁ excited phenylalanine (nanosecond range), reported by Lee et al.,³⁰ indicates a stable S₁ character of isolated Phe. Thus, we have determined the PE profiles of phenylalanine along C_α–C_β bond stretching to obtain more information on the photophysics of this system.

In Fig. 4, the S₀, S₁ PE curves along with the C_α–C_β bond stretching have been presented. From inspection of these results, it is seen that the MEP profile of the S₁ (ππ*) excited state has a rising trend, showing a TS (i.e. a barrier of ∼0.61 eV magnitude) at R_(Cα–Cβ) = 2.0 Å (see ESI† file), then slowly shows a dissociative trend to the end of the reaction coordinate. Meanwhile, the S₀ PE curve has a rising trend, and near the end of the reaction coordinate (R = 2.3 Å) the S₁ (ππ*) crosses with the S₀ electronic state, which develops into the conical intersections (CIs) in a multidimensional picture. Nevertheless, only the wave packet prepared in the ¹ππ* state by optical excitation of adequate excess energy (≃0.60 eV) is able to pass the barrier and reach the CI, which is expected to lead the excited system to the ground state with an internal conversion. Thus, the conical intersection of the S₁–S₀ states at the end of reaction coordinate may mostly play the role of a photostabilizer to protect neutral phenylalanine against UV radiation of λ_ex < 240 nm, via ultrafast internal conversion. Alternatively, from the CI region, the excited system may undergo dissociation along the C_α–C_β bond, producing photoproduct radicals.

Fig. 4 The MEPs profiles of the electronic ground state (circles) and the lowest excited state (squares) as a function of the C_α–C_β stretching coordinate in neutral phenylalanine.

4 Conclusion

Ab initio electronic structure and reaction path calculations, at the RI-CC2 level of theory, have been carried out to characterize the relaxation dynamics in protonated and neutral phenylalanine. The excited state intramolecular proton transfer to the aromatic ring has been predicted to be the most favorable consequence of excitation of PheH⁺ at the origin of the S₁–S₀ transition. The low barrier PT process to the aromatic chromophore consequently leads the excited system to C_α–C_β bond cleavage. In contrast, the PT process from ammonia to the carbonyl group (C=O), the same as the hydrogen detachment reaction coordinate, involves large hindering barriers along the S₁ potential energy curves. When the excited system contains small excess energy above the band origin of the S₁ transition, the excited state PT process to C=O leads the excited system to C_α–C_(COOH) bond breaking. Although relaxation of PheH⁺ along the hydrogen detachment pathway should be extremely unlikely at the band origin of the S₁ transition, it can be responsible for relaxation of the photoexcited system with ∼0.9 eV excess energy above the band origin.

For neutral phenylalanine, the large lifetime of the S₁ state (nanosecond) can be explained by the lack of nonradiative relaxation channels from the origin of the S₁ transition to the ground state. The C_α–C_β bond stretching investigated in this study has a barrier of 0.61 eV magnitude, restricting the excited system from approaching the dissociative region and conical intersection. For definite, this relaxation channel in neutral phenylalanine can be proposed as the UV protection of phenylalanine when excited at λ_ex < 240 nm.

Acknowledgements

The research council of Isfahan University is acknowledged for financial support. The authors are greatly indebted to Prof. C. Jouvet (University of Aix-Marseille) for helpful comments. The use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764) is kindly appreciated.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00630a

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