Gilles Grégoire

Flavins frequently operate as visible-light photoreceptor in both biological and artificial systems. Previous studies (mostly computational) suggest photoinduced protonation-site switching as the fundamental origin of the flavin photoreactivity. In particular, photoexcitation is predicted to change the preferred protonation site to the opposite side of the tricyclic aromatic flavin ring. However, direct experimental identification of such protonation-site switching by photoexcitation has remained an outstanding challenge. Herein, we directly prove the photoinduced protonation-site switching by combining cryogenic ion-trap infrared spectroscopy with controlled microsolvation by water molecules. Using protonated lumiflavin as prototypical model flavin, we show that S₁ ← S₀ photoexcitation triggers proton transfer catalyzed by a water bridge across the aromatic ring. These results provide a general design principle of flavin photochemistry, with implications for developing biological/artificial photoactive molecules.

The global rise in skin cancer incidence has intensified research into the photophysical mechanisms of sunscreen agents such as oxybenzone (OxB), a widely used UV filter. In its neutral form, OxB absorbs UV radiation and relaxes through ultrafast non-radiative pathways, although the detailed mechanisms remain controversial. This work focuses on the oxybenzone radical cation (OxB•+ ), a largely unexplored species that may form in environments where OxB coexists with metal ions, such as swimming pools treated with Ag⁺. Using a combination of photodissociation spectroscopy and theoretical calculations, the D 0 →D 5 electronic transition was identified and excited-state lifetimes on the femtosecond timescale were estimated. Unexpectedly, the vibronic band origin exhibits a shorter lifetime than higher vibronic levels, attributed to an ultrafast excited-state proton transfer. In contrast, out-of-plane vibrational excitation disrupts hydrogen-bond directionality and suppresses this process, leading to longer excited-state lifetimes.

Ultraviolet and infrared photodissociation spectra (UVPD and IRPD) of monohydrated N-methyl-dopamineH⁺ NMeDAH⁺(H₂O)₁ are measured by cryogenic ion spectroscopy and compared with those of monohydrated dopamineH⁺ DAH⁺(H₂O)₁ to reveal the methylation effect on intramolecular excited state proton transfer (ESPT). Whereas the UVPD spectrum of DAH⁺(H₂O)₁ is broad due to the proton transfer from the amine chain to the aromatic ring, that of NMeDAH⁺(H₂O)₁ shows sharp vibronic transitions in addition to a broad background signal. This suggests the coexistence of two or more conformers in which the excited state dynamics are very different. Based on conformer-selected IR spectra and theoretical calculations, sharp bands are assigned to two conformers in which a water molecule is inserted between the amine chain and the aromatic ring and prevents NH → π ESPT. This will lead to slow excited state dynamics. This water-inserted conformer is very minor in DAH⁺(H₂O)_1, and its enhancement by methylation is discussed. The broad UVPD background is attributed to the most stable conformer, in which the N-H bond of the amino group directly interacts with the catechol ring. The reaction path for NH → π ESPT in the most stable conformer shows a low barrier of less than 100 meV, which explains the broad UVPD spectrum by fast ESPT. The results confirm the model based on the relationship between the structure of these hydrated complexes and their photo reactivity associated with the excited state proton transfer.

Flavins are essential biological cofactors and their protonation isomers (protomers) generated upon photoexcitation play a crucial role in their photochemical reactions. Here, we identify the triplet-state (T₁) protomer of protonated lumiflavin (LFH⁺) in a cryogenic ion trap by visible-pump infrared-probe photodissociation spectroscopy using H₂-tagging (H₂-tagging Vis-IRPD). This approach yields high-resolution IR spectra of cold triplet ions which, along with quantum chemical calculations, enables structural assignments and population analysis of the identified O2-cis@T₁ protomer of LFH⁺. Furthermore, the implementation of a tickle radio frequency technique (Vis-tRF-IRPD) effectively removes nonexcited species from the ion trap, allowing vibrational signatures of the T₁ state to be isolated. This work establishes a fundamental platform for monitoring protonation-site switching in photoexcited flavins (S_n or T_n) and demonstrates the broader utility of H₂-tagging Vis-IRPD spectroscopy for electronic-state- and conformation-resolved studies of cold molecular ions.

Nitrophenolate anions are molecules that release nitric oxide (NO) after electronic excitation. This class of molecules is particularly interesting for potential applications in skin cancer treatment, which requires the use of wavelengths in the biological transparency window. Among the various isomers, meta NP^- (mNP^-) has the lowest excitation energy with a band origin at 625 nm. Here, using cryogenic photodissociation spectroscopy, we studied the effect on the electronic spectroscopy of mNP^- of methyl and amino electron-donating groups at the ortho position relative to NO₂. Unlike the bare mNP^- anion, all compounds exhibit broad photodissociation spectra, revealing significant geometric changes between the S₀ and S₁ (ππ_NO2*) states. While methyl substitution causes a blue shift of the absorption band maximum, amino-substituted mNP^- anions show large red shifts, with the absorption band starting below 800 nm for 4NH₂mNP^-, i.e., in the biological transparency window, while still leading to NO photorelease. The spectral shifts induced by the substituents are interpreted by comparison with coupled-cluster calculations performed in the ground state and the first excited state. These electron-donating groups in the ortho position relative to NO₂ perturb the electronic properties of mNP^- through opposing steric and electrostatic effects.

We present visible and infrared (IR) spectroscopy of mono hydrated meta nitrophenolate (mNP) anions. The experiments were conducted using cryogenic photodissociation spectroscopy and interpreted by quantum chemistry calculations. mNP represents a model NO photodonnor system for which solvation effects are important to study for possible biological applications. It turns out that the ionic hydrogen bond (IHB) between the water and phenolate groups is quite strong, as evidenced by a -850 cm^-1 red shift of the IHB stretch and a +1750 cm^-1 solvatochromism of the electronic excitation energy. While time-dependent density functional theory (TD-DFT) calculations provide a good match with the experimental vibronic spectrum, comparisons of the IR vibrational spectrum to harmonic and anharmonic calculations, including vibrational perturbation theory (VPT2) and discrete variable representation (DVR) approaches have raised some issues in theoretical treatments on anharmonicity for IHB. In particular, only discrete variable representation (DVR) method including a 4-modes representation (4MR) and a 6-dimensions (6D) potential energy surface (PES) can reproduce the experimental ν_IHB frequency but fails to recover all of the observed combination bands.

Flavins are multifunctional molecules that serve as essential cofactors for redox reactions and light-sensing proteins, and protonation is often relevant for these biochemical processes. We characterize herein the ground-state protonation sites of lumiflavin, the simplest member of the flavin family, in a cryogenic ion trap using infrared photodissociation (IRPD) spectroscopy as well as various types of isomer-selective vibrational and electronic double-resonance spectroscopies. The spectra are interpreted by complementary (time-dependent) density functional calculations. Infrared spectra in the 3 μm range reveal a predominant population of the most stable O2-cis protomer, while the less stable N1 isomer provides only a minor contribution. The relative abundances of the two protomers depend slightly on the solvent used for electrospray ionization (methanol or acetonitrile), indicating kinetic trapping effects. The optical spectrum observed near 430 nm is assigned to the vibronic transitions into the S₁ state of the O2-cis protomer, while contributions of the N1 protomer suggested previously can be excluded in this spectral range. The IRPD spectra provide the first definitive identification of the N1 protomer in the gas phase.

A droplet beam-laser ablation gas phase source has been developed for the isolation of biomolecular ions ejected from liquid microdroplets under vacuum. Liquid microdroplets containing the biomolecular ions of interest are generated in a primary vacuum chamber and passed through differential pumping stages to interact with an infrared laser pulse in a secondary vacuum chamber. Absorption of the infrared radiation by the solvent causes the explosion of the droplets and the desorption of biomolecular ions into vacuum. These ions are then mass-analyzed by a homemade time-of-flight spectrometer, either through a delayed extraction line or through coupling with a Paul trap. We show that this desorption source allows for the transfer of fragile molecules into the gas phase and preserves small non-covalent complexes. It can be operated in the positive and negative modes. Mass spectrometry analysis by the coupling of the source with a Paul trap allows us to get rid of the broad initial velocity distribution of the desorbed species subsequently to the laser ablation and leads to an improved mass resolution. This is a very promising setup for the production of hydrated biomolecules. It opens the way to the subsequent gas phase structural analysis of the desorbed biomolecular ions.

We use UV and infrared photodissociation spectroscopy to study monohydrated protonated guanine in a dual cryogenic ion trap spectrometer. The monohydrated complexes are formed through helium-mediated collisions between bare electrosprayed protonated guanine and low-pressure water vapor in a clustering trap maintained at 180 K, before being transferred to a quadrupole ion trap at 10 K. The spectrum of the monohydrated complex exhibits sharp vibronic transitions at the band origin and becomes broader and higher in intensity further in blue, which is very similar to protonated guanine but with a notable blue shift of ∼1850 cm^-1 (∼0.23 eV). The UV hole-burning experiments showed that the vibronic bands recorded in the region of the band origin belong to a single conformer under our experimental conditions. The IR photodissociation spectrum in the 3000-3600 cm^-1 range, with the aid of theoretical calculations (SCS-CC2/aug-cc-pVDZ), allowed us to assign the structure to the lowest energy N₇-O conformer.

The electronic and vibrational cryogenic ion spectroscopy of protonated tryptophan (TrpH+) and dopamine (DAH(+)) complexed with methanol has been recorded. These two biological chromophores exhibit ultrafast photochemistry due to excited-state proton transfer (ESPT). We have established the relationship between the structure of the complexes and their photodynamics and compared them with recent results obtained in hydrated complexes. For TrpH(+), there is no substantial change between methanol and water complexes; ESPT is hindered by a single solvent molecule. In the DAH+(MeOH)1 complex, the most stable conformer adopts a structure that prevents the direct interaction of the ammonium group of the side chain with the catechol ring, thus blocking the ESPT reaction. Such a ring structure is indeed a very minor populated conformer in the single-hydrated complex. The change in conformal stability between water and methanol clusters is due to a weak CH-pi attractive interaction of the methyl group of methanol with the catechol.