UV Photoinduced Dynamics of Conformer-Resolved Aromatic Peptides

Abstract

A detailed understanding of radiative and nonradiative processes in peptides containing an aromatic chromophore requires the knowledge of the nature and energy level of low-lying excited states that could be coupled to the bright 1 * excited state. Isolated aromatic amino acids and short peptides provide benchmark cases to study, at the molecular level, the photoinduced processes that govern their excited state dynamics. Recent advances in gas phase laser spectroscopy of conformer-selected peptides have paved the way to a better, yet not fully complete, understanding of the influence of intramolecular interactions on the properties of aromatic chromophores. This review aims at providing an overview of the photophysics and photochemistry at play in neutral and charged aromatic chromophore containing peptides, with a particular emphasis on the charge (electron, proton) and energy transfer processes. A significant impact is exerted by the experimental progress in energy-and time-resolved spectroscopy of protonated species, which leads to a growing demand for theoretical supports to accurately describe their excited state properties.

Excited-state proton transfer in protonated adrenaline revealed by cryogenic UV photodissociation spectroscopy

“…The last fragmentation channel at m/z 32 is assigned to the NH3CH3 + ionic fragment (C-N bond break). It should be stressed here that such fragmentation pattern, competition between CID like fragments and specific UV fragments issued from the C-C bond break, 10 is commonly observed in the UV photodissociation of protonated aromatic amines, 27 amino acids, 28-31 catecholamine 7 and short peptides. [32][33][34][35][36][37] As it can be readily seen in Fig.…”

“…Such photoinduced reaction has previously been assigned in aromatic amines, amino acids and related systems such as synephrine to excited state proton transfer from the ammonium group to the aromatic ring. 10 In the optimized structure of the * state of conformer A, a lengthening of the N-H bond (0.023 Å) pointing to the ring along with an out-of-plane bending of the CH (≈30°) facing the protonated group are observed (see Fig. S8).…”

“…In particular, the specific C-C bond cleavage following UV excitation of protonated aromatic amino acids and peptides has recently been reviewed by Grégoire and coworkers. 10 Such specific photofragmentation patterns could be useful for fast screening and identify biological molecules by mass spectrometry with complementary techniques as low energy collision (CID) or electron capture dissociation (ECD). 11 Furthermore, studying protonated systems cooled at cryogenic temperature reveals possible conformer selectivity which might be of prime importance for understanding the specific molecular recognition of such flexible neurotransmitters.…”

Hydrogen‐Bonded Complexes of Fluorophenylacetylenes: To Fluoresce or Not?

“…[7] In a nutshell, it has been suggested that the nature of coupling (or lack of it) of the initially excited 1 ππ* bright state to one or more low-lying excited states largely determines the nature of excited state dynamics. [8] In the case of the binary complex of phenylacetylene with methylamine, it was earlier reported, that the emissive nature of the initially excited 1 ππ* (S 1 ) state was dependent on the intermolecular structure, wherein the formation of � CÀ H•••N complex leads to fluorescence quenching while the NÀ H•••π complex was fluorescent. [9] The electron transfer mechanism of fluorescence quenching by amines is well known, at least in the condensed phases.…”

Conformational analysis by UV spectroscopy: the decisive contribution of environment-induced electronic Stark effects

“…Conformer-selective UV spectra may be recorded by using UV/UV double resonance techniques, 10 but conformer-selection can also be achieved using ion mobility. 11 Otherwise, UV chromophores take a critical part in the conformer-selection process for other types of spectroscopy, like conformer-selective IR 12–14 or photoinduced dynamics, 15 and in the measurement of the fractional abundances of a conformational mixture, 16 which can be considered as the ultimate goal of conformational analysis.…”