Infrared Spectroscopy as a Probe of Electronic Energy Transfer Valeriu Scutelnic, † Antonio Prlj, ‡ Aleksandra Zabuga, † Cle ́ mence Corminboeuf, ‡ and Thomas R. Rizzo* ,† † Laboratory of Molecular Physical Chemistry, Ecole Polytechnique Fe ́ de ́ rale de Lausanne, Station 6, CH-1015 Lausanne, Switzerland ‡ Laboratory for Computational Molecular Design, Ecole Polytechnique Fe ́ de ́ rale de Lausanne, CH-1015 Lausanne, Switzerland *S Supporting Information ABSTRACT: We have combined electronic and vibrational spec- troscopy in a cryogenic ion trap to produce highly resolved, conformer-selective spectra for the ground and excited states of a peptide containing two chromophores. These spectra permit us to determine the precise three-dimensional structure of the peptide and give insight into the migration of the electronic excitation from phenylalanine to tyrosine because changes in the excited-state infrared spectra are sensitive to localization of the electronic energy in each chromophore. The well-controlled experimental conditions make this result a stringent test for theoretical methods dealing with electronic energy transfer. E lectronic energy transfer (EET) processes are ubiquitous in nature; for instance, they play a central role in photosynthesis 1 and photolyase activity. 2 Fö rster resonance energy transfer (FRET) is widely used in structural biology to measure the distance between donor and acceptor chromo- phores in proteins. 3 Moreover, accurate modeling of EET could assist in the engineering of more efficient solar cells. 4,5 While the majority of EET studies are performed in solution at room temperature, theoretical modeling of EET would significantly benefit from experiments carried out on isolated molecules at low temperature, which would allow for conformer-specific measurements. This is essential because both the distance and orientation of the chromophores play key roles in the energy transfer efficiency. Conformation-dependent EET in the gas phase was first demonstrated in molecular beam experiments by Chattoraj et al. 6 More recently, other research groups have extended EET studies to gas-phase ions inside of a mass spectrometer. 7−9 However, fluorescence detection from ions stored in an ion trap is challenging, owing to the low chromophore density and restricted angle for photon collection. Dugourd and co-workers introduced an “action-FRET” technique 10 that circumvents these obstacles by measuring the EET efficiency by means of specific photofragmentation rather than by fluorescence. However, a detailed picture of the energy dissipation processes that leads to fragmentation is still lacking. The objective of this work is to measure EET rates of a gas- phase peptide of well-defined geometry in a cryogenic ion trap. We use infrared−ultraviolet (IR−UV) double resonance 11 to obtain a ground-state infrared (IR) spectrum of each conformer as well as to determine its contribution to the electronic spectrum. By comparing these highly resolved IR spectra with those computed for the lowest-energy conformers determined by theory, we can de fine the distance between the chromophores and their relative orientation. We then use a UV laser pulse to promote a single conformer to the excited state of a specific chromophore. An IR laser then probes the electronically excited molecules, producing a unique spectral fingerprint that is characteristic of each electronic state. Monitoring specific IR transitions as a function of the delay time between pump and probe pulses provides a measure of the excited-state lifetimes and hence the absolute rates of EET. For this we chose a model peptide, Ac-FAYK-H + . The photochemistry of short peptides containing phenylalanine or tyrosine chromophores has been extensively characterized, 12−18 making Ac-FAYK-H + an ideal system in which to study EET. The C-terminal lysine side-chain induces strong hydrogen bonds with the backbone carbonyls, 19 stabilizing the peptide in a structure similar to the capping motif 20 of a 3 10 helix. 21 This provides a well-defined scaffold for the two chromophores involved. This is not unlike the work of Hendricks et al., who used the polyalanine helical motif to evaluate the EET efficiency from tryptophan to a disulfide bond; 22 however, their studies were not conformer-selective. The controlled conditions of our experimental approach provide a stringent test for models of EET. To begin, we characterize the UV spectroscopy of the peptide of interest. Upon electronic excitation of the cold AcFAYK-H + with a UV laser, a fraction of ions dissociates. Scanning the UV laser and monitoring the ion fragment signal Received: April 18, 2018 Accepted: May 31, 2018 Published: May 31, 2018 Letter pubs.acs.org/JPCL Cite This: J. Phys. Chem. Lett. 2018, 9, 3217-3223 © 2018 American Chemical Society 3217 DOI: 10.1021/acs.jpclett.8b01216 J. Phys. Chem. Lett. 2018, 9, 3217−3223 Downloaded via ECOLE POLYTECHNIC FED LAUSANNE on May 6, 2019 at 11:53:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.