Eur. Phys. J. D 51, 117–124 (2009) DOI: 10.1140/epjd/e2008-00076-4 Regular article T HE EUROPEAN P HYSICAL JOURNAL D Electron photodetachment of trapped doubly deprotonated angiotensin peptides. UV spectroscopy and radical recombination R. Antoine 1, a , L. Joly 1 , A.R. Allouche 1 , M. Broyer 1 , J. Lemoine 2 , and Ph. Dugourd 1 1 Universit´ e Lyon 1, CNRS, LASIM UMR 5579, 43 bd. du 11 Novembre 1918, 69622 Villeurbanne Cedex, France 2 Universit´ e Lyon 1, CNRS, Sciences Analytiques, UMR 5180, 43 bd. du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 28 January 2008 / Received in final form 29 February 2008 / Published online 18 April 2008 c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2008 Abstract. Trapped doubly deprotonated peptides are subjected to electron detachment when irradiated by a UV light. Electron photodetachment experiments as a function of the laser wavelength and laser fluence have been performed on two variants of angiotensin. The electron detachment yield was used to monitor the excited electronic spectrum of the trapped ions. Furthermore, the electron loss leads to the formation of radical ions. The radical recombination after collision activation is discussed. PACS. 87.15.M- Spectra of biomolecules; – 87.14.Ef Peptides; – 34.50.Gb Electronic excitation and ion- ization of molecules; intermediate molecular states – 33.80.Eh Autoionization, photoionization, and pho- todetachment 1 Introduction Absorption properties of gas phase ions are usually ob- tained by recording their photodissociation spectra. Pho- todissociation spectroscopy of molecular ions in traps has been developed in the early 70th in particular by the pio- neering works of Dunbar and Beauchamp in ion-cyclotron resonance cells [1,2]. Since, the development of new ion sources, the advances in ion trap technology, and new tun- able solid-state laser sources have made electronic pho- todissociation spectroscopy an efficient structural probe for isolated molecular ions. However, the use of photodissociation spectroscopy has been usually limited to characterize trapped ions with less than 20 atoms (see for example [3–12]). The diminution of the precursor ion signal or the increase in fragment ion sig- nal after photoexcitation is monitored. As the size of the molecular ion increases, the excitation energy may relax on an increasing number of degrees of freedom, which pre- vents from an efficient dissociation of the ions following a single photon excitation. IR spectroscopy on polypeptides has been achieved with multiphoton excitation [13]. But, to determine linear properties in the visible or near UV range, an observable is needed to monitor the absorption of a single photon by a peptide or a protein. This requires the existence of an observable relaxation channel faster than internal vibration relaxation (IVR). One possibility that has not been fully explored for the spectroscopy of large molecules yet, is to use chromophores that fragment or are cleaved before IVR [14–16], in particular in elec- tronic excited states [17,18]. We have also recently shown a e-mail: rantoine@lasim.univ-lyon1.fr that UV excitation of polypeptides and DNA polyanions induces a resonant electronic excitation of the ions fol- lowed by an efficient electron detachment [19,20]. The elec- tron detachment yield can then be used to monitor the excited electronic spectrum of the precursor ions [19,21]. Electron loss upon UV irradiation leads to the forma- tion of radical biomolecular ions [22]. These odd-electron radical species can be further fragmented by CID and give rise to new fragmentation pathways. These fragmentation pathways are different from those observed for the cor- responding even-electron species, and are interesting for analytical purposes, as recently shown for DNA oligomers and polypeptides [19,23]. 2 Experimental and method The experimental setup consists in a modified commer- cial ion-trap mass spectrometer (LCQ DUO with the MS N option, Thermo Electron, San Jose, CA, USA) equipped with an on-axis electrospray source [4]. The ring electrode of the quadrupole ion trap was drilled to allow the in- troduction of a laser beam. The laser is a nanosecond frequency-doubled tunable Panther TM OPO laser pumped by a PowerLite TM 8000 Nd 3+ :YAG laser (both from Con- tinuum, Santa Clara, CA, USA) operated at a repetition rate of 20 Hz. Before entering the trap, the laser beam goes through a mechanical shutter that is electronically syn- chronized with the mass spectrometer and is collimated with three 3 mm-diameter pinholes. The laser power was controlled using a half wave plate and a polarizer, and was monitored with a power meter located just before the injection in the ion trap.