Electrospray Photochemical Oxidation of Proteins
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J. Am. Soc. Mass Spectrom. (2019) DOI: 10.1007/s13361-019-02313-4
SHORT COMMUNICATION
Electrospray Photochemical Oxidation of Proteins Remilekun O. Lawal, Fabrizio Donnarumma, Kermit K. Murray Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
Abstract. Photooxidation of peptides and proteins by pulsed ultraviolet laser irradiation of an electrospray in the ion source of a mass spectrometer was demonstrated. A 193-nm excimer laser at 1.5-mJ pulse energy was focused with a cylindrical lens at the exit of a nanoelectrospray capillary and ions were sampled into a quadrupole time-of-flight mass spectrometer. A solution containing a peptide or protein and hydrogen peroxide was infused into the spray at a flow rate of 1 μL/min using a syringe pump. The laser creates OH radicals directly in the spray which modify biomolecules within the spray droplet. These results indicate that photochemical oxidation of proteins can be initiated directly within electrospray droplets and detected by mass spectrometry. Keywords: Photochemical oxidation, FPOP, Electrospray, Excimer Online, Ultraviolet Received: 30 April 2019/Revised: 18 July 2019/Accepted: 10 August 2019
Introduction
E
lectrospray droplets can be used both as a source of ions as well as for compound extraction and chemical reactions. The highly charged electrospray droplets serve as small reaction vessels that can be probed by mass spectrometry. With extractive electrospray ionization (EESI), the charged electrospray droplets merge with neutral aerosol particles containing analyte molecules that are extracted to form ions [1–5]. Reactions can be induced in electrospray droplets by exposing them to gaseous reagents. For example, hydrogen/deuterium exchange is observed in proteins within electrospray droplets when the spray is exposed to a deuterating reagent such as D2O [6, 7] and exposure of electrospray droplets to gas-phase acid or base can be used to manipulate protein charge state distribution [8, 9]. Solutions can be mixed in the electrospray cone using microfluidic channel sprayers [10] or dual-channel electrospray emitters such as theta capillaries [1] or coaxial fluidic devices [11]. Reactions with lifetimes as short as 1 μs can be monitored [12] and examples include protein folding
Electronic supplementary material The online version of this article (https:// doi.org/10.1007/s13361-019-02313-4) contains supplementary material, which is available to authorized users. Correspondence to: Kermit Murray; e-mail: [email protected]
[12–14], thermal denaturation [15], and H/D exchange [1, 11, 16]. Photochemical reactions can be initiated in the electrospray using a continuous light source directed at the spray tip [17–19]. Protein footprinting is the process of chemically modifying solvent-exposed regions of proteins in their native state followed by digestion and analysis by mass spectrometry to probe the higher-order structure [20]. Proteins can be oxidized in an electrospray using high voltage and an oxygen nebulizing gas [21, 22]. T
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