Radiation-Induced Transfer of Charge, Atoms, and Energy within Isolated Biomolecular Systems

In biological tissues, ionizing radiation interacts with a variety of molecules and the consequences include cell killing and the modification of mechanical properties. Applications of biological radiation action are for instance radiotherapy, sterilization, or the tailoring of biomaterial properties. During the first femtoseconds to milliseconds after the initial radiation action, biomolecular systems typically respond by transfer of charge, atoms, or energy. In the condensed phase, it is usually very difficult to distinguish direct effects from indirect effects. A straightforward solution for this problem is the use of gas-phase techniques, for instance from the field of mass spectrometry. In this review, we survey mainly experimental but also theoretical work, focusing on radiation-induced intra- and inter-molecular transfer of charge, atoms, and energy within biomolecular systems in the gas phase. Building blocks of DNA, proteins, and saccharides, but also antibiotics are considered. The emergence of general processes as well as their timescales and mechanisms are highlighted.

Direct Observation of Charge, Energy, and Hydrogen Transfer between the Backbone and Nucleobases in Isolated DNA Oligonucleotides

Understanding how charge and energy, as well as protons and hydrogen atoms, are transferred in molecular systems as a result of an electronic excitation is fundamental for understanding the interaction between ionizing radiation and biological matter on the molecular level. To localize the excitation at the atomic scale, it was chosen to target phosphorus atoms in the backbone of gas-phase oligonucleotide anions and cations, by means of resonant photoabsorption at the L- and K-edges. The ionic photoproducts of the excitation process were studied by a combination of mass spectrometry and X-ray spectroscopy. The combination of absorption site selectivity and photoproduct sensitivity allowed the identification of X-ray spectral signatures of specific processes. Moreover, charge and/or energy as well as H transfer from the backbone to nucleobases has been directly observed. Although the probability of one versus two H transfer following valence ionization depends on the nucleobase, ionization of sugar or phosphate groups at the carbon K-edge or the phosphorus L-edge mainly leads to single H transfer to protonated adenine. Moreover, our results indicate a surprising proton-transfer process to specifically form protonated guanine after excitation or ionization of P 2p electrons.

UV-VUV Photofragmentation Spectroscopy of Isolated Neutral Fragile Macromolecules: A Proof-of-Principle Based on a Deprotonated Vancomycin-Peptide Noncovalent Complex

The gas phase offers the possibility to analyze organic molecules by ultraviolet-vacuum ultraviolet (UV-VUV) spectroscopy without any solvent effect or limitation in terms of spectral range due to absorption by the solvent. Up to now, the size and chemical composition of neutral molecular systems under study have been limited by the use of vaporization methods based on thermal heating. Soft sources of gas-phase thermolabile molecular systems such as electrospray or matrix-assisted laser desorption ionization are appealing alternatives to heating-based techniques, but they lead to the production of ions. In such cases, UV-VUV action spectroscopy is then the method of choice to study the electronic structure and corresponding photodynamics of these gas-phase molecular ions. However, previous investigations have shown that the UV-VUV action spectrum of a given molecular ion depends on the charge state, which in many cases might be a caveat. Here, by means of synchrotron radiation coupled to mass spectrometry and through the test case of the glycopeptide antibiotic vancomycin noncovalently bound to a deprotonated small peptide, we show that the UV-VUV photofragmentation spectrum of neutral thermally fragile organic molecules can be obtained via charge-tagging action spectroscopy.

Mass Spectral Signatures of Complex Post-Translational Modifications in Proteins: A Proof-of-Principle Based on X-ray Irradiated Vancomycin

Characterizing post-translational modifications (PTM) of proteins is of key relevance for the understanding of many biological processes, as these covalent modifications strongly influence or even determine protein function. Among the different analytical techniques available, mass spectrometry is attracting growing attention because recent instrumental and computational improvements have led to a massive rise of the number of PTM sites that can be identified and quantified. However, multiple PTM occurring at adjacent amino acid residues can lead to complex and dense chemical patterns that are a challenge to characterize. By means of X-ray synchrotron radiation coupled to mass spectrometry, and through the test-case of the glycopeptide antibiotic vancomycin, we show that such a pattern has a unique and robust signature in terms of photon energy and molecular environment. This highlights the potential of this technique in proteomics and its value as a tool to understand the biological roles of PTM.