Water acting as a catalyst for ionization induced molecular ring-break

Ionizing radiation has always been a part of our lives. This process can create energetic secondary electrons and affect destructively the structure of the system, e.g. chemical bond breaking. Studies will show that hydrogen-bonding can considerably affect the stability of molecular covalent bonds.
Water acting as a catalyst for ionization induced molecular ring-break

All matter in our environment is exposed to permanent bombardment by ionizing radiation from various sources. If a molecule gets ionized it loses electrons from chemical bonding orbitals or more strongly bound inner electronic shells. In any case, energy is deposited and the molecular structure is changed. Therefore, strong vibration is induced and regularly the molecule breaks apart. 

Naturally, the question arises on how the complex and, therefore, vulnerable biomolecular structures in living organisms can sustain and survive such damaging reactions. One stabilization mechanism is provided by the local aqueous environment, namely the hydration shell in which biomolecules are embedded. Water strongly interacts with biomolecules via hydrogen bonds. As a result, internal molecular energy (vibration/heat) can be quickly transferred to the surrounding and dissipated before any internal molecular damages can form. This protective effect was regularly observed for ionization of hydrated organic molecules like for building blocks of DNA or for proteins. If single molecules in the gas phase were ionized the product analysis with mass spectrometers showed various small molecular fragments. In contrast if the molecules were embedded in water shells or more molecules were aggregated to clusters the mass spectra showed mainly intact molecular ions. Strikingly, for small systems consisting of just two weakly bound partners, the situation was different and additional molecular fragmentation reactions were observed which so far were not understood. 

We have now identified the underlying mechanisms for biological relevant pairs (dimers) consisting of two tetrahydrofuran (THF) molecules or one THF and one water molecule. THF is a molecular ring and an analog of the deoxyribose sugar in the DNA backbone. For electron-impact ionization of monomers, dimers and larger clusters we measured the energy deposited in the system and, simultaneously, the ion mass to see if there is a fragmentation of THF. For the dimers, it turned out that THF ring-break occurs even for the smallest possible energy transfer which results in stable ring-structures for the other systems, single molecules as well as larger clusters. 

Computer simulations which we performed show that ionization of one constituent in the dimers and clusters leads to spatial rearrangement of the molecules in the vicinity of the ion resulting in strong vibrational excitation. The larger clusters can cool down by redistribution of the energy and possibly by evaporation of molecules. These energy dissipation processes are not acting in the dimers and as a consequence, the vibrational energy is sufficient to overcome the reaction barrier for the THF ring-opening and this inevitably leads to the complete ring-break. Considering a THF-H2O dimer, for ionization of the THF the neighboring water molecule catalyzes the ring-break reaction which would not occur in THF monomers. A schematic view of the present studies can be seen in Fig. 1. These findings show how important the efficient coupling to a heat bath in the form of an aqueous environment is for the stability of biological systems under irradiation. 

Fig. 1. Schematic representation illustrating the initial structure of a THF-H2O dimer, the rearrangement and ring opening and the final ring-break.

You will find more details from the paper “Water acting as a catalyst for electron-driven molecular break-up of tetrahydrofuran” in Nature Communications 11, 2194 (2020).

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