Surprising insights into the chemistry of hydroxyl radicals at BESSY II

How do radicals form in aqueous solutions when exposed to UV light? This question is important for health research and environmental protection, for example with regard to the overfertilisation of water bodies by intensive agriculture. A team at BESSY II has now developed a new method of investigating hydroxyl radicals in solution. By using a clever trick, the scientists gained surprising insights into the reaction pathway.

Hydroxyl radicals (OH·) are found everywhere, from the troposphere to the cells of the human body. There, they cause oxidative stress and accelerate the ageing process. They are also increasingly present in rivers and lakes, where they are formed by the photolysis of nitrogen oxides that have entered the water from over-fertilised soils. When UV radiation from sunlight strikes nitrogen oxides, hydroxyl radicals and a range of other radicals are generated. The chemistry of these radicals is extremely difficult to characterise accurately, as they react very quickly

A team led by Professor Alexander Föhlisch of the HZB has investigated the chemistry of hydroxyl radicals formed from nitrogen oxides in water using X-ray absorption spectroscopy at the BESSY II X-ray source.

‘We work with a liquid jet sample cell, in which we can study molecules in solution under highly realistic conditions. This is only possible at BESSY II,’ says Leo Cordsmeier, the study's first author. The scientists exploited the fact that certain molecules can ‘capture’ radicals. The molecule TEMPO is known as a radical scavenger for both carbon and nitrogen oxide radicals. ‘We use TEMPO here as a kind of trick,’ explains Cordsmeier. ‘It serves here as a “sensor” because this molecule is directly involved in the reaction and can be detected very easily in our experiment. This enables us to analyse the reaction of the OH radicals directly.'

In this way, they could observe, step by step, how radicals form in aqueous solutions containing nitrogen oxides when irradiated with UV light and how they are bound by TEMPO. In doing so, they also succeeded in measuring an unexpected intermediate state, enabling the reaction pathway to be reconstructed with precision. Their results show that the proton of the hydroxyl radical reacts with TEMPO first. ‘For the scavenging of hydroxyl radicals by TEMPO, we show that the mechanism does not proceed through a bound intermediate state between the two molecules, as has been proposed in the literature, but instead through an electron transfer. This is a surprising finding,' says Alexander Föhlisch. This study has enabled his team to develop a new method for investigating radicals in solution, allowing them to selectively observe where bonds break and new ones form.