“The beauty of nature is that it often decides to use similar chemistries throughout a system, but we never thought that we would find a common link between atmospheric chemistry, and the chemistry of our bodies and food,” said Kevin Wilson, the deputy director of Berkeley Lab’s Chemical Sciences Division who led the study. “Our study is the first to explore another chemical pathway that might affect how well the cells in our bodies – and even our food – can respond to oxidative stress, such as pollution, over time.”
Human bodies and some foods have much in common: that is being made of organic molecules including unsaturated fats that are important building blocks for cell walls. Unsaturated lipids and other organic molecules will slowly degrade over time due to a chain reaction called autoxidation that is started by oxygen and hydroxyl radicals; these reactive oxygen species attack the unsaturated lipids in our bodies and food. In food this will cause the freshest of avocados to brown for example, but the damage to a human body is more devastating. With age the decades of exposure to hydroxyl radical and other reactive oxygen species will slowly debilitate the body’s unsaturated lipids, this irreversible damage will increase oxidative stress and elevate the likelihood of developing cancers and age related chronic diseases.
Hydroxyl radicals were believed to work alone when attacking unsaturated lipids, but the team has revealed that Criegee intermediates, which are highly reactive exotic molecules play along with the hydroxyl radicals. These secondary ozonide chemical species molecules were found to contain carbon, hydrogen and oxygen during a hydroxyl reaction with unsaturated lipids at the lab’s Advanced Light Source. Typically secondary ozonides are not associated with unsaturated lipids, they are products of a Criegee intermediate reaction with atmospheric aldehydes that are organic compounds derived from alcohols.
Criegee intermediates don’t exist for very long making them hard to study directly, meaning that a process of elimination had to be used to zero in on them. Mass spectroscopy was used to illuminate lipid nanodroplets under ultraviolet lighting, when scavenger alcohol molecules known to react only with Criegee intermediates was added to the nanodroplets the lipids were observed to degrade more slowly, which was due to the scavenger molecules reactivity with the intermediates rendering them inert according to lead author Meirong Zeng. Once the intermediates were disabled by the scavenger molecules the reaction produced products similar to peroxide and did not release secondary ozonides.
Findings are believed to provide evidence of a new lipid degradation pathway in which lipid hungry hydroxyl generates Criegee intermediates that give birth to a new batch of hydroxyl that send off a new generation of Criegee intermediates, and this cycle keeps replicating.
“This surprised us because hydroxyl radicals were known to cause oxidative damage to cells, but what wasn’t known before our study is that hydroxyl does this via the formation of Criegee intermediates,” added Kevin Wilson, the deputy director of Berkeley Lab’s Chemical Sciences Division who led the study.
As chronic disease, cancers and food spoilage are linked to cell damage caused by hydroxyl radicals, Criegee intermediates are believed to also play similar roles in the molecular degradation that makes humans vulnerable to age related diseases, and the discovery may be the foundation for a new class of antioxidants according to the scientists.
“It’s an exciting discovery. This gave us a fuller picture of the mechanisms behind cellular degradation and disease that was completely unexpected,” said Zeng.
“To complete this work took years of hard work by Nadja and Meirong, and the unique capabilities of the Advanced Light Source to probe complex chemistry,” Wilson said. “We hope that the results from our study will inspire researchers to further explore the biochemistry of Criegee intermediates, lipids, and antioxidants, which are needed to help people in a number of ways: from the prevention of disease to the preservation of food.”
The study was supported by the DOE Office of Science through the Gas Phase Chemical Physics Program, and the next phase will be to work with theorists to study the quantum properties that may be at work in the Criegee intermediate hydroxyl reaction to better explore how the cycle may operate in human cells, food and in materials containing unsaturated lipids such as fuels and plastics.
