How a plant molecule is a “superb” sunscreen

October 30, 2014 § Leave a comment



Plants make their own sunblock to protect against UV damage. Image from

Like us, plants worry about sunburn. Research now shows that a molecule called sinapoyl malate found in plants is one hardworking sunblock. It captures some of the most damaging radiation from the sun.

Ultraviolet radiation is split into nine groups. One of the groups is UV-B radiation in the 280 to315 nm range. Overexposure to UV-B radiation in humans causes sunburn and some forms of skin cancer. In plants, the radiation can cause DNA damage and interfere with photosynthesis.

Timothy Zwier at Purdue University, who led the recent study published in the Journal of the American Chemical Society, says he got intrigued by sinapoyl malate when a Purdue colleague, Clint Chapple, discovered that the molecule was responsible for acting as a sunscreen in plants. While sinapoyl malate generally was known to absorb UV-B radiation in aqueous solution, the inherent spectral properties of the bare molecule were unknown.

Zwier’s lab’s specialty is studying the UV spectroscopy of isolated molecules in the gas phase. But the problem is that sinapoyl malate, like most other biological molecules, can’t be heated into a gas because it breaks down.

So Zwier and colleagues zapped the molecule off a thin film with a laser to get it into the gas phase. Once the molecule was in the gas phase, the investigators cooled it down to a temperature within a few degrees of absolute.

“This gave us a chance to study the inherent spectroscopic signatures of this UV-B-sunscreen molecule as an isolated molecule, free from the effects of solvent and cooled to low temperatures, where it can best reveal its secrets,” says Zwier.

However, sinapoyl malate had a surprise. Even in the stripped-down experimental conditions, the molecule’s spectrum was inherently broad, absorbing continuously over all wavelengths encompassed in the UV-B radiation range. Zwier notes that in its natural environment in the plant cell’s aqueous condition, sinapoyl malate probably works in conjunction with other factors to cover even more of the UV spectrum.

“Nevertheless, at the most fundamental quantum mechanical level, sinapoyl malate has what it takes to be a superb UV-B sunscreen — large absorption cross-section in the right wavelength range and complete spectral coverage,” says Zwier. “Nature has chosen a molecule to serve as its UV-B sunscreen that is extraordinarily well-suited for the task.”

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