DURHAM, N.C.--Experimenting with lasers, a biophysical chemistry team now at Duke University has discovered rare, hard-to-detect interactions between skin molecules and sunlight that eventually could cause the uncomplimentary changes characteristic of "photoaging."
A group led by John Simon, Duke's George B. Geller professor of chemistry, used special sensors to analyze how molecules in the skin, called "chromophores," respond to a range of different wavelengths of ultraviolet light.
The investigators found that wavelengths in the ultraviolet-A (UV-A) range can cause sporadic weak molecular "transitions" in the chromophore urocanic acid. Those changes, they also discovered, can raise the energy level of oxygen molecules to a damaging "singlet" state.
UV-A, a commonplace group of wavelengths in sunlight that reaches Earth's surface, has already been linked to skin photoaging, which produces "deep lines," a "leathered appearance" and a "sagging of the skin's surface typically associated with old age," Simon and another researcher wrote in a report on the findings.
Authored by Simon and Kerry Hanson, his former doctoral student now at the University of Illinois in Champagne-Urbana, that report is published in the Sept. 1 issue of the Proceedings of the National Academy of Sciences. Their research, supported by the National Institute of General Medical Sciences, began at the University of California at San Diego, where Simon's team previously worked.
Urocanic acid, a breakdown product formed as the outer layer of cells in skin die, is among the chromophores known to absorb approximately the same wavelengths of sunlight as does DNA in living skin, Simon said in an interview. Urocanic acid thus was thought to help protect DNA in living skin from ultraviolet light damage, which can lead to cancer.
The molecule was even used for a time as a sun screen ingredient. But manufacturers in the United States agreed to a voluntary ban, Simon said, after George Washington University researchers discovered that a rearrangment of urocanic acid, called the cis-isomer, is formed by the absorption of sunlight and may impair the immune system.
Separate medical research also had established that normal exposure to the UV-A wavelengths of sunlight can cause the photochemical formation of the cis-isomer.
"There were many features of urocanic acid's photobiology that were not understood," Simon added. "One of them was that the chemistry of the molecule depends on the light wavelength that hits it, especially in the UV-A. We've spent the last two years sorting that out."
Using a special neodynium-YLF (yttrium lithium fluoride) laser, Simon's group began a detailed study of urocanic acid's "absorption spectrum" essentially what percentage of each different wavelength the molecule actually absorbs. The researchers also employed sensors that can detect how the molecule uses the light energy it absorbs over different scales of time.
With those tools, the team learned that urocanic acid responds to ultraviolet-A in very complex ways. Starting at UV-A wavelengths of about 320 billionths of a meter, urocanic acid retains an increasing amount of the sunlight energy it absorbs, reaching a maximum at 340, Simon's and Hanson's report said.
At its maximum, the absorbed energy is "almost three times the energy difference between the two isomers of UV-A," the authors wrote. "Consequently, we can conclude that photoisomerization is not the sole photochemical process that is initiated by UV-A exposure."
That additional energy, the authors' evidence suggests, forms an extra intermediate state of the molecule lasting longer than hundreds of billionths of a second. "Essentially, the absorption spectrum is comprised of transitions to two different electronic states, and those states react differently with different rates," Simon said.
The new transition that they discovered leads to an intermediate state that can transfer its energy to a nearby oxygen molecule to form damaging "singlet" oxygen molecules, the PNAS report added.
The term "singlet" refers to the spin states of a molecule's most excited electrons. With electrons in the singlet state, oxygen is likely to break existing chemical bonds and form new ones, Simon said. "That's why singlet oxygen is so devastating. It's highly reactive."
Examining the medical literature on the "action spectra" of trans-urocanic acid, action spectra are those wavelengths of light shown to induce physical changes, Simon's team concluded that singlet oxygen may be responsible for initiating chemical processes that lead to photoaging.
"Singlet oxygen has always been thought to be involved in photoaging," he added. "But there was never any direct link between a physiological action spectrum for that type of behavior and a molecular absorption process. Now there is."
The observation that singlet oxygen formation involves weak transitions may explain why photoaging is a long-term process, Simon said.
"Urocanic acid in UV-A doesn't absorb very strongly, so most of the photons (particles of light) will pass through," he said. "But, every now and then, it will grab a photon. And when it does, there's a good chance that's going to make singlet oxygen.
"The initiating molecule has a weak efficiency for absorbing the light, but when it does, it's highly efficient in causing the damaging event."