Study Pinpoints Rare Molecular "Transitions" as Possible Cause of Skin "Photoaging"
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
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
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
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,
"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