How ultraviolet burns you

In What nuclear means I drew a line at about ten electron volts. A photon above the line can knock an electron off a molecule. A photon below it cannot. The ultraviolet in sunlight is below the line: the photons that burn you carry about 4 electron volts. Yet they damage DNA, cause sunburn and cause most skin cancer. This is how they do it without removing a single electron.

Two things a photon can do to an electron

An electron in a molecule cannot have just any energy. It sits on one of a set of allowed levels, like rungs on a ladder, and the gaps between the rungs are fixed by the molecule’s shape. The lowest rung is where the electron normally lives. Far above the top rung is the point where the electron is no longer attached at all.

A photon can do one of two things. If its energy matches a gap, it lifts the electron to a higher rung. That is excitation. The electron is still part of the molecule, but it now sits in a different orbit, and the molecule’s bonds are arranged differently for as long as it stays there. If the photon carries more than the whole ladder, it lifts the electron clean off. That is ionization, and for a molecule it costs 7 to 10 electron volts. If the photon matches no gap and is too weak to ionize, nothing happens: the molecule is transparent to it, and at most it picks up a little heat.

Excitation is the one that matters here. An excited molecule is, for a moment, a different chemical. Most of the time it drops back to the ground rung within a trillionth of a second and hands the energy on as heat, which is why a dark shirt is warm in the sun. But while it is up there, it can react in ways the ground-state molecule never would. This is the chemistry behind vision, photosynthesis and photographic film. It is also the chemistry behind sunburn.

Excite or ionize. One electron on a ladder, drawn for a DNA base. Set the photon's energy and send it. Below about 4 electron volts the molecule is transparent. Between 4 and 8 the electron is lifted to a higher rung and falls back a moment later. Above 8 it leaves. Sunlight at ground level, the band marked on the slider, only ever reaches the middle case.

The molecule that catches the light

DNA is a chain of four bases, adenine, thymine, guanine and cytosine, each a flat ring of carbon and nitrogen. All four absorb ultraviolet, most strongly at a wavelength of 260 nanometres, which is 4.8 electron volts. The absorption falls off steeply by 300 nanometres and is gone by 320. Visible light passes straight through, which is why a solution of DNA is colourless.

Ozone in the upper atmosphere removes almost everything below 290 nanometres before it reaches the ground. So the sun works on the tail of DNA’s absorption, the band from 290 to 320 nanometres called UV-B. Not much of it gets through, and DNA does not absorb it strongly. The product of the two is small. It is still enough.

Here is what happens when thymine absorbs a photon. The electron is lifted to a higher rung, and in that state the double bond in thymine’s ring is loosened. If the next base along the strand is another thymine, stacked just above it as bases are in a helix, the two loosened rings can bond to each other. Four carbon atoms, two from each base, close into a square. The two bases are now stapled together. The thing is called a cyclobutane pyrimidine dimer, and it forms about one picosecond after the photon arrives.

Count the electrons before and after. None left. They moved from one set of bonds into another. That is all excitation ever does, and it is the whole difference from ionizing radiation.

Most absorbed photons do not make a dimer. The neighbouring base has to be at the right angle at the instant the photon lands, and usually it is not, so the excited thymine simply falls back and the energy becomes heat. The yield is a few per cent at best. But sunlight delivers a very large number of photons, and a few per cent of a very large number is a great many staples per skin cell in an hour of midday sun.

Stapling thymines. One strand of DNA under a lamp. Choose the wavelength, or put something between the lamp and the strand. A photon that is absorbed next to a pair of pyrimidines sometimes staples them; the rest fall back as heat. Switch the repair enzymes on and watch each dimer get cut out with a patch of its neighbours and copied back from the other strand. The staple yield is exaggerated so you can see it; in a real cell it is a few per cent.

Why a staple matters

A dimer bends the helix by about thirty degrees and locks two bases that are supposed to be read one at a time. When the cell copies that strand, the copying enzyme stalls at the staple and, if it pushes through, it tends to read a cytosine as a thymine. The result is a specific error, C changed to T at a place where two pyrimidines sat side by side. This is the ultraviolet signature, and it is found in the p53 gene of most skin cancers and almost no internal ones. The lesion tells you which photons did it.

The cell knows about this and has a dedicated crew for it. Nucleotide excision repair finds the kink, cuts the strand on both sides of it, lifts out a patch of 24 to 32 bases, and copies the gap back from the undamaged strand opposite. A cell with too many staples to fix kills itself instead. Sunburn is what you feel when a layer of skin does that at once: the redness is blood flow to the damage, the peeling is the dead cells. People born without working excision repair, a condition called xeroderma pigmentosum, burn in minutes and get skin cancer at more than a thousand times the usual rate. That is the control experiment. Take away the repair and the dimer is the disease.

Two ways to break a molecule

Put this next to what an X-ray does, and the point of the ten electron volt line becomes clear.

An X-ray photon knocks an electron off whatever it happens to pass, which in a cell is mostly water. The damaged water makes a reactive fragment that attacks whatever is nearest, DNA included, at random: any base, either strand, sometimes both strands at once. That is a blow. Repair has to cope with every kind of break.

Ultraviolet makes the same lesion every time, in a predictable place, on one strand, with the other strand intact as a template. That is a key in a lock. It is why evolution could build a crew for it, and why skin, which gets the dose, and only skin, gets the cancer. UV-B does not reach past the outer tenth of a millimetre of you.

One more thing the ladder explains. UV-A, 320 to 400 nanometres, carries 3.1 to 3.9 electron volts and DNA barely absorbs it. It still damages skin, more slowly, by exciting other molecules that then pass the energy to oxygen. That is the slow route to tanning and wrinkles. Ordinary window glass stops nearly all UV-B and passes most UV-A, which is why you do not burn through a window but your skin still ages beside one.

Takeaway

A photon can excite an electron, lifting it to a higher rung in its molecule, or ionize it, removing it altogether. Molecules ionize at 7 to 10 electron volts.
Sunlight at ground level tops out near 4.3 electron volts. It never ionizes anything in you.
DNA absorbs ultraviolet. An excited thymine next to another thymine bonds to it within a picosecond, and the two are stapled together. No electron leaves.
The staple blocks copying and causes one specific mutation, C to T, the fingerprint of sunlight in skin cancer.
Cells cut the staple out with a patch of 24 to 32 bases. Sunburn is cells that gave up and killed themselves instead.
Ionizing radiation damages at random. Ultraviolet damages in one way, and the cell has a crew for exactly that one way.