MadSci Network: Biochemistry

Re: how will the decay of a carbon 14 in a amino acid affect the organism?

Date: Fri Mar 10 01:29:28 2000
Posted By: Mike Conrad, Post-doc/Fellow, Microbiology, UNC
Area of science: Biochemistry
ID: 950121444.Bc

How does the decay of a C14 affect the amino acid?  What happens if
the amino acid is in a protein?

That's a good question. Many people use radioactive materials in chemistry and biochemistry without thinking about what happens after the radioactive atom disintegrates. Briefly, in an amino acid (alpha carbon), the carboxy and amines would be released and, in water, would end up as CO2 and ammonia. The side chain could rearrange or settle down as RH or ROH. In a protein, the situation is more complicated. The reactive ends of the peptide could end up as COOH and NH2. But they could also rearrange or react with more protein to produce all kinds of complicated, ill-defined products.

But hey, chemistry is interesting! Let's take closer look at what would happen.

C14 has a half life of 5730 years. About one carbon atom per trillion is a C14. It's produced in the upper atmosphere when cosmic rays hit nitrogen. The rate of production equals the rate of decay so the level in the air stays relatively constant. That allows C14 to be used for dating, the less C14 left in a sample, the older it is. But note that the levels of C14 went up a few percent in the 60's with all the atom bomb tests - adjust your C14 dating accordingly!

When C14 decays, a neutron emits an electron (a beta particle) and the carbon turns into a nitrogen atom. You ask what would happen if the alpha carbon of an amino acid is C14 and it decays. The alpha carbon is the carbon that everything on the amino acid hangs on. First, the newly formed nitrogen can not make any of the chemical bonds that carbon does. Second, when the C14 decays, it has just emitted a beta particle with an energy of up to 154,000 election volts (154KeV). That literally blows the nitrogen right out of the molecule. No molecule can withstand the recoil from a nuclear disintegration like that. Chemical bonds have an energy of about 30 to 40 electron volts and it takes only about 10 to 20 eV to strip an electron off of an atom. The "Szilard-Chalmers Effect" is the term for atoms being ejected from molecules after nuclear emissions.

Then what happens? Well, everything hangs on the alpha carbon. This breaks the amino acid. If the amino acid is in a protein, it causes a break in the amide chain, the backbone of the protein. The protein most likely will become unstable and not be able to carry on its normal functions.

What's left? The molecular fragments formed at the break can have unpaired electrons, known as "free radicals". They can rearrange, or react with other molecules, or pick up a hydrogen from water. More likely, the molecule will break with too many electrons on one side and too few on the other. These charged radicals will react quickly with other molecules or pull H+ or H- or OH- from water.

But it's worse than that. Remember the 154 KeV beta particle?

The beta particle will tear along, colliding into molecules, knocking out electrons, ionizing water and making hydrogen peroxide and other reactive molecules. Much of the electon's energy will be dissipated from just exciting molecules, but still thousands of radicals can be formed. And high concentrations of OH- formed from ionized water can also damage organic molecules.

A 154 KeV beta particle can only travel about 0.3 mM through tissue. The dead layer of you skin absorbs 90% of them. But if you eat or inhale C14 or any radioactive nuclide, the emitted rays can do direct damage. And more dangerous than damage to protein molecules is damage to your DNA. Disintegrating C14 can cause breaks and mutations in DNA molecules. 100 rads of radiation in single exposure will give you radiation sickness and 400 rads has a 50% mortality. Cancer can originate from one cell growing out of control. Thus, you don't want to increase the chance that some unlucky beta particles do the type of DNA damage that can trigger cancer. All radioactive material should be handled properly with due care.

Mike Conrad

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