Re: Radioactive decay and long-term preservation

Robert J. Bradbury (
Thu, 21 Oct 1999 07:21:55 -0700 (PDT)

I'm correcting any mis-statements I may have made by providing the exact data (this is why we need our brains hardwired to the data!).

C12 generation:
n + 14N --> 14/6C + p

The neutrons coming from intereactions of cosmic rays in the upper atmosphere. [Interesting from the perspective of nearby supernovas since that will "bump" the production of 14C and make things look younger than they would typically be dated as.]

C12 breakdown:
14/6C --> B- (an electron w/ 0.15658 MeV) + 14/7N (stable) Abundance: 1 in 1,000,000 (10^-6); (Spike's recollection) Half life: 5715 or 5730 years (different sources). In so far as the N causes interesting bond rearrangements in the DNA, Spike may be onto something.

K40 breakdown:
40/19K --> B- (e- w/ 1.32 MeV) + 40/20Ca (stable) Natural abundance: 0.0117% (10^-4)
Half life: 1.26*10^9 years.

Looking at the DNA bases themselves, C count is: A(5); C(4); G(5), T(5); U(4) -- Average C count for DNA: 4.75 N count is:
A(5); C(3); G(5); T(2); U(2) -- Averge N count for DNA: 3.75

Seems interesting that we already have a lot of N, though this could simply be due to the fact that we started from a protein soup.

In addition you have 1 ribose per base in the backbone with 5 more C's.

Now the biological effect of C-->N in a DNA base is going to vary depending on the atom. It is the N's and O's that are involved in cross-strand hydrogen bonding, so this will not impact on that. If you change a C-->N in a methyl group (CH3), you probably end up with NH2, which probably isn't going to effect much. If you change a C that has no attached H's, then you might break one of the C-N rings in the bases which would definately be messy.

So looking at a duplex genome size of 6 billion bases it looks like you have (4.75+5)*6*10^9 = 58.5*10^9 C atoms. If we take Spikes estimate of 14C as 1*10^-6 (which I don't have confirmation of), then that gives us a count of 58,500 14C atoms in the genome of each cell.

However, the DNA backbone and bases in your non-dividing cells *does not* turn over very much (damage & repair occur at some moderately low fixed rate. [We could probably look at the Ames estimates for oxidative "hits"/day as a start on this, but that would require a trip to the library since there is some controversy on getting those assays to be accurate.] My guess is that Spike's suggestion will not work because unless the DNA recycling rate is very high, the only way to replace the 12C atoms with 14C atoms will be to wait for most of them to decay (which will be tens of thousands of years). The other alternative is nuclear abortion and replacement with nanobots (as I suggested at Extro4 based on the capabilities outlined in Nanomedicine). Since the nanobots can weigh the bases to atomic accuracy, they can replace the DNA with pure, unadulterated 12C DNA.

When considering the relative danger of radition, it is important to consider the "relative biological effectiveness" which according to Van Nostrand's Scientific Encyclopedia is as follows:

       Type            rbe
   X- & gamma rays	 1
   Beta rays             1
   Alpha rays           20
   Fast neutrons        10
   Slow neutrons         5

Radiation is measured in roentgens (R) which equals an energy value of 83.8 ergs/g of air or ~93.8 ergs/g in body tissue. The REM values (roentgen equivalent man) are derived from the rbe * R.

So, now how do 14C and 40K stack up in overall radiation dose?
>From Nanomedicine, Chp 3 (online at the Foresight Inst), you
have 365x as many C atoms as K atoms in your body. So we have more C atoms but radioactive K is more abundant and gives a greater radiation dose. Looks like:
(1.32/0.1536) * ((1.117*10^-4)/10^-6) * ((2.2*10^24)/(8.03*10^26)) So *if* they decayed at the same rate we would have 2.63 x the radiation dose from 40K vs 12C. But that isn't the case since the 12C is decaying about million times faster than 40K. So my previous "guess" was incorrect, 12C is more significant than 40K.

However, my comments on other radiation sources seem to have some merit. Radon is nasty stuff, various isotopes have half lives of seconds to minutes and mostly decay through Beta particles. Iodine and Cesium fall seem to follow similar patterns. So even though their abundances are lower, the RBE of alpha particles and the much more rapid decay rates make them good candidates for causing more of the real damage.

In theory we could take the energy of the beta, alpha, etc. particles, from the various radioactive isotopes in the body and compute the REM values delivered for all of the atoms in the body. But this sounds like about a weekend of work so we will have to wait until one of us gets really ambitious.

Now, I'm going to put the 6 books spread across my desk back in my library and go do something useful.