Re: UV Absorbance difference between Purines and Pyrimidines

On Sep 26, 1:17=A0pm, Tom Hendricks <tom-hendri...@xxxxxxx> wrote:
On Sep 24, 12:18=3DA0pm, Alan Meyer <amey...@xxxxxxxxx> wrote:


If purines and pyrimidines both have differences
two hydrogen bonds versus three
different UV absorbance
then that must account for some selection differences.
Only if the difference absorbance corresponded to different
photodecomposition. Some chromophores can absorb radiation without
decomposition. The benzyl-ring chromophore in aromatic amino acids is
an example of a chromophore that absorbs UV without a corresponding
decomposition. I suspect that the pyradine-ring chromophore in
pyradines is the same way.
In many posts you repeat the assertion that
absorption=3Ddecomposition. This is not always true.
Also, you do realize that a lot of chemical environments break
down double bonds without UV radiation. In fact, some chemical
processes break down the double a lot better than UV absorption. The
differential selection that you ascribe to UV may be instead caused by
a high or low pH.
You also have this belief that the only seasonal change
possible are changes in UV irradiance. Seasonal cycles, even though
originating in the sun, express themselves in different ways. The
seasonal cycle could, for instance, cause an oscillation in pH. Even
if you think oscillations are necessary for abiogenesis, and I do not
believe so, the oscillations can be due to changes in the chemical
environment.
Maybe the surface where life started was near a beach, which was
periodically covered with salt water by the tide. A dry-salt water-dry
cycle twice a day. Or maybe once every 15 days, as happens wide
supertidal zones. Then the cycles would be associated with the moons
gravity. The cycle, if you need one, may not be solar at all. It
certainly doesn't have to involve UV. It very well may involve solar
UV, but it doesn't have to.

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