> As far as anyone knows, the choice of chirality is arbitrary.

Many theorise that it is not arbitrary:
http://www.abc.net.au/science/news/stories/s924661.htm >>

This one suggested Serine pushed
all the other amino acids into a specific chirality
much like a alpha male dancer pushes females
into certain arm positions. Interesting.

I looked up the coding for Serine (the most 'observered frequency" of all amino
acids according to a chart from Non Darwinian Evolution, King, L.L. in Science
16, May 1978) 

And noted that it has 6 ways to be coded.
AGA (all purines)
AGG (all purines) or

UCU (all pyrimidines)
UCC (all pyrimidines) or

UCA (pyr, pyr, pur in wobble) or
UCG (pyr, pyr, pur in wobble)

Though I don't have an exact point,
I note the coding seems to really
be almost all purine, or all pyrimidine, and with my recent 
ideas, that seems to fit in there
somewhere - sort of.
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