I don't know much about organic chemistry, but amino acids are fairly simple molecules, and if memory serves they've been found free-floating in molecular clouds. I'd tentatively bet at 100:1 odds that that the right isomers of at least the simpler ones will be found conditional on the experiment being run, since a comet's basically a giant slushball of compounds left over from those molecular clouds.
On the other hand, I'd put the odds at well under 1% that we'll see terrestrial-like isomer ratios. That would be tantamount to finding a biosphere -- an active one, not just the remains of one, since amino acids are photosensitive and tend toward an equilibrium isomer ratio unless biological processes are leaning on it.
since amino acids are photosensitive and tend toward an equilibrium isomer ratio
Well, amino acid racemization rates is high (under 1M years) at room temperature, but probably much lower at 3K or whatever the comet ambient temperature is, and the warm layers of the comet are regularly blown off, anyway. Not sure about the effects of X- and gamma rays on the racemization rate, but if it is anything significant, then yeah, little chance of finding anything but 50/50.
EDIT: actually, having looked a bit more, a deviation from equal ratio is quite common in extraterrestrial objects, and is an open problem, so the odds of finding something non-racemized are much higher than 1%. D'oh.
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