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Spent a half hour googling this and the best I could find was this:

Now for some rhymes: Arginine = R. R we having fun yet? Asparagine = N The kNights of Ne say "Ne". Glutamine is a cute amine = Q I say "glutamate"/a former vice president says "glutEmate" = E ditto with AsparDic acid = D Fenylalanine makes tasty italien sausage = F Theres are two rings in tryptophan, and there are two v's in W = W Tyrosine = Y, Y? Because we love biochemistry. Source: One letter codes, rhyme and reason

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    $\begingroup$ Because glycine took G? $\endgroup$ – kmm Feb 26 '13 at 16:05
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    $\begingroup$ Did you want a mnemonic or did you want to know why it was assigned that particular code? The cute-amine was the mnemonic I learned. It may be related to the rationale behind the code, but that could be something else entirely. $\endgroup$ – blep Feb 26 '13 at 16:24
  • $\begingroup$ @dd3 I want to know why it was assigned that particular code $\endgroup$ – Robot-Scott Feb 26 '13 at 17:18
  • $\begingroup$ @Kevin That reasoning makes sense for Lysine (since its the letter before L) but Q is several letters away from G. Though I suppose Q could have been the next available. That may be the simplest answer, thanks. $\endgroup$ – Robot-Scott Feb 26 '13 at 17:24
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The Biology Project from the University of Arizona provides the best explanation of Dr. Margaret Oakley Dayhoff's logic for the non-obvious single letter AA abbreviations:

Glutamine ~ Q-tamine therefore, Q

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Initial letters of the names of the amino acids were chosen where there was no ambiguity. There are six such cases: cysteine, histidine. isoleucine, methionine, serine and valine. All the other amino acids share the initial letters A, G, L, P or T, so arbitrary assignments were made. These letters were assigned to the most frequently occurring and structurally most simple of the amino acids with these initials, alanine (A), glycine (G), leucine (L), proline (P) and threonine (T).

Other assignments were made on the basis of associations that might be helpful in remembering the code, e.g. the phonetic associations of F for phenylalanine and R for arginine. For tryptophan the double ring of the molecule is associated with the bulky letter W. The letters N and Q were assigned to asparagine and glutamine respectively; D and E to aspartic and glutamic acids respectively. K and Y were chosen for the two remaining amino acids, lysine and tyrosine, because, of the few remaining letters, they were close alphabetically to the initial letters of the names. U and O were avoided because U is easily confused with V in handwritten material, and O with G, Q, C and D in imperfect computer print-outs, and also with zero. J was avoided because it is absent from several languages.

Two other symbols are often necessary in partly determined sequences, so B was assigned to aspartic acid or asparagine when these have not been distinguished; Z was similarly assigned to glutamic acid or glutamine. X means that the identity of an amino acid is undetermined, or that the amino acid is atypical. "Nomenclature and Symbolism for Amino Acids and Peptides"

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    $\begingroup$ U is the code for selenocysteine. $\endgroup$ – terdon Feb 27 '13 at 10:35
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    $\begingroup$ IUPAC-IUBM add it in 1999.onlinelibrary.wiley.com/doi/10.1046/j.1432-1327.1999.news99.x/… $\endgroup$ – sviter Feb 27 '13 at 16:04
  • $\begingroup$ I know, it's just that I did my PhD on selenoproteins and everyone forgets selenocysteine and that makes me sad :(. $\endgroup$ – terdon Feb 27 '13 at 16:08

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