If I execute:

ChemicalData["L-Cysteine", "MolarMass"]

I get the following error:

ChemicalData::notent: "L-Cysteine" is not a known entity, class, or tag for ChemicalData. Use ChemicalData[] for a list of entities.


ChemicalData["L-Alanine", "MolarMass"]

works fine.

Is this a bug? How can I retrieve the molar mass of L-Cysteine?

  • 3
    $\begingroup$ Use a wildcard character: ChemicalData["L*Cysteine"] $\endgroup$
    – Bob Hanlon
    Apr 19, 2016 at 19:35
  • $\begingroup$ That is most unusual. ChemicalData["LCysteine", #] & /@ {"Name", "StandardName", "FormattedName"} gives {"L-cysteine", "LCysteine", "L-cysteine"} but ChemicalData["L-cysteine", "MolarMass"] also does not work. (It tells me that L-cysteine is not a known entity). In addition, the three letter codes (Ala, for example) work with Wolfram Alpha, but do not work with Mathematica. $\endgroup$
    – user1066
    Apr 20, 2016 at 8:46

2 Answers 2


One way to try to figure such things out is to use Free-Form Input.

Type a '=', type a phrase, and execute. You'll get an output cell, which you can click to turn into input cell. Then InputForm will often show you what you're looking for.

Mathematica graphics

We can see that, just as @becko has already found out, that the Mathematica name for L-cysteine is "LCysteine".

  • 1
    $\begingroup$ I think Ctrl-= is a better tool for this, as the output is the desired entity. $\endgroup$
    – Carl Woll
    Sep 18, 2018 at 20:48

I figured it out:

ChemicalData["LCysteine", "MolarMass"]

Turns out ChemicalData["LAlanine", "MolarMass"] also works. It seems that ChemicalData["L-Alanine", "MolarMass"] with the dash is a notational exception allowed only with alanine.

  • 2
    $\begingroup$ It seems to me that the "Name" and "FormattedName" used by Mathematica for the amino acids are of the form L-lowercase-name. ChemicalData["L-Alanine", #] & /@ {"Name", "StandardName", "FormattedName", "IUPACName", "AlternateNames", "CASNumber"} gives {"L-alanine", "LAlanine", "L-alanine", "(2S)-2-aminopropanoic acid", \ {"(2S)-2-aminopropionic acid"}, "CAS56-41-7"}. But the inconsistencies are frustrating, to say the least. $\endgroup$
    – user1066
    Apr 20, 2016 at 8:55

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