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In the following two examples, is there any difference as to how amino acid should be hyphenated?

  • There are twenty amino acids.
  • The amino acid content is 80%.

My intuition would be to hyphenate in the second example (where amino acid is a modifier) and not in the first, but I couldn't back it up.

Is there a general rule that would apply?

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    Google hyphen to verify your intuition.
    – Richard Kayser
    Commented Nov 18, 2016 at 14:35
  • @RichardKayser As a chemist you should know that some people use "amino-acids" indiscriminately.
    – Greg Slodkowicz
    Commented Nov 18, 2016 at 14:37
  • Googling "amino acid content" shows a reasonably even split between the hyphenated and unhyphenated forms. The rule 'hyphenate compounds when used to pre-modify' is nowadays usually relaxed by adding 'if this adds clarification'. I wouldn't, here.
    – Edwin Ashworth
    Commented Nov 18, 2016 at 15:07
  • Not trying to look smart. Just suggesting that you do some research on the use of hyphens before coming here. You can't go wrong with no hyphen in your first example and hyphen in you second example. Agree with @EdwinAshworth's comment on your second example.
    – Richard Kayser
    Commented Nov 18, 2016 at 15:18

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Your intuition is correct.

According to the International Union of Pure and Applied Chemistry's Guide to Nomenclature and Symbolism for Amino Acids and Peptides:

When the phrase 'amino acid' is a qualified noun it contains no hyphen; a hyphen is inserted when it becomes an adjective so as to join its components in qualifying another noun, e.g. amino-acid sequence

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    And, indeed, this is generally good practice. Why? Because it can prevent ambiguity ‘false scent’, especially where nouns are being used as adjectives.
    – David
    Commented Nov 18, 2016 at 17:58
  • Yes, but this is a domain-specific guide and not binding to general English usage. There was a dog's breakfast a few tears ago when the BrE spelling of sulphur had to be marked wrong in science exams in the UK, but the AmE (and IUPAC) spelling had to be marked wrong in English exams. ELU is not science-orientated, so here, either spelling is acceptable.
    – Edwin Ashworth
    Commented Nov 18, 2016 at 18:58

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