Amino acid vs. amino-acid [duplicate]

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?

Solution 1:

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