Proper nouns are usually capitalized - a primary-school lesson that all of us have learned.

Let's look at a substance found in a human body.

Arginine (/ˈɑrɡɪniːn/, abbreviated as Arg or R)1 is an α-amino acid. It was first isolated in 1886

Another one, this time a kind of poison

Solanine - Solanine is a glycoalkaloid poison found in species of the nightshade family (Solanaceae), such as the potato (Solanum tuberosum) and the tomato (Solanum lycopersicum).

Being a healthcare provider, I read several research papers, documents and webpages on enzymes or those sort of substances. They all have name, a proper name but none of those written content ever capitalize them. Why?

I also see some exceptions.

Hemp seeds are high in Arginine, an essential amino acid known to improve heart health and protect from heart disease.

But is it due to the comma there? As the following sentence is introducing what 'Arginine' is? But then, is there any rule that if you define a proper noun in the middle of the sentence, you capitalize its first letter and if you don't, you don't! I don't think so. :)

  • I gave in to the urge :)
    – oerkelens
    Aug 20, 2014 at 9:42
  • 6
    They aren't proper nouns at all. They're ordinary mass (uncountable) nouns, just like "water", "gold", "tea" or "furniture". If you think about it, every noun is a name, but not all are proper names. Aug 20, 2014 at 13:07
  • I'm quite sure the title part "Why are things the like not capitalized" isn't grammatical. 'The like' is a noun. Not an adjective or any other form.
    – user26486
    Aug 20, 2014 at 20:28

3 Answers 3


These names are usually formed according to some kind of system. Especially in chemistry "new" compounds are often named like that. Even a common word like alcohol gives rise to -ol compounds (like methanol) to indicate to a scientist that the two have certain chemical properties.

The compounds they name may or may not be in common use as nouns, and as such have lost their "name"-feeling.

If you want to capitalize arginine, you should, by rights, also capitalize alcohol, methanol and polypropylene. These are "names" given to substances, but they do not function like proper names.

In extremis, if you want to capitalize them, you should capitalize Water as well. It is, after all, the "proper name" given to dihydrogenoxide.

Of course, when a compound is given a trade mark name, it does get capitalized, even if the name seems to be systematically derived. However, if the name becomes very commonly used, it is possible that the trade mark name loses its "proper name" feel again, and we stop capitalizing it again.

Examples of that are Aspirin and Heroin (trademarks of Bayer), which are now written as aspirin and heroin. Something similar happened with nylon and rayon (trademark of duPont) - their Teflon seems to be going the same way.

  • 1
    It's irrelevant that the names come from a system. They're not capitalized because they're not proper nouns. They don't feel like proper nouns because they're not proper nouns. Aug 20, 2014 at 17:16
  • And, by the way, it's possible to have proper nouns that come from a system; these are still capitalized. For example, army regiments. Aug 20, 2014 at 17:40
  • That last bit is obvious, as Aspirin and Heroin and other Bayer trademarks followed a system, as did the duPOnt trademarks :)
    – oerkelens
    Aug 20, 2014 at 17:50
  • Perhaps you see nucleic acids and nucleotides capitalized sometimes because they are so often abbreviated. Some writers may be trying to emphasize those first letters so that the reader will recognize them later when they see them in abbreviated contexts (ACGT, for bases). I think it's still wrong, but might explain the discrepancy.
    – Jenn D.
    Aug 20, 2014 at 20:23

They're not capitalized because they're not proper nouns. A proper noun is one that is used to refer to a unique entity; "arginine" is used to refer to any sample of that particular chemical. It's no more a proper noun than "water".

  • It is funny to note that the same goes for animal species, but their Latin names are (often) capitalized. Random example: dung beetle. Yet those names is used to refer to any member of that species, not a unique entity (unless we call our cat Felix...). Just like with chemicals, we refer to the name of a species or a chemical compound. So it doesn't seem to be all that straight-forward...
    – oerkelens
    Aug 20, 2014 at 17:57
  • 2
    The convention is that only the genus of a binomial name is capitalized (so "Homo sapiens", not "Homo Sapiens" or "homo sapiens"). That's not because it's a proper noun; it's just an orthographic convention. Likewise, ornithologists seem to capitalize bird species names ("I saw a Golden Eagle") even though they're not proper nouns. Aug 20, 2014 at 18:43

This is not an issue regarding enzymes as such, it is common to all chemistry.

It is unusual for compounds to be written with capital letters. You do not see Sodium Chloride or Toluene but sodium chloride and toluene. Complex organic compounds such as enzymes are no different.

The presence of a comma in your example should make no difference.

Where you see capital letters used, I would expect this to be either a feature of the writing style of the journal or an error.

The only time I would expect to see capitalization is for branded compounds such as proprietary drugs. Perhaps the writer has just continued using an initial capital out of habit.

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