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I'm reading some text (purchase request) that has the following line in it.

(-)-Norepinephrine

What does the minus sign mean in front that is in parenthesis? Is this some kind of standard way of describing charge of the molecule or something? Or is this someone's way of representing something else? Or does this mean nothing?

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The minus sign in parentheses here indicates the optical rotation, (+) means the molecule rotates linear polarized light clockwise, (-) means it rotates counterclockwise. Typically you would also write exactly which stereoisomer is meant (R or S), which is generally the more useful information.

The following are all valid names for the natural stereoisomer of norepinephrine:

  • D-norepinephrine
  • D-(-)-norepinephrine
  • (R)-norepinephrine
  • (R)-(-)-norepinephrine

I'd recommend that you take a look at the concept of Chirality if you're not already familiar with it.

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