An epimer is a term in chemistry referring to relations between organic molecules. One organic molecule is an epimer of another if it differs by only one chiral carbon. Epimers are different from enantiomers in that they are not mirror images. Epimers can only be formed with organic molecules of more than three carbons, so amino acids generally don't have them, while carbohydrates possess them in spades. Here's an example in Hallworth projection form:

   H   O
    \ //
     C
     |
   H-C-OH
     |
  HO-C-H
     |
   H-C-OH
     |
   H-C-OH
     |
     CH2OH

  D-Glucose, one of whose epimers is...

   H   O
    \ //
     C
     |
   H-C-OH
     |
  HO-C-H
     |
  HO-C-H
     |
   H-C-OH
     |
     CH2OH
  
  ...D-Galactose.

They only differ at the third, γ chiral center, making them epimers of each other. Epimers cannot be warped into each other without a chemical reacion, and they are not chemically similar. They really are different molecules altogether, even though the difference is only of alignment. This caveat is a classic example of the necessity to know more than just the chemical formula of a molecule.

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