Every amino acid except glycine is chiral, existing as L and D forms that are mirror images. Some synthetic peptides contain D-amino acids by design, often to resist enzymatic breakdown in vitro.
D-forms can also appear as impurities when a residue racemises during synthesis. The resulting epimer has exactly the same mass as the intended peptide, so mass spectrometry cannot detect it; it is separated, where possible, by HPLC.
See also
For in vitro laboratory research only. Not for human or veterinary use. This library describes chemistry, analytical methods and published laboratory research; it gives no guidance on use in people or animals.