text: Most commonly, amino acids coordinate to metal ions as N,O bidentate ligands, utilizing the amino group and the carboxylate.  They are "L-X" ligands.  A five-membered chelate ring is formed.  The chelate ring is only slightly ruffled at the sp-hybridized carbon and nitrogen centers.
  For those amino acids containing coordinating substituents, the resulting complexes are more structurally diverse since these substituents can coordinate.  Histidine, aspartic acid, methionine, and cysteine sometimes form tridentate N,N,O, N,O,O, S,N,O, and S,N,O complexes, respectively.
  Using kinetically inert metal ions, complexes containing monodentate amino acids have been characterized.  These complexes exist in either the N or the O linkage isomers.  It can be assumed that such monodentate complexes exist transiently for many kinetically labile metal ions (e.g. Zn).