New calcium indicators and buffers with high selectivity against magnesium and protons: design, synthesis, and properties of prototype structures

Biochemistry. 1980 May 27;19(11):2396-404. doi: 10.1021/bi00552a018.


A new family of high-affinity buffers and optical indicators for Ca2+ is rationally designed and synthesized. The parent compound is 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a relative of the well-known chelator EGTA [ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] in which methylene links between oxygen and nitrogen are replaced by benzene rings. BAPTA and its derivatives share the high (greater than 10(5)) selectivity for Ca2+ over Mg2+ of EGTA but are very much less affected by pH changes and are faster at taking up and releasing Ca2+. The affinity of the parent compound for Ca2+ (dissociation constant 1.1 x 10(-7) M in 0.1 M KCl) may be strengthened or weakened by electron-releasing or -withdrawing substituents on the aromatic rings. The Ca2+ and Mg2+ affinities may further be altered by replacing the ether oxygens by heterocyclic nitrogen atoms. The compounds described are fluorescent Ca2+ indicators absorbing in the ultraviolet region; the very large spectral shifts observed on binding Ca2+ fit the prediction that complexation should hinder the conjugation of the nitrogen lone-pair electrons with the aromatic rings. Derivatives with quinoline nuclei are notable for their high sensitivity of fluorescent quantum yield to the binding of Ca2+ but not of Mg2+. Preliminary biological tests have so far revealed little or no binding to membranes or toxic effects following intracellular microinjection.

Publication types

  • Comparative Study

MeSH terms

  • Calcium*
  • Egtazic Acid* / analogs & derivatives
  • Egtazic Acid* / chemical synthesis
  • Ethylene Glycols* / chemical synthesis
  • Indicators and Reagents
  • Magnesium*
  • Methods
  • Spectrophotometry, Ultraviolet
  • Structure-Activity Relationship


  • Ethylene Glycols
  • Indicators and Reagents
  • Egtazic Acid
  • Magnesium
  • 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid
  • Calcium