title: Interaction of calcium channel antagonists with calcium: structural studies on nicardipine and its Ca2+ complex.
abstract: Conformational features of nicardipine in acetonitrile, in the absence and presence of Ca2+, were investigated by one-dimensional NMR and difference absorption spectroscopy techniques. The data show that in acetonitrile solution the antiperiplanar form of nicardipine is dominant. The addition of Ca2+ to the drug solution caused marked changes in the difference absorbance spectra in the 200-400 nm region and in many of its 1H and 13C NMR resonances. The changes were most significant up to a ratio of 0.5 Ca2+:drug. Analysis of the binding data showed the predominant species to be a 2:1 drug:Ca2+ "sandwich" complex with an estimated dissociation constant of 100 microM at 25 degrees C. One-dimensional nuclear Overhauser effect (NOE) experiments revealed through-space connectivities in the drug before and after Ca2+ binding. These changes in conjunction with the changes in 1H and 13C chemical shifts suggest a structure in which the 4-aryl ring substitute of the pyridine moiety moves closer to the C3-side chain in the presence of Ca2+. This attraction is achieved via the chelation of the Ca2+ ion by the oxygen atoms in the m-NO2 of the aryl group and the COOCH2 group in the side chain of the dihydropyridine ring, and gives rise to a stable synperiplanar conformation. A preference for this conformation was also observed in the Ca2+ complex of nifedipine in acetonitrile as inferred from the rather limited NOE data obtained. Our study provides a detailed solution structure for nicardipine and also leads to a suggestion of a role for Ca2+ in the action of this and possibly other dihydropyridines.
authors.affiliation: Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada.
authors.forename: M P
authors.initials: MP
authors.lastname: Belciug
authors.forename: V S
authors.initials: VS
authors.lastname: Ananthanarayanan
mesh_terms: D000097:Acetonitriles; D002118:Calcium / Q000737:chemistry / Q000378:metabolism*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D009529:Nicardipine / Q000737:chemistry* / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D000097:Acetonitriles; D009529:Nicardipine; D002118:Calcium; C032159:acetonitrile