Modeling and spectroscopic studies of bisphosphonate-bone interactions. The Raman, NMR and crystallographic investigations of Ca-HEDP complexes

Raman spectroscopy was used to study the interactions of bovine bone, hydroxyapatite (HA, as a model of bone) and calcium hydrogen phosphate (CaHPO4) with 1-hydroxyethylidene-1,1-diphosphonic acid, CH3C(OH)(PO3H2)2 (HEDP, the oldest known member in the class of bisphosphonates (BPs) that is commonly used as (i) a reference compound for BP activity, a scale of a BP's potency, and (ii) a pain palliative agent). Raman spectra with diminished background fluorescence were obtained using a visible laser line of 514.5 nm. The Raman spectra of the products from the reaction of HEDP with bone, HA and CaHPO4 could be considered virtually identical. This strongly suggests that CaHPO4 forms first from the reaction of bone or HA with HEDP (which also acts as a strong acid), upon which free Ca2+ ions become available for complexation reactions with HEDP. Two complexes were observed using Raman spectroscopy for each of the interactions of HEDP studied here. This shows that HA can be substituted for bone in studies concerned with the interaction of bone with chemical compounds. Also, Raman spectroscopy can be utilized to distinguish between different complexes formed at the solid/solution interface. One of the two complexes has been further characterized using Nuclear Magnetic Resonance (NMR) spectroscopy, as well as single crystal and powder X-ray diffraction (XRD). This complex has been found to be calcium dihydrogen ethane-1-hydroxy-1,1-diphosphonate dihydrate (Ca(CH3C(OH)(PO3H)2)·2H2O). Molecular modeling of this calcium complex using Gaussian03 software confirmed the assignments of the Raman vibrational bands. © 2007 Elsevier Inc. All rights reserved.