engleski

The interaction of small and macromolecules with growing calcium hydrogenphosphate dihydrate crystals

Calcium hydrogenphosphate dihydrate (DCPD, CaHPO4 . 2H2O) is a frequent constitutent of kidney stone and is used as additive to cattle feed and in the cosmetics and pharmaceutical industries for different applications (e.g. abrasive in tooth paste). In most applications the size and shape of the crystals is of utmost importance. Some of us have shown earlier (Lj. Brecevic et al., Colloids and Surfaces, 11(1984) 55-68) that the growth morphology of DCPD may be changed by the addition of di- and tricarboxylates to the crystallizing solution. In this work we report on the influence of a number of small and macromolecular potential inhibitors, i.e. glutamic and asparticacid, citrate and polyaspartic acid (MW 5-10 kD) on the growth morphology of DCPD crystals. The crystals were prepared under controlled conditions by fast mixing of the anionic and cationic reactant solutions, and growing them without further stirring in the course of 24h. The initial conditions were: pH 5.5, reactant concentrations (in mol dm-3): c(Ca) =c(PO4) = 0.021 and c(NaCl) = 0.3. The respective additive was added to the anionic component prior to pH adjustment. The affected crystal faces were identified by light and scanning electron microscopy and electron diffraction. In addition, the presence of carbon at different locations of the affected crystals was determined by wavelength dispersive X-ray spectrometry. In the control systems crystals grew slowly and mostly large (app. 200 ?m) platelets with prominent (010) faces were obtained. Glutamic and aspartic acid had no significant influence on the rate of growth and growth morphology. However both citrate and polyaspartic acid significantly inhibited DCPD crystallization and changed the morphology of the crystals by specifically affecting different crystal faces. When grown in the presence of citrate ions the crystals appeared rod - like, while in the presence of polyaspartic acid the basic orientation was as in the controls, but new faces with Miller indices (h0l) appeared on acount of the (001) and (201) faces, that developed in the controls. The results are discussed taking into account the ionic structure of the affected crystal faces.

crystal growth; calcium hydrogenphosphate; molecular recognition

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