Influence of polyaspartic acid and phosphophoryn on octacalcium phosphate growth kinetics

Polyaspartic acid (PAA) and phosphophoryn (PPn) have been suggested to adsorb specifically on the (100) and (010) faces, respectively, of octacalcium phosphate (Ca₄H(PO₄)₃ · 2.5H₂O, OCP). In this study, the extent of adsorption and influence of these molecules on OCP crystal growth has been investigated. For kinetic studies, protein effects on crystal growth were examined in solutions sustained at a constant level of supersaturation at pH 6.00 and ionic strength of 0.08 mol l⁻¹. The maximum adsorbed mol surface concentration for PPn was 100-fold less than that for PAA. Inhibitory effects interpreted in terms of mol surface coverage showed PPn to retard OCP growth more effectively than PAA. However, when considering percentage of crystal face covered by protein, PAA and PPn showed similar maximum adsorption concentrations onto the (100) and (010) crystal faces, respectively. PAA inhibited OCP growth by 20% when only 1% of the (100) face (1% total crystal area) was covered. PPn had to reach over 200% (010) face coverage (or 28% total crystal area) before a similar level of crystal growth inhibition was obtained. This difference in inhibitory effect may be the result of a more effective β-strand conformation of the shorter PAA molecule or may indicate that growth at the (100) face is rate controlling and, therefore, less than 1% coverage of this face is needed before a significant decrease in rate is observed.