Modulation of Hydroxyapatite Nanocrystal Size and Shape by Polyelectrolytic Peptides

Interactions of synthetic peptides containing charged amino acids with hydroxyapatite (HAP) have been used as models of non-collagenous proteins' interaction with bone's minerals. While there is some knowledge of the affinities of these peptides with HAP, the effects of the type of charged peptide, peptidic molecular weight, and concentration oil the crystal morphology have not been systematically investigated. Such knowledge could be useful for identifying the peptides ideal for in vitro or in vivo administration to customize crystal growth or for coating of implant surfaces for improving anchorage to neighboring bone tissue. This study used charged peptides of arginine (Arg), aspartate (Asp), glutamate (Glu), and lysine (Lys) to influence the HAP morphology. A systematic approach evaluated the polyelectrolytic peptides' affinity for HAP as well as their effects on mineral nucleation/growth kinetics and crystal morphology. Negatively charged polymers (Asp, Glu) had greater affinity for hydroxyapatite than positively charged polymers (Arg, Lys). Polymers of higher molecular weight (HMW) had greater affinity for HAP than their lower molecular weight (LMW) counterparts. Negatively charged polymers at lower concentrations created more mineral mass at greater crystallinity. Therefore, polymers of greater affinity have the most effect on crystal growth kinetics. However, these effects are inhibited at higher concentrations. Generally, the addition of any polymer decreased the dimensions of the HAP crystals formed with respect to control. Poly-L-Asp LMW, poly-L-Glu LMW, and poly-L-Lys HMW were found to significantly affect crystal morphology increasing the aspect ratio in comparison to the control. In conclusion, these results Suggest that polyelectrolytic peptides may be useful in vitro or in vivo administration to customize size and shape of HAP nanocrystals or for coating of implant surfaces for modulation of implant attachment to neighboring bone tissue.