Oligo(l-glutamic acids) in Calcium Phosphate Precipitation: Mechanism of Delayed Phase Transformation

Proteins and their mimics that contain negatively charged sequences are important in natural and biomimetic mineralization. The mechanism by which these sequences affect calcium phosphate mineralization is not well understood. Here, peptides containing different numbers of repeat units of contiguous glutamic acid residues, oligo(l-glutamic acid) (n = 3, 7, 8, 10), were investigated with regards to the mechanism in delaying the crystallization of amorphous calcium phosphate (ACP) while holding the amount of carboxylic acid groups in solution constant. Increasing peptide chain length increases the stability of ACP at a certain total amount of carboxylic acid groups in solution. This effect is shown to be due to stronger binding as well as binding to more calcium ions per peptide by the longer oligopeptides compared to the shorter ones. It is proposed that these associations delay the structural rearrangement of calcium ions and the dehydration of ACP, which are required for the crystallization of hydroxyapatite. The initial nucleation and the local structure of ACP, however, do not vary with chain length. This second part of a two-part series provides an improved mechanistic understanding of how organic additives, especially those with contiguous acidic amino acid sequences, modulate the kinetics of calcium phosphate precipitation and phase transformation.