Influence of Calcium Binding on Conformations and Motions of Anionic Polyamino Acids. Effect of Side Chain Length

Abstract

Investigation of the effect of CaCl2 salt on conformations of two anionic poly(amino acids) with different side chain lengths, poly-(α-l glutamic acid) (PGA) and poly-(α-l aspartic acid) (PASA), was performed by atomistic molecular dynamics (MD) simulations. The simulations were performed using both unbiased MD and the Hamiltonian replica exchange (HRE) method. The results show that at low CaCl2 concentration adsorption of Ca2+ ions lead to a significant chain size reduction for both PGA and PASA. With the increase in concentration, the chains sizes partially recover due to electrostatic repulsion between the adsorbed Ca2+ ions. Here, the side chain length becomes important. Due to the longer side chain and its ability to distance the charged groups with adsorbed ions from both each other and the backbone, PGA remains longer in the collapsed state as the CaCl2 concentration is increased. The analysis of the distribution of the mineral ions suggests that both poly(amino acids) should induce the formation of mineral with the same structure of the crystal cell.

Effects of Amino Acid Side Chain Length and Chemical Structure on Anionic Polyglutamic and Polyaspartic Acid Cellulose-Based Polyelectrolyte Brushes

“…The brush height decreases rapidly with the addition of salt at low concentrations of CaCl2 and partially recovers at higher concentrations. This behavior is similar to what has been reported for chain sizes of free polyelectrolytes in multivalent solutions [32,69,70]. Moreover, similar to the behavior of anionic poly (amino acid) chains [32], glutamic acid brushes remain folded over a wider concentration range than aspartic acid brushes.…”

“…The differences in the side chains also lead to difference in brush structures in CaCl2 solutions. In the case of low grafting density at which the grafted chains do not interact by steric interactions, the behavior of the chains is similar to the behavior of polyelectrolyte chains in a multivalent salt [32,69,70]. The longer side chain of glutamic acid helps to spatially distribute the adsorbed ions inside the brush.…”

“…Calcium ions in the brush tend to interact with two carboxyl groups connecting them and form long-living bridges [32]. At 25% surface modification, the distance between the grafting points is 1 nm (see Figure 3c), which is very close to the distance between the backbones of the grafted glutamic acid oligomers connected by a Ca 2+ bridge.…”

“…Recent results show, however, that this issue needs more attention. For example, despite the very similar chemical structures of the α,L-oligomers of glutamic and aspartic acids (Figure 1a, b) it has been shown that they interact differently with multivalent salts [32][33][34], and Thula et al have demonstrated a significant difference in organic matrix mineralization by various anionic poly(amino acids) [34]. Similarly, Picker et al have shown that aspartic and glutamic acids have qualitatively different effects on calcium carbonate crystallization [33].…”

Effects of Amino Acid Side-Chain Length and Chemical Structure on Anionic Polyglutamic and Polyaspartic Acid Cellulose-Based Polyelectrolyte Brushes

“…Recent results show, however, that this issue requires further attention. For example, despite the very similar chemical structures of the α,L-oligomers of glutamic and aspartic acids ( Figure S1a,b ), it has been shown that they interact differently with multivalent salts [ 32 , 33 , 34 ], and Thula et al have demonstrated a significant difference in organic matrix mineralization by various anionic poly(amino acids) [ 34 ]. Similarly, Picker et al have shown that aspartic and glutamic acids have qualitatively different effects on calcium carbonate crystallization [ 33 ].…”