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.
Highlights
Polyelectrolytes, such as anionic poly(amino acids), are widely used in diverse applications including water treatment and purification [1], anticorrosion agents [2], drug delivery [3,4,5,6] and tissue engineering [7,8,9,10,11]
We have previously shown that the behavior of both poly-(α-l glutamic acid) (PGA) and poly-(α-l aspartic acid) (PASA) depends on the counterion type [57]
Due to the restrictions of conformational transitions caused by the side-chain lengths of PASA and PGA, and the differences in the long-lived Ca2+ bridges, the complete statistical ensemble cannot be reached by classical unbiased number of possible conformational states
Summary
Polyelectrolytes, such as anionic poly(amino acids), are widely used in diverse applications including water treatment and purification [1], anticorrosion agents [2], drug delivery [3,4,5,6] and tissue engineering [7,8,9,10,11]. Polarizable force fields bridges in proteins [58] and between charged groups in polyelectrolytes [59] could reach 1 s, which are difficult to parameterize, are not available for all compounds and their use involves a considerably larger computational cost [53,54,55] Another solution to this problem is to add corrections into the electrostatic interactions of the existing classical force fields [47,48,49,56]. Lifetimes of ion bridges in Polymers 2020, 12, 1279 proteins [58] and between charged groups in polyelectrolytes [59] could reach 1 s, which is about six orders of magnitude longer than current typical MD simulations and three orders of magnitude longer than the current longest MD simulation performed on a special-purpose computer [60] This leads to the problem of correct sampling in classical MD [45]. In addition to studying polymer conformations, we provide a detailed description of the performance of the HRE method and give practical recommendations for simulations of similar systems
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