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

Abstract

We used atomistic molecular dynamics (MD) simulations to study polyelectrolyte brushes based on anionic α,L-glutamic acid and α,L-aspartic acid grafted on cellulose in the presence of divalent CaCl₂ salt at different concentrations. The motivation is to search for ways to control properties such as sorption capacity and the structural response of the brush to multivalent salts. For this detailed understanding of the role of side-chain length, the chemical structure and their interplay are required. It was found that in the case of glutamic acid oligomers, the longer side chains facilitate attractive interactions with the cellulose surface, which forces the grafted chains to lie down on the surface. The additional methylene group in the side chain enables side-chain rotation, enhancing this effect. On the other hand, the shorter and more restricted side chains of aspartic acid oligomers prevent attractive interactions to a large degree and push the grafted chains away from the surface. The difference in side-chain length also leads to differences in other properties of the brush in divalent salt solutions. At a low grafting density, the longer side chains of glutamic acid allow the adsorbed cations to be spatially distributed inside the brush resulting in a charge inversion. With an increase in grafting density, the difference in the total charge of the aspartic and glutamine brushes disappears, but new structural features appear. The longer sides allow for ion bridging between the grafted chains and the cellulose surface without a significant change in main-chain conformation. This leads to the brush structure being less sensitive to changes in salt concentration.

Keywords: cellulose; mineralization; molecular dynamics simulation; poly(amino acids); poly-(α,L-aspartic acid); poly-(α,L-glutamic acid); polyelectrolyte brushes.

Figure 1

Grafted cellulose model. Side view of a model cellulose layer in a simulation box (a). Spatial distribution of grafted chains at 12% (b) and 25% (c) substitution. Green surface: cellulose crystal. Red: oxygen; cyan: carbons; white: hydrogens; blue: nitrogen; pink: K⁺ ions. Water molecules are not shown for clarity.

Figure 2

Number density profiles of the brush based on the oligomers of aspartic (solid lines) and glutamic (dashed lines) acids. The density profiles of cellulose, brush, water, and K⁺ ions are shown by green, black, blue, and red lines, respectively. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution. Insets show K⁺ ion distributions on a larger scale for clarity.

Figure 3

Number density profiles of the chain ends of the grafted oligomers of aspartic (solid lines) and glutamic (dashed lines) acids. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution. The green line indicates the density of the cellulose layer.

Figure 4

Average cosine of the angle between the vectors connecting Cα atoms of neighbor residues and the axis perpendicular to the surface. Solid lines: aspartic acid oligomers; dashed lines: glutamic acid oligomers. (a) Systems with 12% substitution of primary hydroxyl groups, (b) systems with 25% substitution, and (c) scheme illustrating the angles.

Figure 5

Illustrations of two typical conformations of a glutamic acid oligomer: (a) chain lying on the cellulose surface and (b) chain directed away from the surface. Cellulose surface is green, the side chains are illustrated by lines, CPK representation is used for the backbone, and Cα atoms of the last residue are visualized using a larger size.

Figure 6

Radial distribution functions between the cellulose surface (hydrogens of surface hydroxyl groups) and amino acid backbone (O of amide groups). Solid lines: aspartic acid oligomers; dashed lines: glutamic acid oligomers. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution.

Figure 7

Density profiles of the chain ends of the grafted glutamic acid oligomers for the system with 12% substitution of primary hydroxyl groups by glutamic acid oligomers. Red line: System without partial charges on the cellulose molecules. Blue lines: with partial charges (unmodified force field).

Figure 8

Number density profiles of the brush for (a,b) aspartic acid brushes and (c,d) glutamic acid brushes. The density profiles of cellulose, brush, Ca²⁺, K⁺, and Cl⁻ ions are shown by green, purple, black, red, and blue lines, respectively. Insets show close-ups of brush distributions. (a,c) Systems with 12% substitution of primary hydroxyl groups and (b,d) systems with 25% substitution. CaCl₂ concentration is 0.15 mol/kg.

Figure 9

Dependence of the number of ions per carboxyl group (Ca²⁺, K⁺, Cl⁻) on CaCl₂ concentration. Ca²⁺, K⁺, and Cl⁻ ions are shown by black, red, and blue lines, respectively. Solid lines: aspartic acid brushes; dashed lines: glutamic acid brushes. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution.

Figure 10

Total charge of the brush as a function of CaCl₂ concentration. Solid lines: aspartic acid brushes; dashed lines: glutamic acid brushes. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution.

Figure 11

Dependence of the brush height on CaCl₂ concentration. Solid lines: aspartic acid brushes; dashed lines: glutamic acid brushes. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution.

Figure 12

Snapshots illustrating Ca²⁺ bridges in (a) glutamic acid and (b) aspartic acid brushes.

Figure 13

Ca²⁺-Ca²⁺ radial distribution functions for the systems with 0.94 mol/kg CaCl₂ concentration. Solid lines: aspartic acid brushes; dashed lines: glutamic acid brushes. (a) Systems with 12% substitution of primary hydroxyl groups and (b) systems with 25% substitution.