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A density functional theory study of amino acids on Mg and Mg-based alloys

John Bolin, Amanda Goold, Olof Hildeberg, Alva Limbäck, Elsebeth Schröder

TL;DR

The paper addresses improving Mg-based degradable implants by using biocompatible amino-acid coatings to control corrosion. It uses density functional theory with the vdW-DF-cx functional to quantify adsorption of glycine, proline, hydroxyproline, and a Gly-Hyp-Pro collagen snippet on Mg(0001), plus the effect of sparse alloying with Li, Zn, and Al, and a water environment modeled by SCCS. Adsorption energies for amino acids on clean Mg(0001) lie around $-1.0$ to $-1.6$ eV, with Hyp showing strong O-Mg coordination; alloying generally increases adsorption energy by up to about $0.11$ eV per molecule, especially when O groups are near the alloy. In a water-like environment the energies decrease by roughly $0.4$–$0.5$ eV but binding persists, and solvation effects largely cancel when comparing full and isolated-molecule energies. The results support amino-acid coatings as a biocompatible route to slow Mg corrosion and suggest extending the study to other Mg surfaces and defects.

Abstract

Magnesium (Mg) has mechanical properties similar to bone tissue, and Mg ions take part in the metabolism. This makes Mg of interest for biocompatible degradable body implants, provided that its high corrosion rate can be inhibited. Slightly alloying Mg and adding surface coatings can slow down the corrosion processes without significantly changing the mechanical properties. Use of coating molecules that are native to the body increase the likelihood of making the surface biocompatible, for example by use of amino acids. We here present a density functional theory (DFT) study of the adsorption on Mg(0001) of the amino acids glycine, L-proline, and L-hydroxyproline (Hyp), the main amino acid content of collagen. We investigate how binding of the functional groups of Hyp are affected when Mg(0001) is slightly alloyed with zinc, lithium or aluminium, and we also model the immersion of the systems in a water environment to see how this affects the binding.

A density functional theory study of amino acids on Mg and Mg-based alloys

TL;DR

The paper addresses improving Mg-based degradable implants by using biocompatible amino-acid coatings to control corrosion. It uses density functional theory with the vdW-DF-cx functional to quantify adsorption of glycine, proline, hydroxyproline, and a Gly-Hyp-Pro collagen snippet on Mg(0001), plus the effect of sparse alloying with Li, Zn, and Al, and a water environment modeled by SCCS. Adsorption energies for amino acids on clean Mg(0001) lie around to eV, with Hyp showing strong O-Mg coordination; alloying generally increases adsorption energy by up to about eV per molecule, especially when O groups are near the alloy. In a water-like environment the energies decrease by roughly – eV but binding persists, and solvation effects largely cancel when comparing full and isolated-molecule energies. The results support amino-acid coatings as a biocompatible route to slow Mg corrosion and suggest extending the study to other Mg surfaces and defects.

Abstract

Magnesium (Mg) has mechanical properties similar to bone tissue, and Mg ions take part in the metabolism. This makes Mg of interest for biocompatible degradable body implants, provided that its high corrosion rate can be inhibited. Slightly alloying Mg and adding surface coatings can slow down the corrosion processes without significantly changing the mechanical properties. Use of coating molecules that are native to the body increase the likelihood of making the surface biocompatible, for example by use of amino acids. We here present a density functional theory (DFT) study of the adsorption on Mg(0001) of the amino acids glycine, L-proline, and L-hydroxyproline (Hyp), the main amino acid content of collagen. We investigate how binding of the functional groups of Hyp are affected when Mg(0001) is slightly alloyed with zinc, lithium or aluminium, and we also model the immersion of the systems in a water environment to see how this affects the binding.
Paper Structure (14 sections, 2 equations, 6 figures, 3 tables)

This paper contains 14 sections, 2 equations, 6 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Material: Molecules and surfaces
  3. Molecules
  4. Surfaces
  5. Methods
  6. Density functional theory calculations
  7. Alloying considerations
  8. Water environment
  9. Results and discussion
  10. Mg(0001) surfaces and alloys
  11. Amino acids on clean Mg(0001)
  12. Adsorption on alloy surfaces
  13. In a water environment
  14. Conclusion

Figures (6)

  • Figure 1: Top panels: The three amino acids studied here - glycine (Gly, C$_2$H$_5$NO$_2$), L-proline (Pro, C$_5$H$_9$NO$_2$), and L-hydroxyproline (Hyp, C$_5$H$_9$NO$_3$). Bottom panel: Gly-Hyp-Pro snippet of a strand in the collagen (Col-I) triple helix. Shown atoms are oxygen (red), nitrogen (blue), carbon (black), and hydrogen (white).
  • Figure 2: Top panel: Positions of the alloying atoms in Mg(0001) with adsorbed L-hydroxyproline (Hyp), investigated alloy positions (one at a time) are indicated by green, Mg atoms are grey. Naming of the alloy positions are atom 63, 88 and 118, left to right. Hyp is shown in its relaxed position on top of a clean Mg(0001) surface, after relaxation with an alloying atom in one of the three indicated positions all atoms move, both surface and Hyp atoms. Bottom panel: Relaxed position of Hyp on Mg(0001) with Zn (violet) in atom position 88. Compared to the starting position in the top panel Hyp has rotated to keep one Mg-O binding and move the O of the hydroxy group loop from on top of the Zn atom to on top of a neighbouring Mg atom of the top layer. A similar rotation is seen when Zn is in the position of atom 118, thus avoiding O on top of Zn.
  • Figure 3: Left to right: Glycine in two energetically equivalent positions, L-proline, and L-hydroxyproline, adsorbed on Mg(0001) in vacuum, top (top panels) and side view (bottom panels). Only the central part of Mg(0001) is shown. Color coding as in Figures \ref{['fig:molecules']} and \ref{['fig:dopingMg']}.
  • Figure 4: Glycine adsorbed partly into Mg(0001), side and top view.
  • Figure 5: Sketch of the two top layers of Mg (gray) in Mg(0001), with an alloying atom indicated (green), and exaggerated vertical position changes. The $\Delta h$ values are negative if the measured atoms are below the plane.
  • ...and 1 more figures