Interfacial Modeling of Fibrinogen Adsorption onto LiNbO3 Single Crystal-Single Domain Surfaces

Abstract

For the development of next-generation protein-based biosensor surfaces, it is important to understand how functional proteins, such as fibrinogen (FBG), interact with polar substrate surfaces in order to prepare highly sensitive points of medical care diagnostics. FBG, which is a fibrous protein with an extracellular matrix, has both positively and negatively charged regions on its 3-dimensional surface, which makes interpreting how it effectively binds to polarized surfaces challenging. In this study, single-crystal LiNbO₃ (LNO) substrates that have surface charges were used to investigate the adsorption of FBG protruding polar fragments on the positively and negatively charged LNO surfaces. We performed a combination of experiments and multi-scale molecular modeling to understand the binding of FBG in vacuum and water-solvated surfaces of LNO. XPS measurements showed that the FBG adsorption on LNO increased with increment in solution concentration on surfaces independent of charges. Multi-scale molecular modeling employing Quantum Mechanics, Monte Carlo, and Molecular Mechanics addressed the phenomenon of FBG fragment bonding on LNO surfaces. The binding simulation validated the experimental observation using zeta potential measurements which showed presence of solvated medium influenced the adsorption phenomenon due to the negative surface potential.

Keywords: LiNbO3; XPS; adsorption; biosensor; fibrinogen; molecular dynamics.

Figure 1

Zeta potential results of the single crystal-single domain LNO substrates with positive (001) and negative surfaces (00−1) (front and back side) after 10-min acid etching.

Figure 2

N/Nb ratio of LNO substrate by the XPS measurement as a function of the FBG concentration adsorbed on the surface.

Figure 3

(a) Atomistic structure model of FBG. The color code for atoms are as follows: grey (carbon), blue (nitrogen), red (oxygen), yellow (sulfur). (b) FBG electrostatic potential map and protruding positive (red part) and negative (blue part) charged surface fragments used for surface simulation studies, which corresponds to the αC and E domains as described in the text.

Figure 4

Morphology of LNO with possible surfaces; (001) and (00−1) labeled. The color code for atoms are as follows: cyan (niobium), pink (lithium), and red (oxygen).

Figure 5

(a)The adsorption configuration of positive protein fragment over the (00−1) LNO surface. The color code is cyan (niobium), pink (lithium), red (oxygen), grey (carbon), blue (nitrogen), yellow (sulfur). (b) The adsorption configuration of negative protein fragment over (00−1) LNO surface. The color code for atoms are as follows: cyan (niobium), pink (lithium), red (oxygen), grey (carbon), blue (nitrogen), yellow (sulfur).

Figure 6

(a) Electrostatic potential map of the LNO Nb rich (00−1) surface. The color code for atoms are as follows: cyan (niobium), pink (lithium), red (oxygen). (b) Electrostatic potential with Nb poor LNO (001) surface. The color code for atoms are as follows: cyan (niobium), pink (lithium), red (oxygen). (c) The optimized structure of the positive protein fragment over the LNO (001) surface. The distance between the amino hydrogen to oxygen of LNO surface and the distance between the oxygen moieties to niobium is shown. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen).

Figure 6

(a) Electrostatic potential map of the LNO Nb rich (00−1) surface. The color code for atoms are as follows: cyan (niobium), pink (lithium), red (oxygen). (b) Electrostatic potential with Nb poor LNO (001) surface. The color code for atoms are as follows: cyan (niobium), pink (lithium), red (oxygen). (c) The optimized structure of the positive protein fragment over the LNO (001) surface. The distance between the amino hydrogen to oxygen of LNO surface and the distance between the oxygen moieties to niobium is shown. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen).

Figure 7

(a) Simulation of positive protein fragment in the presence of water molecules with Nb poor (001) (001) surface. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen). (b) Simulation of the positive fragment in the presence of water molecules with Nb rich LNO (00−1) surface. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen). (c)-only-XYZ: mean square displacement of the positive fragment in the presence of water with Nb rich LNO (00−1) surface with the view only with checked directions in x, y, z. (d)-only-XYZ: mean square displacement of the negative fragment in the presence of water with Nb poor (001) surface with the view only with checked directions in x, y, z.

Figure 7

(a) Simulation of positive protein fragment in the presence of water molecules with Nb poor (001) (001) surface. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen). (b) Simulation of the positive fragment in the presence of water molecules with Nb rich LNO (00−1) surface. The color code for atoms are as follows: grey (carbon), blue (nitrogen), yellow (sulfur), cyan (niobium), pink (lithium), red (oxygen). (c)-only-XYZ: mean square displacement of the positive fragment in the presence of water with Nb rich LNO (00−1) surface with the view only with checked directions in x, y, z. (d)-only-XYZ: mean square displacement of the negative fragment in the presence of water with Nb poor (001) surface with the view only with checked directions in x, y, z.

Figure 8

Illustration of proposed binding of FBG binding on a chemical etched LNO surface on the Nb sites (not to scale).

Figure 9

(a) Atomistic structure of LNO (001) 3 × 3 × 1 surface model (b) LNO (00−1) 3 × 3 × 1 surface model created from LNO bulk structure. The color code for atoms are as follows: cyan (niobium), pink (lithium), and red (oxygen).