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Xi Cheng, Peter M. Pinsky; A Molecular-level Model For Swelling Pressure In The Corneal Stroma. Invest. Ophthalmol. Vis. Sci. 2012;53(14):6794.
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It is proposed that structural interactions between collagen fibrils and proteoglycans (PGs) play a dual role in the corneal stroma. The fixed charges on the glycosaminoglycan (GAG) chains induce osmotic pressure which contributes to the swelling pressure. Some GAG chains bridge neighboring fibrils allowing them to provide entropic elastic forces that (negatively) contribute to the swelling pressure and restore collagen fibrils that are disturbed from their lattice locations. We describe a molecular-level model for the mechanics of the fibril-PG system and compare predictions for swelling pressure to experimental data including variation of hydration and bath ionic concentration.
An x-ray scattering investigation on drying corneas has elucidated the general spatial organization of the PGs in relation to the fibrils over a range of hydration. Stromal PGs appear to form a relatively dense coating around each fibril with markedly lower density in the interstitial space. We introduce a unit cell model for the corneal stroma in thermodynamic equilibrium in which negative charges associated with the PGs are partitioned into a fibril coating region and the interstitial space. Free energy contributions arise from electrostatic, entropic elastic and molecular mixing sources and the electrostatic potential is determined from Poisson-Boltzmann theory. The swelling pressure is obtained from the gradient of the free energy with respect to the swelling volume.
We compare swelling pressure predictions to experimental results for varying hydration values and emphasize the increasing importance of electrostatic repulsion as the PG coatings come into closer proximity at lower hydration. The model shows that osmotic pressure is only a partial contributor to the swelling pressure, particularly at low hydration. We provide predictions for swelling pressure under varying ionic bath concentrations, where ion binding and ion shielding effects are important.
The charge density resulting from the PG-dense coating on collagen fibrils is essential for explaining swelling pressure at low hydration where electrostatic effects dominate osmotic pressure. The calculation of collagen restoring force suggests that the PG-dense coating also plays an important role in maintaining the collagen lattice at lower hydration through electrostatic repulsion effects.
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