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37- Self Assembly of Artificial Proteins.docx

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CHEM 204
Christopher Barrett

CHEM 204 04/12/2013 Lecture 37 Martin M. PHGY Tutor Self Assembly of Artificial Proteins A problem with implants is that their surfaces are not bio-compatible and quickly rejected by our bodies. An electrode in the brain can only be there for 2 hours before rejection. The goal is to coat the surfaces of contact lenses, sutures, stents, artificial limbs, bone grafts, dentistry, etc. It's not a chemistry problem, the charge/surface ions/protein absorption/water content/physical morphology are all important. We need:  Tunable chemistry  Hydrophilicity  Thermodynamic minimum  Stable layers  Time stability  Controlled water content  Ion content  Modulus  Application Originally people couldn't wear contacts for more than a day. We thought they were just dirty so they put on a chemical paint but that didn't work either. We need to change the electrochemistry and water content! Implant materials are way too dry and too hydrophboic and too hard -- by a factor of a million! Biology is wet, gooey, soft, charged, ion rich, and pliable! How does nature do it? Theory: How a protein is spread or absorbed onto a surface depends on its Gibbs energy. Recall that a polymer chain of N units of size "a" has an equilibrium shape of Ro = aN .3/5 The surface its bonding to has 'stickers' of bond strength δ J/mol. Now we have to find the radius at equilibrium. Well, the exposed surface 2 area under the coil is π r , and assuming the stickers are 'b' apart, then the number of CHEM 204 04/12/2013 Lecture 37 Martin M. PHGY Tutor stickers the area it covers is = π r b–2  The higher the coil is the more wet and soft it is. 2 –2 3 each bond has H gain –δ, ΔH tot = –δ π r b J/mol. The initial coil volume = Ro and the final volume V = hr . If we consider the density to be conserved, we define h in terms of Ro & r: h = –2 3 –2 V r = Ro r (it is easiest to keep track of the coil height) the coil entropy ΔS = –3/2 [ h /R 0 R /h0] R2 4 4 4 4 ΔS = –3/2 [ r /R 0 + R /0 ] R We have defined our Δ adsorb and Δ deform, so Δ spread = Δ adsorb — TΔ deform 2 –2 4 4 4 4
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