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III - Hydrophobic Surface Hydration

As polypeptides emerge from ribosomes, series of peptides which bear only hydrophobic non-hydrogen-bonding aliphatic and aromatic side chains, force surface water into covalent bonding all around the polypeptide chain. At any instant, only five or six water molecules are covalently-bonded together as a linear element and that element lasts for only an instant, but they generate such extreme order around the chain that it immediately begins absorbing energy from surface water into its peptide bonds to rotate the chain and decrease linear order. By rotating through numerous unstable, water-bridged conform-ational states (which will be illustrated later), polypeptide chains, like the one shown on the left below, will form a thermodynamically-stable coil.  In the process, all covalent linear bonding above and below the linear chain will be lost.  Destabilizing covalent structuring may still be present on hydrophobic surfaces of the coil but it will be dramatically reduced from that present on the linear chain and almost lost on the finished protein.

On the other hand, chains which emerge from ribosomes with hydrophobic peptides on one side and peptides which hydrogen-bond directly with surface water on the other side, retain their linear form long enough to form stable assemblies. With water forming relatively rigid layering on one side and dynamic hydrogen-bonding on the other side, chains are driven spontaneously toward the ordering side in a search of a complimentary ordering surface with which to combine, release ordered water and increase stability.

If the combining surface is a coil, a linear segment/coil assembly will form; if the surface is another linear chain, a beta sheet will form. With a distribution of polar, nonpolar and ionic peptides on the sides, the initial beta-sheet will search for other chains with which it can hydrogen-bond, release bridging water and widen the sheet. Remember, polypeptide folding from random linear states, as they are released from ribosomes, into single functional proteins is driven by the spontaneous movement of surface water from order toward disorder.

 Download PDF Book  Download PDF of this page  Coil Formation The absorbtion of energy from hydrophobic surfaces of polypeptides as water moves from order toward disorder drives polypeptides toward order.