SOLVENT HYDROGEN-BOND NETWORK IN PROTEIN SELF-ASSEMBLY - SOLVATION OFCOLLAGEN TRIPLE HELICES IN NONAQUEOUS SOLVENTS

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers i n reconstituted films is controlled by the concentration of soluble po lymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between th e triple helices as a function of the polymer concentration is measure d by x-ray diffraction. Exponential-like changes in the spacing with i ncreasing osmotic stress, qualitatively similar to the forces previous ly found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming th ree-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen- bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some thresh old where removal of the solvent becomes energetically favorable. No s olvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2- methoxyethanol, or N-methylformamide, the change in spacing with polym er concentration is intermediate between exponential-like and hard-wal l. These results provide direct evidence that the exponential repulsio n observed between collagen helices at 0-8-Angstrom surface separation s in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen sur faces.