Disagreement between calorimetric and van't Hoff enthalpies of assembly of protein supramolecular structures.

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Abstract
The effect of temperature on the extent of association of self-assembling protein polymers is expressed mathematically in terms of the van''t Hoff enthalpy of polymerization, .DELTA.Hv.H.. This quantity is experimentally defined in 2 ways.sbd.from the respective temperature derivatives of the critical polymerization concentration and of the fractional conversion of protein into polymer. These 2 definitions are shown not to be identical, except in certain limits. In terms of both definitions, it is shown that .DELTA.Hv.H. depends not only upon the enthalpy changes but also upon the corresponding equilibrium constants for the various equilibria involved in polymer formation. This has 2 consequences: large .DELTA.Hv.H. values may result from reactions having small calorimetric enthalpy changes; .DELTA.Hv.H. can depend strongly on temperature. These considerations are applied to 2 systems for which there exist considerable experimental data.sbd.namely, Hb S and tubulin. The large discrepancy between the calorimetric and van''t Hoff enthalpies for the polymerization of tubulin is explicable in terms of these considerations.