The association reaction of collagen model polypeptides (Pro‐Pro‐Gly)n
The characterization of recently synthesized (Pro‐Pro‐Gly)n, n = 7, 8 is described, along with melting profile studies of its association equilibrium, and thermal quenching studies of the kinetics of its association reaction. The order of the kinetic reaction is about 3, implying that three peptide chains are involved in the activated state of the rate‐limiting step. The reaction rate was found to exhibit a negative temperature coefficient. With the (Pro‐Pro‐Gly)7 peptide, the concentration dependence of the (Pro‐Pro‐Gly)n association equilibrium was observed for the first time. Detailed thermodynamic analysis for these n = 7, 8 data, together with literature data for n = 10, 15, 20 were carried out for both the simple “all‐or‐none” binding model and for a series of complex equilibrium models. For the latter, all of the (Pro‐Pro‐Gly)n data (in 10% acetic acid) are fit best with a maximally cooperative near‐neighbor model with a standard enthalpy change ΔH = −650 cal/mole of residues, and a standard entropy change ΔS = −14.63 −10/n cal/deg‐mole of residues, wherein the −10 eu represents an end‐effect contribution to the binding free energy. With regard to optical rotatory properties and thermodynamic parameters, the data for the new n = 7, 8 peptides match rather well with the literature data for the n = 10, 15, and 20 peptides. The enthalpic stabilization per residue of the triple‐helical form of (Pro‐Pro‐Gly)n was nearly an order of magnitude smaller than the enthalpic stabilization per additional proline obtained from direct calorimetric measurements on native collagens of different (and much lower) proline contents by Privalov and Tiktopulo. [Biopolymers (1970) 9, 127–139.] Possible explanations for this phenomenon are discussed. Copyright © 1975 John Wiley & Sons, Inc.
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