Structure, Dynamics, and Hydration of a Collagen Model Polypeptide, (l-Prolyl-l-ProlylGlycyl)10, in Aqueous Media: a Chemical Equilibrium Analysis of Triple Helix-to-Single Coil Transition

Abstract

The structure, dynamics, and hydration behavior of a collagen model polypeptide, (L-prolyl-L-prolylglycyl)(10) (PPG10), were investigated in pure water and dilute acetic acid over a wide temperature range using broadband dielectric relaxation (DR) techniques that spanned frequencies from 1 kHz to 20 GHz. All samples showed pronounced dielectric dispersion with two major relaxation processes around 3 MHz and 20 GHz. Because DR measurements sensitively probe dipoles and their dynamics, the structures and ionization states of the carboxy and amino termini of aqueous PPG10 were precisely determined from the relaxation times and strengths in the 3 MHz frequency range. In solution, PPG10 formed mixtures of monodisperse rods as triple helices with lengths and diameters of 8.6 and 1.5 nm, respectively, and monomeric random coils with radii of approximately 1.4 nm. Ionization of the C-terminus was suppressed by the addition of acetic acid in both states. The fraction of random coils (f(coil)) was found to be a function of temperature (T) and the concentration of PPG10 (c). At low temperatures, small f(coil) values were found, which increased with temperature to reach f(coil) = 1 at approximately 60 degrees C, irrespective of c. This phenomenon, well-known as a triple helix-to-single coil transition, is discussed on the basis of the chemical reaction, (PPG10)(3) <==> 3PPG10, with an equilibrium constant of K = 3(c/55.6)(2)f(coil)(3)(1 - f(coil))(-1). The standard enthalpy change evaluated from Arrhenius plots (ln K versus T(-1)) was found to change dramatically at the same transition temperature that was previously determined by using optical rotation experiments. The other major DR process, observed at approximately 20 GHz, was assigned to free and hydrated water molecules and used to determine the average hydration number (m) per PPG10. The m values for the triple helix and random coil state at 25 degrees C were evaluated to be m(th) = 60-70 and m(coil) = 250-270. The m(th) value was in reasonable agreement with the number of hydrated water molecules in crystals of (PPG10)(3) residing in the first and second hydration shells around the amino acid residues. This agreement suggests that the structure of the triple helix in crystals is very similar to that in aqueous solution, including the location of hydrated water molecules.