Effect of the molecular weight on the dynamics of the conformational transition of poly(methacrylic acid)

Stop flow pH jump experiments were carried out on poly(methacrylic acid), P MA, solution of Various molecular weights. The fluorescence changes of the probe pyrene, which can locate in the hydrophobic PMA coil or in the aqueou s phase, were used to monitor pH-induced events in the PMA-water system. At least two well-separated time regions for the pH-induced conformational ch ange of PMA were observed: one with tau = 1-2 s and another with tau = 50-1 00 s. A theoretical model to quantitatively explain the observed kinetic ch anges is given. This is based on a model developed by Schwarz for the helix -coil transition of polypeptides and can be briefly described as follows. A ny configuration of the polymer is characterized by a statistical distribut ion of segments in polar and hydrophobic regions. Then a conformational tra nsition involves the change of a number of segments from one state to the a nother, and its kinetics corresponds to a relaxation process with the pH ju mp as the initial perturbation. Several details pertinent to unambiguous in terpretation are also described briefly. This includes the rate of neutrali zation of the acid groups on PMA and the rate of solution of small pyrene c rystals in a colloidal system. The above confusing effects were established and thus could be avoided in the actual pH-induced conformational changes of PMA.