Thermodynamic and spectroscopic studies of protonation with a optically active polyampholite

The protonation of an optically active polymer containing an α-alanine residue was studied in aqueous solution by spectroscopic (FT-IR, circular dichroism) and thermodynamic (potentiometric, calorimetric, viscometric) techniques. For the polyampholyte, which undergoes a conformational transition in the alkaline region, both basicity constants follow the modified Henderson-Hasselbalch equation. log K1relative to the protonation of the tertiary nitrogen atom decreases with α (the degree of protonation of the whole macromolecule) while the uptake of the proton from the carboxylate group is greatly facilitated as the number of negative charges on the macromolecule decreases. Similar behavior was also observed in polyampholytes of comparable structure. Circular dichroism and FT-IR spectral examination strongly supports an interaction between the protonated nitrogen and the carbonyl groups which gives rise to a locally ordered structure involving the optically active center in the dissymmetrical perturbation. © 1989, American Chemical Society. All rights reserved.