Hydrogen-bonded network structure of water in aqueous solution of sulfobetaine polymers

Poly(N,N-dimethylaminopropyl methacrylamide) (poly(DMAPMA)) was incubated with 1,3-propanesultone and 1,4-butanesultone to afford polymers with various contents of N,N-dimethyl-N-(3-sulfopropyl)-3'-methacrylamidopropanaminium inner salt residues and NN-dimethyl-N-(4-sulfobutyl)-3'-methacrylamidopropanaminium inner salt residues, respectively. The structure and hydrogen bonding of water in an aqueous solution of the sulfobetaine polymers were analyzed using the contours of the O-H stretching in the polarized Raman spectra. With an increase in the content of the sulfobetaine residue, the relative intensity of the collective band (C value) corresponding to a long-range coupling of the O-H stretching in the aqueous polymer solutions became larger and approached the C value of pure water. The number of hydrogen bonds disrupted because of the presence of one monomer residue (N value) for the polymers with a large sulfobetaine content was a small positive value and comparable to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This is in significant contrast with the largely positive N values for the precursor polymer (poly(DMAPMA)), and ordinary polyelectrolytes such as sodium polyethylenesulfonate, poly-L-lysine hydrobromide, sodium polyacrylate, and poly(acrylic acid). The N value for a small molecular weight zwitterionic compound, 3-aminopropanesulfonic acid, was also slightly positive, which is consistent with the tendency observed for the sulfobetaine polymers. The present results clearly indicate that the zwitterionic polymers do not significantly disturb the hydrogen-bonded network structure of water, probably because of the counteraction of the electrostriction effect by the proximity between the anionic and cationic groups.