Structure of water in the vicinity of phospholipid analogue copolymers as studied by vibrational spectroscopy

The structure and hydrogen bonding of water in the vicinity of phospholipid analogue random copolymers [poly(2-methacryloyloxyethyl phosphorylcholine-r-n-butyl methacrylate), Poly(MPC-r-BMA)] with various molecular weights were analyzed in their aqueous solutions and thin films with contours of O-H stretching of Raman and attenuated total reflection infrared (ATR-IR) spectra, respectively. The relative intensity of the collective band (C value) corresponding to a long-range coupling of O-H stretchings of the Raman spectra for the aqueous solution of Poly(MPC-r-BMA) was very close to that for pure water, which is in contrast with the smaller C value in the aqueous solution of ordinary polyelectrolytes. The number of hydrogen bonds collapsed by the presence of one monomer residue (N-corr value) of Poly(MPC-r-BMA) (M-w 1.3 x 10(4), 3.0 x 10(4), and 9.3 x 10(4)) was much smaller than those for ordinary polyelectrolytes and close to those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). Furthermore, water-insoluble Poly(MPC-r-BMA) with a large molecular weight (4.2 x 10(5)) could be cast as a thin film (thickness, ca. 10 mum) on a ZnSe crystal for the ATR-IR spectroscopy. At an early stage of sorption of water into the Poly(MPC-r-BMA) film, the O-H stretching band of the IR spectra for the water incorporated in the film was similar to that for free water, which is in contrast with the drastic change in the O-H stretching band of water incorporated in polymer films such as poly(2-hydroxyethyl methacrylate), poly(methyl methacrylate), and poly(n-butyl methacrylate). These results suggest that the phospholipid analogue monomer residues with a zwitterionic structure do not significantly disturb the hydrogen bonding between water molecules in either the aqueous solution or the thin film systems.