Ultrafast infrared spectroscopy of biological membranes and biological water
Shattuck, Jeffrey T.
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Ultrafast infrared pump-probe and photon echo spectroscopy is used to provide insight into the differences ofthe hydrogen bonding network ofwater in neat liquid, aqueous solutions and lipid membrane environments. Due to water's fundamental role in biology and biological processes, it is essential to understand its interactions with these environments. The vibrational energy relaxation (VER) mechanism ofthe libration-bend combination band of neat water was investigated using pump-probe spectroscopy. Previous studies concentrate on the kinetics stretch and bend normal modes. Two concerted pathways were needed to describe the energy relaxation. In the first pathway vibrational energy leaves the excited combination band directly (140 fs) populating low frequency modes. In the second pathway the combination band decays to the bend normal mode which subsequently relaxes to librations and finally to low frequency modes (840 fs). Pump-probe spectroscopy was used to determine perturbations to the VER of nitrous oxide (N20) dissolved in water caused by ionic solutes and membranes. Altering the cation for a number ofchloride salts showed the VER rate ofN20 to follow a Hofmeister series trend. Kosmotropes Ca2+, and Mg2+, increased the lifetime of the v3 mode ofN20 while chaotropes (Cs+) decreased the lifetime. The v3 lifetime of N20 also showed that charged lipid headgroups alter the hydrogen bonding network of interlamellar water. The v3 lifetime ofN20 dissolved in oriented water near the lipid headgroups was 20 ps and changed by over a factor of two compared to its bulk value of 9 ps. Both experiments show that strongly oriented water slows the N20 VER by changing the bulk water structure of the intramolecular hydrogen bonding network. Homodyne photon echoes of N20 in water and octanol, both model environments for head and tail portions of lipids, showed the timescales of spectral diffusion for each solvent. Spectral diffusion timescales for N20 in water is caused by inertial rotational motions, 130 fs, and hydrogen bond breaking, 1.5 ps. In octanol spectral diffusion is due to inertial rotation (230 fs), hydrogen bond breaking (3.5 ps), and solvent reorientation (35 ps). Anisotropy measurements are consistent with this interpretation.
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