Lipid/water interfaces and associated interfacial water are vital for various biochemical reactions, but the molecular-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine, monolayer/water interface using heterodyne-detected vibrational sum frequency generation spectroscopy. Isotopically diluted water was utilized in the experiments to minimize the effect of intra/intermolecular couplings. It was found that the OH stretch band in the Imχ((2)) spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ((2)) spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ((2)) spectrum of the zwitterionic surfactant/water interface clearly shows both positive and negative bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we observed a drastic change in the Imχ((2)) spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that, when the positive and negative charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the respective charged sites. In addition, it was found that a positive Imχ((2)) appears around 3600 cm(-1) for all the monolayer interfaces examined, indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ((2)) spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water associated with the negatively charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the positively charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation.

Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ((2))]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ((2)) [Imχ((2))] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ((1))] and Raman [Imχ((3))] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ((2))|(2)] spectra. The Imχ((2)) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation and structure of interfacial water at charged, neutral, and biorelevant water interfaces.

Cell membrane/water interfaces provide a unique environment for many biochemical reactions, and associated interfacial water is an integral part of such reactions. A molecular level understanding of the structure and orientation of water at lipid/water interfaces is required to realize the complex chemistry at biointerfaces. Here we report the heterodyne-detected vibrational sum frequency generation (HD-VSFG) studies of lipid monolayer/water interfaces. At charged lipid/water interfaces, the orientation of interfacial water is governed by the net charge on the lipid headgroup; at an anionic lipid/water interface, water is in the hydrogen-up orientation, and at the cationic lipid/water interface, water is in the hydrogen-down orientation. At the cationic and anionic lipid/water interfaces, interfacial water has comparable hydrogen bond strength, and it is analogous to the bulk water.

Dynamics of the excited singlet (S(1)) state of curcumin has been investigated in a wide varieties of solvents using subpicosecond time-resolved fluorescence and absorption spectroscopic techniques. As a consequence of extra stability of the cis-enol conformer due to the presence of an intramolecular hydrogen bond, it is the major form existing in the ground-state and the excited-state processes described here has been attributed to this form. Steady-state absorption and fluorescence spectra suggest significant perturbation of the intramolecular hydrogen bond and the possibility of formation of intermolecular hydrogen-bonded complex with the hydrogen-bonding solvents. Both the time-resolved techniques used here reveal that solvation is the major process contributing to the relaxation dynamics of the S(1) state. Solvation dynamics in protic solvents is multimodal, and the linear correlation between the longest component of the solvation process and the longitudinal relaxation time of the solvent suggests the specific hydrogen-bonding interaction between the solute and the solvent. However, a good correlation between the experimentally determined average solvation time and that predicted by the dielectric continuum model in all kinds of solvents also suggests that the dielectric relaxation of the solvent is also an important contributor to the solvation process. The lifetime of the S(1) state is very short in nonpolar solvents (∼44 ps in 1,4-dioxane) because of efficient nonradiative deactivation of the S(1) state, which is an important consequence of the ultrafast excited-state intramolecular hydrogen transfer (ESIHT) reaction in the six-membered hydrogen-bonded chelate ring of the cis-enol form. However, it has not been possible to monitor the ESIHT reaction in real time because of the symmetrical structure of the molecule with respect to the hydrogen-bonded chelate ring. In polar solvents, dipole-dipole interaction perturbs the intramolecular hydrogen bond leading to the reduced efficiency of the nonradiative deactivation process. However, stretching vibration in the intermolecular hydrogen bonds formed in the hydrogen-bonding (both donating and accepting) solvents induces another efficient channel for the nonradiative relaxation of the S(1) state of curcumin.

Raman spectroscopy in combination with multivariate curve resolution (Raman-MCR) is used to explore the interaction between water and various kosmotropic and chaotropic anions. Raman-MCR of aqueous Na-salt (NaI, NaBr, NaNO3, Na2SO4, and Na3PO4) solutions provides solute-correlated Raman spectra (SC-spectra) of water. The SC-spectra predominantly bear the vibrational characteristics of water in the hydration shell of anions, because Na(+)-cation has negligible effect on the OH stretch band of water. The SC-spectra for the chaotropic I(-), Br(-), and NO3(-) anions and even for the kosmotropic SO4(2-) anion resemble the Raman spectrum of isotopically diluted water (H2O/D2O = 1/19; v/v) whose OH stretch band is largely comprised by the response of vibrationally decoupled OH oscillators. On the other hand, the SC-spectrum for the kosmotropic PO4(3-) anion is quite similar to the Raman spectrum of H2O (bulk). Comparison of the peak positions of SC-spectra and the Raman spectrum of isotopically diluted water suggests that the hydrogen bond strength of water in the hydration shell of SO4(2-) is comparable to that of the isotopically diluted water, but that in the hydration shell of I(-), Br(-), and NO3(-) anions is weaker than that of the latter. Analysis of integrated area of component bands of the SC-spectra reveals ∼80% reduction of the delocalization of vibrational modes (intermolecular coupling and Fermi resonance) of water in the hydration shell of I(-), Br(-), NO3(-), and SO4(2-) anions. In the case of trivalent PO4(3-), the vibrational delocalization is presumably reduced and the corresponding decrease in spectral response at ∼3250 cm(-1) is compensated by the increased signal of strongly hydrogen bonded (but decoupled) water species in the hydration shell. The peak area-averaged wavenumber of the SC-spectrum increases as PO4(3-) < SO4(2-) < NO3(-) < Br(-) < I(-) and indeed suggests strong hydrogen bonding of water in the hydration shell of PO4(3-) anion.