We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm(-1). We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm(-1) all show vibrational relaxation with a time constant of T(1) = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm(-1) show distinctly faster relaxation dynamics, with T(1) < 80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm(-1)) and absent (3200 cm(-1)). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm(-1) that are decoupled from the bulk water system.

Duramycin is a small tetracyclic peptide which binds specifically to ethanolamine phospholipids (PE). In this study, we used lipid monolayers consisting of 1-palmitoyl-2-oleoyl phosphatidylethanolamine (POPE) and various phosphatidylcholines (PC) to investigate the effect of duramycin on the organization of lipids and its influence on surrounding water molecules, using vibrational sum-frequency generation spectroscopy in conjunction with surface pressure measurements and fluorescence microscopy. The results show that while duramycin has no effect on the PC lipid monolayers, it induces significant disorder of PE molecules and causes an increase of the PE monolayer surface pressure. Duramycin adopts a β-sheet conformation and is well-ordered at the air-water interface as well as after binding to PE. Our results are consistent with duramycin inserting into the PE monolayer via its hydrophobic end, exposing phenylalanine residues to the lipid. Binding of duramycin to PE broadens the hydrogen-bond distribution of lipid-bound water molecules, notably increasing the fraction of the less strongly hydrogen-bonded, possibly undercoordinated, water molecules. Fluorescence microscopy reveals that the interaction of duramycin with PE causes a change in the shape of the liquid-condensed domains of the PE monolayer from circular to horseshoe-like, indicating a reduction of line tension at the boundary of the two lipid phases. These results reveal that the first steps in the disruption of membrane integrity by duramycin consist of a reduction of the line tension, a decrease in the lipid order, and a weakening of the hydrogen bonding network of water around PE.

Understanding the molecular mechanism of DNA/lipid interaction is critical in optimizing the use of lipid cofactors in gene therapy. Here, we address this question by employing label-free vibrational sum frequency (VSF) spectroscopy to study the interaction of DNA with lipid monolayers of the cationic lipids DPTAP(1,2-dipalmitoyl-3-trimethylammonium-propane) and diC14-amidine as well as the zwitterionic lipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) in the presence and absence of calcium. Our approach has the advantage both of allowing us to explicitly probe intermolecular interactions and of providing insight into the structure of water and lipids around DNA at the lipid interface. We find, by examination of the OD stretch of interfacial D(2)O, that water structure differs markedly between systems containing DNA adsorbed to cationic and those that contain DNA adsorbed to zwitterionic lipid monolayers (in the presence or absence of Ca(2+)). The spectral response of interfacial water in the cationic system is consistent with a highly structured, undercoordinated, structural 'type' of water. Further, by investigation of CH stretch modes of the diC14-amidine lipid tails, we demonstrate that the adsorption of DNA to this lipid leads to increased ordering of lipid tails.

We report a numerical algorithm, the maximum entropy method (MEM), to obtain the absolute phase of the sum frequency signal from vibrational sum frequency generation (VSFG) spectra, without the need for phase-sensitive measurements. From the phase of the VSFG susceptibility, we can determine the molecular orientation, i.e., whether molecular groups are pointing "up" or "down", with respect to the interface. Furthermore, with previous knowledge of the nonresonant phase, the real and imaginary parts of second-order susceptibility can also be deter-mined. The phase retrieval algorithm is successfully applied to spectra obtained from three distinct samples: (1) water vibrations of the SiO2-water interface, (2) methyl vibrations of a dodecanol monolayer on water, and (3) methyl vibrations of self-assembled dodecanethiol monolayers on a gold substrate. These results demonstrate that the approach is applicable to a wide range of spectra, with varying resonance widths and nonresonant background levels. For the SiO2-water interface at high pH, we find that the water molecules are oriented with their hydrogen atoms toward the surface, and we show that the procedure demonstrated here provides information on the interfacial vibrations that cannot be obtained from a multiresonance fit. For surfactant monolayers, we find, as expected, that the methyl groups point away from the substrate. Possible complications and limitations in determining the phase spectrum of the nonlinear susceptibility using MEM are also discussed.

The last decade has seen a transformation in understanding of the role of membrane-bound interfacial water. Whereas until recently water was treated principally as a continuum (primarily screening charges of lipids and proteins), it has become apparent recently that consideration of water's molecular-level properties is critical in understanding a variety of biochemical and biophysical processes. Here we investigate the structure and dynamics of water in contact with a monolayer of artificial lung surfactant, composed of four types of lipids and one protein. We probe this water using frequency-domain sum-frequency generation (SFG) spectroscopy, and a newly developed time-domain, three-pulse technique, in which the vibrational relaxation of interfacial water molecules is followed in real time. We characterize interfacial water in three systems: a monolayer of the pure lipid that is the majority of the lung surfactant mixture, a monolayer of the four lipids constituting the mixture, and a monolayer of the four lipids and the protein. We find subtle differences in the water structure and dynamics that depend on the mixture density and composition. In particular, frequency-domain measurements suggest that in the lipid mixture and the lipid mixture + protein, the relatively bulky lipids (those that have either three or unsaturated hydrocarbon tails) tend to be squeezed out at higher pressure. Measurements using the time-domain, three-pulse technique make clear that structural relaxation of interfacial water is significantly slowed down upon adding small amounts of protein to the lipids. Both results are consistent with prior measurements using other techniques in which more fluid lipids were shown to be 'squeezed out' of lung surfactant at high compression and the role of protein in the mixture was demonstrated to be a catalyst for the formation of multilayers under compression that are subsequently reintegrated into the monolayer on expansion.

We demonstrate that marked variations exist in hydrogen bonding interactions of interfacial water at different aqueous interfaces. The average hydrogen bond strength and its distribution are inferred from surface sum-frequency generation (SFG) spectra through the center frequency and width, respectively, of the O-D stretch vibration of isotopically diluted HDO in H2O. The use of partially deuterated water prevents complications due to intramolecular vibrational coupling, which we show gives rise to features in the SFG spectra that are unrelated to hydrogen bonding interactions. At the water-air interface, the SFG spectrum in the hydrogen-bonded region strongly resembles the bulk Raman spectrum, indicating that, at this interface, the interfacial hydrogen bonding properties are very similar to those in bulk water. In contrast, for silica-water and lipid-water interfaces, interfacial hydrogen bonding is substantially stronger, with a larger degree of heterogeneity. (C) 2009 Elsevier B.V. All rights reserved.

Erratum to ‘‘Ultrafast vibrational dynamics of interfacial water” [Chem. Phys. 350 (2008) 23] Avishek Ghosh , Marc Smits , Maria Sovago , Jens Bredenbeck , Michiel Muller , Mischa Bonn a,* a FOM-Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, NL-1098 SJ, Amsterdam, The Netherlands b Swammerdam Institute for Life Sciences, University of Amsterdam, P.O. Box 94062, 1090 GB Amsterdam, The Netherlands

We investigate the structure and orientation of water molecules at the water-lipid interface, using vibrational sum-frequency generation in conjunction with a maximum entropy phase retrieval method. We find that interfacial water molecules have an orientation opposite to that predicted by electrostatics and thus are likely localized between the lipid headgroup and its apolar alkyl chain. This type of water molecule is observed for phospholipids but not for structurally simpler surfactants.

We report investigations on the ultrafast vibrational dynamics of water molecules at model biological interfaces and the neat water/air interface, using a newly developed surfacespecific 4th-order femtosecond infrared pump-probe spectroscopic technique. The vibrational relaxation rates and mechanisms depend strongly on the nature of the interface. Whereas water at the neat water/air interface exchanges vibrational energy rapidly with the bulk, the water molecules at model biological interfaces are energetically decoupled from the bulk.

A Reply to the Comment by C. S. Tian and Y. R. Shen.Received 26 August 2008DOI:https://doi.org/10.1103/PhysRevLett.101.139402©2008 American Physical Society

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (approximately 5 mN/m), the presence on Ca2+ results in the unexpected appearance of ordered domains. For pressures between approximately 5 and approximately 25 mN/m, Ca2+ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca2+ with the lipids is evident from Ca2+-induced changes in the signals corresponding to area changes of 4 A2/lipid-precisely the surface area of a Ca2+ ion, with evidence for a change in lipid Ca2+ complexation at high pressures.

A novel implementation of broad-bandwidth sum-frequency generation (SFG) spectroscopy is presented, which allows for the simultaneous recording of SFG spectra with different polarization combinations of the SFG, visible, and infrared beams. This method is particularly advantageous for studies in which surface properties are time-dependent, such as kinetic studies. The technique is illustrated by a study that mimics lung surfactant relaxation during the breathing cycle. The time-dependent molecular order of lung surfactant lipids is quantified through the vibrational response of the terminal CH3 group.

Cationic lipids are promising candidates as transporters of genetic material into cells. Therefore, detailed knowledge on the molecular interaction between polynucleotides (DNA) and cationic lipids is needed to improve our understanding of these so-called lipoplexes. Here, for the first time, we have investigated the role of water in DNA-lipid interaction using vibrational sum-frequency generation (VSFG). We show that the DNA-lipid complexation induces a dramatic restructuring of interfacial water, with less than one monolayer of water remaining between the DNA and the lipid; we also observe that the lipid-bound water reorients upon interaction with DNA. This is apparent from the rather dramatic changes in the vibrational response of interfacial water. The approach presented here has a very high (picomolar) sensitivity and allows for the quantification of the DNA-lipid association constant.