Sensitive and Quantitative Probe of Molecular Chirality with Heterodyne-Detected Doubly Resonant Sum Frequency Generation Spectroscopy

Heterodyne-detected vibrationally electronically doubly resonant chiral sum frequency generation (HD-DR chiral SFG) spectroscopy has been developed for the study of chiral molecules with chromophores. The method enables us to detect and distinguish chiral molecules with high sensitivity and to obtain information on molecular vibrations. Strong enhancement due to the electronic resonance improves the sensitivity, and heterodyne detection ensures that the signal intensity is linear to the sample concentration. Detection of HD-DR chiral SFG signal from a dilute solution of binaphthol with 20 mM concentration and tens of nanometers thickness was demonstrated. Taking advantage of the enantiomer-dependent sign and linearity of the signal to the concentration, molecular concentrations and enantiomeric excesses were accurately evaluated. HD-DR chiral SFG is expected to have widespread application in the study of molecular chirality of thin films or samples of a very small quantity

Chirality Discriminated by Heterodyne-Detected Vibrational Sum Frequency Generation

We first demonstrated chiral vibrational sum frequency generation (VSFG) in the heterodyne detection, which enables us to uniquely determine chiral second-order nonlinear susceptibility consisting of phase and amplitude and distinguish molecular chirality with high sensitivity. Liquid limonene was measured to evaluate the heterodyne-detected chiral VSFG developed in this study. <i>R</i>-(+)- and <i>S</i>-(−)-limonene showed clearly opposite signs in the complex spectra of the second-order nonlinear susceptibility in the CH stretching region. This is the first report of the chiral distinction by VSFG without any a priori knowledge about chiral and achiral spectral response. Furthermore, from the phase of the chiral VSFG field measured in the heterodyne detection, the origin of the chiral signal was ascribed to the bulk limonene. The heterodyne detection also improves detection limits significantly, allowing us to observe weak chiral signals in reflection. The heterodyne-detected chiral VSFG can provide information on absolute molecular configuration

Heterodyne-Detected Achiral and Chiral Vibrational Sum Frequency Generation of Proteins at Air/Water Interface

We present complex achiral and chiral vibrational sum frequency generation (VSFG) spectra at the air/water interface of protein solutions by using heterodyne-detected VSFG. Bovine serum albumin, pepsin, concanavalin A, and α-chymotrypsin were measured as model proteins. The obtained achiral Im­[χ⁽²⁾] spectra gave us insights into the molecular orientation of protein molecules and water at the interface. From the chiral Im­[χ⁽²⁾] spectra in the NH stretching and amide I regions, the secondary structures of the interfacial proteins were deduced. We attributed the chiral signals in the amide I and NH stretching regions to the interface on the basis of the phase of the signals. All the achiral and chiral spectra in each region showed the same sign despite different secondary-structure contents of the examined proteins. Real-time observation of the spectral change of α-chymotrypsin was also performed by heterodyne-detected chiral VSFG. The signal intensity of the chiral Im­[χ⁽²⁾] spectra in the NH stretching and amide I regions decreased on the scale of 10 min, originating from the decrease of the portion of antiparallel β-sheet conformation in the molecule. The conformational change occurred not in the bulk but at the interface. Heterodyne-detected achiral and chiral VSFG are capable of addressing the molecular orientation and conformation of proteins at air/water interfaces

Development of Heterodyne-Detected Total Internal Reflection Vibrational Sum Frequency Generation Spectroscopy and Its Application to CaF₂/Liquid Interfaces

We present heterodyne-detected total internal reflection vibrational sum frequency generation (HD-TIR VSFG) spectroscopy for CaF₂/liquid interfaces. With this technique, absolute orientations at solid/liquid interfaces can be determined by measuring complex χ⁽²⁾ spectra of the buried interfaces. We applied the technique to CaF₂/sodium dodecyl sulfate (SDS) solution interfaces, and directly determined the polar orientations of the water molecules and surfactants at the interface based on the phase information on VSFG spectra, which is unavailable in conventional homodyne-detected TIR VSFG spectroscopy. The reorientation of the interfacial water molecules was observed as a change of the sign of Im­[χ⁽²⁾] depending on the surfactant concentration

Symmetric Raman Tensor Contributes to Chiral Vibrational Sum-Frequency Generation from Binaphthyl Amphiphile Monolayers on Water: Study of Electronic Resonance Amplitude and Phase Profiles

Heterodyne-detected vibrationally electronically doubly resonant chiral sum-frequency generation (HD-DR chiral SFG) spectra were observed for the first time on monolayers. Langmuir monolayers of <i>R</i>- and <i>S</i>-binaphthyl amphiphiles on water exhibited complex chiral vibrational sum frequency generation (VSFG) spectra whose amplitudes were approximately the same, but whose phases were different by π. By comparing the electronic resonance profiles of the chiral VSFG signal amplitude and phase with simulations based on a nonadiabatic resonance Raman theory, we concluded that the chiral VSFG signals from the monolayers originated not from the antisymmetric Raman tensor but from the symmetric one. This is the first experimental evidence that the symmetric Raman tensor indeed contributes to the chiral VSFG signals from oriented monolayer systems

The concentration dependence of d₂₅-SDS cell lysis efficiency; ++ corresponds to clear cell lysis, +: moderate cell lysis, −: remaining stable.

<p>The concentration dependence of d₂₅-SDS cell lysis efficiency; ++ corresponds to clear cell lysis, +: moderate cell lysis, −: remaining stable.</p

SDS molecules are condensed in a CHL cell.

<p><b>A</b>. The molecular structure of d₂₅-SDS. <b>B</b>. Im[χ⁽³⁾] spectrum obtained from one point of a CHL cell indicated as the cross in the inset several minutes after the addition of d₂₅-SDS. <b>C</b>. The expanded spectrum of <b>B</b>. <b>D</b>. Im[χ⁽³⁾] spectrum of 1% d₂₅- SDS aqueous solution. The exposure time for <b>B</b>–<b>D</b> is 50 msec and <b>B</b>–<b>D</b> are measured under the same experimental condition.</p

Accumulation of SDS in a CHL cell and subsequent cellular death.

<p><b>A</b>. Time-resolved Im[χ⁽³⁾] spectra obtained with the summation over all the spectra in the cell shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0093401#pone-0093401-g003" target="_blank">Fig. 3</a>. Time-profiles of band amplitudes at 2100 cm⁻¹ (<b>B</b>), 2930 cm⁻¹ (<b>C</b>), 2850 cm⁻¹ (<b>D</b>), 1655 cm⁻¹ (<b>E</b>), 1446 cm⁻¹ (<b>F</b>) and 1003 cm⁻¹ (<b>G</b>).</p

Im[χ⁽³⁾] spectra and images from a CHL cell.

<p>Im[χ⁽³⁾] spectra from the two points of the CHL cell. <b>A</b> and <b>B</b> are obtained from the points indicated as × and + in <b>C</b>, respectively. The inset of each spectrum is the expanded spectrum in the fingerprint region. The exposure time is 50 msec. Im[χ⁽³⁾] images at 2930 cm⁻¹ (<b>C</b>), 2850 cm⁻¹ (<b>D</b>), 2655 cm⁻¹ (<b>E</b>), 2446 cm⁻¹ (<b>F</b>) and 1003 cm⁻¹ (<b>G</b>), respectively. The scale bar in the image is 10 µm. The image consists of 91×81 pixels and the exposure time for each pixel is 50 msec. Each image is normalized at the intensity maximal of each band.</p

Time-resolved Im[χ⁽³⁾] images of the CHL cell with the surfactant.

<p>The scale bar in the image is 10 µm. The image consists of 71×51 pixels and the exposure time for each pixel is 50 msec. Each row of the CARS images is measured every 3.5 min. Each column is normalized at the intensity maximal of each band.</p