Vibrational Sum‐Frequency Generation Activity of a 2,4‐Dinitrophenyl Phospholipid Hybrid Bilayer: Retrieving Orientational Parameters from a DFT Analysis of Experimental Data

Abstract

The vibrational nonlinear activity of films of 2,4-dinitrophenyl phospholipid (DNP) at the solid interface is measured by sum-frequency generation spectroscopy (SFG). Hybrid bilayers are formed by a Langmuir-Schaefer approach in which the lipid layer is physisorbed on top of a self-assembled monolayer of dodecanethiol on Pt with the polar heads pointing out from the surface. The SFG response is investigated in two vibrational frequency domains, namely, 3050-2750 and 1375-1240 cm(-1). The first region probes the CH stretching modes of DNP films, and the latter explores the vibrational nonlinear activity of the 2,4-dinitroaniline moiety of the polar head of the lipid. Analysis of the CH stretching vibrations suggests substantial conformational order of the aliphatic chains with only a few gauche defects. To reliably assign the detected SFG signals to specific molecular vibrations, DFT calculations of the IR and Raman activities of molecular models are performed and compared to experimental solid-state spectra. This allows unambiguous assignment of the observed SFG vibrations to molecular modes localized on the 2,4-dinitroaniline moiety of the polar head of DNP. Then, SFG spectra of DNP in the 1375-1240 cm(-1) frequency range are simulated and compared with experimental ones, and thus the 1,4-axis of the 2,4-dinitrophenyl head is estimated to have tilt and rotation angles of 45±5° and 0±30°, respectively.

Unique Vibrational Features as a Direct Probe of Specific Antigen–Antibody Recognition at the Surface of a Solid‐Supported Hybrid Lipid Bilayer

“…This makes sense because cross‐polarization leads to different efficiencies for the tensor excitation, and therefore, to a distinct signature of the same vibrational modes . However, the low amplitude of the methylene vibrations (m2, 2903 cm‐1) for both polarizations indicates an absolute weakness, which is typical for a high conformational order of the aliphatic chains within the ss‐HBL, suggesting an all‐ trans configuration of the methylene units . The phase value of the methyl symmetric stretching has been deliberately fixed to zero.…”

“…A 1 mg mL −1 chloroform solution of lipid was spread at the water surface of a Langmuir trough (MiniMicro System, KSV Instruments). The surface area was compressed at a rate of 5 mm min −1 until a surface pressure of 46 mN m −1 was reached . Hence, the DNP layer was deposited onto the SAM film through the Langmuir–Schaefer technique, which consists in horizontally lowering the SAM film onto the DNP monolayer at the liquid/air interface and then picking‐up the DNP film onto the SAM‐modified surface, at a transfer rate of 3 mm min −1 .…”

“…The surface area was compressed at ar ate of 5mmmin À1 until as urface pressure of 46 mN m À1 was reached. [36] Hence, the DNP layer was deposited onto the SAM film through the Langmuir-Schaefer technique, which consists in horizontally lowering the SAM film onto the DNP monolayer at the liquid/air interface and then picking-up the DNP film onto the SAM-modified surface, at at ransfer rate of 3mmmin À1 .T his procedure has been proved to form DNP-antigen ss-HBL owing the DNP active polar head pointing out from the surface and therefore available to anti-DNP antibody. [36,41] As chematic representation of the structure and the procedure to grow the bioactive platform is shown in Scheme 1.…”

“…[35] However,t he low amplitude of the methylene vibrations (m2, 2903 cm-1) for both polarizations indicates an absolute weakness, which is typical for ah igh conformationalo rder of the aliphatic chains within the ss-HBL, suggesting an all-trans configuration of the methylene units. [36] The phase value of the methyl symmetric stretching has been deliberatelyf ixed to zero. We consider here that the active methyl vibrations point towards the surface (see the film structure in Scheme1 and the top-left inset of Figure1).…”

“…At higherw avenumbers, OH and NH vibrations appear,n amely,t he OH stretchings of the water molecules at 3149 (m5) and 3371 cm À1 (m7) (Table 1a nd Figure 1a,b lue color), which are organized at the interface, [37,38,39,40] and the NH stretching of the amine group of the DNP polar head at 3330 cm À1 (m6) (Table1and Figure 1a,o range color). [36] Bio-recognition by Anti-DNP Antibody Figure 1b shows the SFG spectra recordeda fter 90 min of injection of mouse anti-DNPa ntibody,w hileF igure 1c shows the spectrald ifferencea fter anti-DNP interaction, that is, the difference between Figure 1b and 1a as from their reconstructed peak modes. The mosts triking change can be identified in the appearance of an ew vibration at 3230 cm À1 (m12) ( Table 1 and Figure 1b -c, red color), which unequivocally corresponds to the frequency of peptide bonds from the antibody.C hanges can also be seen in the CH and OH regions.…”