Raman spectroscopy of model membrane monolayers of dipalmitoylphosphatidic acid at the air-water interface using surface enhancement from buoyant thin silver films

In Situ Raman Study of Neurodegenerated Human Neuroblastoma Cells Exposed to Outer-Membrane Vesicles Isolated from Porphyromonas gingivalis

The aim of this study was to elucidate the chemistry of cellular degeneration in human neuroblastoma cells upon exposure to outer-membrane vesicles (OMVs) produced by Porphyromonas gingivalis (Pg) oral bacteria by monitoring their metabolomic evolution using in situ Raman spectroscopy. Pg-OMVs are a key factor in Alzheimer's disease (AD) pathogenesis, as they act as efficient vectors for the delivery of toxins promoting neuronal damage. However, the chemical mechanisms underlying the direct impact of Pg-OMVs on cell metabolites at the molecular scale still remain conspicuously unclear. A widely used in vitro model employing neuroblastoma SH-SY5Y cells (a sub-line of the SK-N-SH cell line) was spectroscopically analyzed in situ before and 6 h after Pg-OMV contamination. Concurrently, Raman characterizations were also performed on isolated Pg-OMVs, which included phosphorylated dihydroceramide (PDHC) lipids and lipopolysaccharide (LPS), the latter in turn being contaminated with a highly pathogenic class of cysteine proteases, a key factor in neuronal cell degradation. Raman characterizations located lipopolysaccharide fingerprints in the vesicle structure and unveiled so far unproved aspects of the chemistry behind protein degradation induced by Pg-OMV contamination of SH-SY5Y cells. The observed alterations of cells' Raman profiles were then discussed in view of key factors including the formation of amyloid β (Aβ) plaques and hyperphosphorylated Tau neurofibrillary tangles, and the formation of cholesterol agglomerates that exacerbate AD pathologies.

Structure and Interaction of Ceramide-Containing Liposomes with Gold Nanoparticles as Characterized by SERS and Cryo-EM

Due to the great potential of surface-enhanced Raman scattering (SERS) as local vibrational probe of lipid-nanostructure interaction in lipid bilayers, it is important to characterize these interactions in detail. The interpretation of SERS data of lipids in living cells requires an understanding of how the molecules interact with gold nanostructures and how intermolecular interactions influence the proximity and contact between lipids and nanoparticles. Ceramide, a sphingolipid that acts as important structural component and regulator of biological function, therefore of interest to probing, lacks a phosphocholine head group that is common to many lipids used in liposome models. SERS spectra of liposomes of a mixture of ceramide, phosphatidic acid, and phosphatidylcholine, as well as of pure ceramide and of the phospholipid mixture are reported. Distinct groups of SERS spectra represent varied contributions of the choline, sphingosine, and phosphate head groups and the structures of the acyl chains. Spectral bands related to the state of order of the membrane and moreover to the amide function of the sphingosine head groups indicate that the gold nanoparticles interact with molecules involved in different intermolecular relations. While cryogenic electron microscopy shows the formation of bilayer liposomes in all preparations, pure ceramide was found to also form supramolecular, concentric stacked and densely packed lamellar, nonliposomal structures. That the formation of such supramolecular assemblies supports the intermolecular interactions of ceramide is indicated by the SERS data. The unique spectral features that are assigned to the ceramide-containing lipid model systems here enable an identification of these molecules in biological systems and allow us to obtain information on their structure and interaction by SERS.

Multivariate Analysis of Mixed Lipid Aggregate Phase Transitions Monitored Using Raman Spectroscopy

The phase behavior of aqueous 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) mixtures between 8.0 ℃ and 41.0 ℃ were monitored using Raman spectroscopy. Temperature-dependent Raman matrices were assembled from series of spectra and subjected to multivariate analysis. The consensus of pseudo-rank estimation results is that seven to eight components account for the temperature-dependent changes observed in the spectra. The spectra and temperature response profiles of the mixture components were resolved by applying a variant of the non-negative matrix factorization (NMF) algorithm described by Lee and Seung (1999). The rotational ambiguity of the data matrix was reduced by augmenting the original temperature-dependent spectral matrix with its cumulative counterpart, i.e., the matrix formed by successive integration of the spectra across the temperature index (columns). Successive rounds of constrained NMF were used to isolate component spectra from a significant fluorescence background. Five major components exhibiting varying degrees of gel and liquid crystalline lipid character were resolved. Hydrogen-bonded water networks exhibiting varying degrees of organization are associated with the lipid components. Spectral parameters were computed to compare the chain conformation, packing, and hydration indicated by the resolved spectra. Based on spectral features and relative amounts of the components observed, four components reflect long chain lipid response. The fifth component could reflect the response of the short chain lipid, DHPC, but there were no definitive spectral features confirming this assignment. A minor component of uncertain assignment that exhibits a striking response to the DMPC pre-transition and chain melting transition also was recovered. While none of the spectra resolved exhibit features unequivocally attributable to a specific aggregate morphology or step in the gelation process, the results are consistent with the evolution of mixed phase bicelles (nanodisks) and small amounts of worm-like DMPC/DHPC aggregates, and perhaps DHPC micelles, at low temperature to suspensions of branched and entangled worm-like aggregates above the DMPC gel phase transition and perforated multi-lamellar aggregates at high temperature.

Interfacial assembly of a phospholipid Langmuir monolayer by electrodeposited silver colloids

Two-dimensional nanostructured silver films were electrodeposited at the surface of a silver nitrate subphase coated by a negatively charged dimyristoylphosphatidylglycerol (DMPG) Langmuir monolayer. The modifications of the phospholipid interfacial organization generated by the growing colloidal silver film were investigated using surface pressure-time isotherms and grazing incidence X-ray diffraction experiments (GIXD). A decrease in the initial surface pressure of the DMPG monolayer is observed outside of the growing silver film, followed by a stabilization of the surface pressure when the radius of the metallic layer reaches its plateau value. This behavior is attributed to the compression of the DMPG molecules above the silver film and to the correlated relaxation and expansion of those outside the silver film area as recorded by a Wilhelmy pressure sensor. GIXD experiments further evidenced the contraction of the phospholipid monolayer above the electrochemically growing films. Indeed, the diffraction spectra show a shift in the peak position toward higher values of the in-plane component of the wave-vector transfer, indicating a closer packing of the DMPG alkyl chains. This is also in agreement with the observed loss of the chain tilt angle, suggesting that the colloidal silver film induces interfacial structuring of the DMPG monolayer.

Surface-enhanced Raman scattering from oleate-stabilized silver colloids at a liquid/liquid interface

Oleate-stabilized silver colloids of 5-nm-diameter were adsorbed to a toluene/water interface, and surface enhanced Raman scattering (SERS) spectra from these colloids were measured under the total internal reflection (TIR) condition. From the observed spectra, we examined the states of oleate ions and toluene molecules on silver colloids at the liquid/liquid interfacial region. The TIR-SERS spectra of oleate ions showed stronger peaks of the carboxylate group and the ethylene group than those of alkyl chains. From these results, it was found that the oleate ions were adsorbed on the silver surface in two different ways at the liquid/liquid interface; the carboxylate group adsorbed in the organic phase side, while the ethylene group adsorbed in the aqueous phase side. The shifts of the toluene in the interfacial SERS spectra were identical to those of bulk toluene, though the relative intensities among the peaks were not same. This result suggested that the toluene was adsorbed with a weak interaction, but was significantly enhanced by the local electromagnetic field at the colloid surface.