Feng Wei

The Lethal 7 (Let-7) miRNA family is attracting more and more attention in recent decades due to its capability in post-transcriptional regulation and the modulation of various physiological and pathological processes. Yet, the underlying mechanism that controls the conformation and biofunctions of Let-7 molecules during their translocation in and out of the cell membrane is still elusive. In the present study, the sum frequency generation vibrational spectroscopy (SFG-VS), a second-order nonlinear spectroscopic technique, was applied to examine the interfacial behavior of Let-7a at the lipid membrane interface. By creating the lamellar membrane environment in vitro with different ionic solutions, we were able to capture conformational changes in the Let-7a molecule based on the SFG spectra collected from the membrane interface after the miRNA-membrane interaction. Our results indicate that the ribose guanosine (rG) residues and ribose uridine (rU) residues of the Let-7a molecule show versatile changes as the concentration of ionic solutions increases. The analysis results also indicate that the hydrogen-bonding networks around the Let-7a molecules are greatly changed by the salt as the concentration of the NaCl and CaCl₂ solutions increases. As NaCl and CaCl₂ solution concentrations rose during experiments, we noted a marked reconfiguration of the hydrogen-bonding network around Let-7a molecules, driven by strengthened interactions between salt ions, water molecules, and Let-7a's functional groups, disrupting the original network and altering its connectivity/stability. This research offers valuable molecular-level insights into the salt-dependent conformation and behavior changes of miRNA at the lamellar lipid membrane interface. These findings also provide new perspectives for miRNA-related therapy and may help researchers in the improvement of innovative and pertinent drug delivery systems.

Alumina is one of the most abundant oxides in nature. Hence, the molecular-level elucidation of the alumina/water interface is important not only for a fundamental understanding of the oxide surface, but also for applications of oxide-related materials in extensive research fields. In the present study, the water structure in the electrical double layer (EDL) at the alumina-water interface is investigated through the interface-selective vibrational spectra (χ⁽²⁾) in the OH stretch region, measured with heterodyne-detected vibrational sum-frequency generation spectroscopy. The imaginary part of χ⁽²⁾ (Imχ⁽²⁾) spectra is collected while salt concentrations are varied (0.01-5.0 M) in the acidic and neutral pH range (pH 2.0-8.0), where the alumina surface is positively charged. The spectral additivity of the Imχ⁽²⁾ spectra enables the decomposition of the spectra into the contributions of the diffuse layer (DL) and Stern layer (SL) to discuss them separately. The surface charge density at each pH is evaluated from the salt-concentration dependence of the amplitude of the DL spectra using the modified Gouy-Chapman theory, providing a single pK_a value of ∼3.7 ± 0.2 due to the Al₂OH₂⁺/Al₂OH equilibrium at the alumina surface. Moreover, it is found that the spectra of the DL and SL are very similar to each other, indicating that the water in the SL is bulk-like water with highly concentrated salt, without specific interaction with the alumina surface. This implies that the water structure of the EDL at the alumina/water interface is much simpler than that at the silica/water interface, where the water molecule has specific interactions with the silica surface. These findings reveal that the alumina/water interface provides another distinct prototype of the EDL structure at oxide/water interfaces.

The density evolution of polystyrene (PS) adsorbed layers on phenyl-modified SiO₂-Si substrates was investigated. The thickness and density of flattened layer on substrates with above 75% phenyl content increased over annealing time and could approach 4.7 nm and 1.37 g/cm³ at equilibrium, respectively, which were much higher than those on SiO₂-Si. The annealing time for flattened chains to reach equilibrium increased with an increasing phenyl content on the substrate. The interface sensitive sum frequency generation vibrational spectroscopy (SFG) technique revealed that both the amount and the strength of the interfacial π-π interaction between the phenyl groups of substrates and in PS chains increased with annealing time. This resulted in more stretched chains perpendicularly, leading to a denser and thicker adsorbed layer with a closest-packing structure, driven by favorable enthalpy processes. Our work provides important insight into the densification mechanism of adsorbed flattened layers.

The silica/water interface is one of the most abundant charged interfaces in natural environments, and the elucidation of the water structure at the silica/water interface is essential. In the present study, we measured the interface-selective vibrational (χ⁽²⁾) spectra in the OH stretch region of the silica/water interface in a wide pH range of pH 2.0-12.0 while changing the salt concentration by heterodyne-detected vibrational sum-frequency generation spectroscopy. With the help of singular value decomposition analysis, it is shown that the imaginary part of the χ⁽²⁾ (Imχ⁽²⁾) spectra can be decomposed into the spectra of the diffuse Gouy-Chapman layer (DL) and the compact Stern layer (SL), which enables us to quantitatively analyze the spectra of DL and SL separately. The salt-concentration dependence of the DL spectra at different pH values is analyzed using the modified Gouy-Chapman theory, and the pH-dependent surface charge density and the pK_a value (4.8 ± 0.2) of the silica/water interface are evaluated. Furthermore, it is found that the pH-dependent change of the SL spectra is quantitatively explained by three spectral components that represent the three characteristic water species appearing in different pH regions in SL. The quantitative understanding obtained from the analysis of each spectral component in the Imχ⁽²⁾ spectra provides a clear molecular-level picture of the electric double layer at the silica/water interface.

In the present study, the effects of magnesium ions on the conformational changes of the deuterated 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (D54-DMPE) monolayer were elucidated by frequency-resolved sum frequency generation vibrational spectroscopy (SFG-VS) and surface pressure-area isotherm measurements. It is found that the tilt angles of the methyl in tail groups decrease, while the tilt angles of the phosphate and methylene in head groups increase during the compression of the DMPE monolayers at both the air/water interface and the air/MgCl2 solution interfaces. It is also shown that the tilt angle of the methyl in the tail groups slightly decreases, while the tilt angles of the phosphate and methylene in the head groups significantly increase as the MgCl2 concentration increases from 0 to 1.0 M. These results indicate that both the tail groups and the head groups of the DMPE molecules become closer to the surface normal, as the MgCl2 concentration increases in the subphase.

Aptamers have been widely used in the detection, diagnosis, and treatment of cancer. Owing to their special binding affinity toward cancer-related biomarkers, aptamers can be used for targeted drug delivery or bio-sensing/bio-imaging in various scenarios. The interfacial properties of aptamers play important roles in controlling the surface charge, recognition efficiency, and binding affinity of drug-delivering lipid-based carriers. In this research, the interfacial behaviors, such as surface orientation, molecular conformation, and adsorption kinetics of conjugated AS1411 molecules at different cationic lipid bilayer interfaces were investigated by sum frequency generation vibrational spectroscopy (SFG-VS) in situ and in real-time. It is shown that the conjugated AS1411 molecules at the DMTAP bilayer interface show a higher binding affinity but with slower binding kinetics compared to the DMDAP bilayer interface. The analysis results also reveal that the thymine residues of cholesteryl conjugated AS1411 molecules show higher conformational ordering compared to the thymine residues of the alkyl chain conjugated AS1411 molecules. These understandings provide unique molecular insight into the aptamer–lipid membrane interactions, which may help researchers to improve the efficiency and safety of aptamer-related drug delivery systems.

The conformation and adsorption kinetics of oligonucleotides at lipid membrane interfaces are crucial to their biological functions, but are yet not clearly understood. Poly-dT oligonucleotide molecules have been widely used as primers for reverse translation of RNA molecules, as well as a surface recognition agent for mRNA purification and extraction. In this research, the adsorption processes of poly-dT₂₅ on lipid membranes in different ionic solutions were investigated by sum frequency generation vibrational spectroscopy (SFG-VS) together with a single molecule tracking technique in situ and in real time. These systematic studies provide us with molecular insight into the chemical and physical nature of oligonucleotide–membrane interactions, and show us how the electric double layer (EDL) structure changes the conformation and adsorption kinetics of oligonucleotides. The SFG-VS results indicate that an increase of ionic concentration not only decreases the adsorption density of oligonucleotides but also changes the conformation of oligonucleotides from an elongated conformation to a coiled conformation, causing stronger thermodynamic interactions with membranes, as demonstrated by single molecule tracking techniques. It is also shown that the ionic solution can tune the balance between the surface diffusion rate and solution diffusion rate of oligonucleotides significantly. These results demonstrated that the spectra and kinetics collected by in situ label-free SFG-VS detection and the single molecular tracking technique can provide new molecular insights into the mechanisms of oligonucleotide–membrane interactions. These new understandings may help researchers to control the assembly of oligonucleotide–liposome complexes and to improve the efficiency of transportation and delivery of oligonucleotide molecules.