Grand Séminaire d'Institut

Vendredi 7 Juin 2024 à 11h00.

(1) Observing Molecular Motions at Living Biological Cell Membranes through Nonlinear Light Scattering and Microscopy, and (2) Nuclear quantum effects on vibrational relaxation at aqueous-oxide interfaces.

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(1) Nonlinear light scattering in the form of Second Harmonic Generation, due to its symmetry properties, has been proven effective for observing molecular adsorption and transport at the surfaces of colloidal objects, including living biological cells. This method affords membrane specificity, real time resolution, and the ability to image single cells in examining molecule-membrane interactions.

This talk will lay out the basic principles of Second Harmonic Light Scattering (SHLS) and illustrate how SHLS can be applied to examine the surface of colloidal objects and molecular transport at cell membranes. This method has been used to determine the fundamental mechanism of the century-old Gram stain for classifying bacteria. Examples illustrating effects of molecular structure and the membrane structure in influencing molecular adsorption and transport at living cell membranes will be presented. SHLS applied in the imaging modality shows that molecular transport can be examined with spatial resolution and that the transport rate varies greatly from regions to regions at a cell membrane. Furthermore, it will be shown that this second harmonic microscopic tool can be used to determine membrane phase transition and membrane asymmetry.

(2) Nuclear quantum effects, including zero-point energy, tunneling, and nuclear delocalization, affect the properties of water. Recent simulations established that nuclear quantum effects can impact intermolecular and intramolecular interactions and are more distinct in H2O compared to D2O. However, the existence and impact of such effects at oxide-water interfaces, that are key to diverse chemical and physical processes relevant to the environment and geochemistry, are not well known. Our ultrafast time-resolved vibrational sum-frequency generation (vSFG) spectroscopy experiments show that in the absence of surface charge (pH 6), H2O/Al2O3 surfaces exhibit significantly faster OH stretch vibrational relaxation (~100 fs) compared to bulk water (~200 fs). In addition, the dynamics do not depend on whether strongly or weakly hydrogen bonded species are probed. However, slower OD stretch vibrational relaxation is observed at D2O/Al2O3 interfaces where the strongly hydrogen bonded OD relaxes more rapidly (~200 fs) than in bulk D2O (~400 fs) while weakly hydrogen bonded interface OD relaxes on a timescale comparable to the bulk. While it is unclear whether these vibrational dynamics are due to changes in the local hydrogen bonding environment or some other effect (e.g., vibrationally assisted proton transfer), there is evidence of interfacial nuclear quantum effects.

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