Fig. 1: Divalent cations induce transient domains of ordered interfacial water.
a Two counter-propagating beams (190 fs, 1030 nm, ω, red arrows) overlap in space and time to illuminate the lipid bilayer membrane. SH photons (2ω, green arrow) are collected (magnification: 50×, NA = 0.65) in the phase-matched direction. SH images of b a symmetric membrane composed of 70:30 mol% DPhPC:DPhPA and c an asymmetric membrane composed of 70:30 mol% DPhPC:DPhPA (top leaflet), and DPhPC (bottom leaflet) in contact with a 150 µM pH neutral KCl solution. d–f SH images of a symmetric membrane composed of 70:30 mol% DPhPC:DPhPA with the bottom leaflet in contact with a 150 µM pH neutral KCl solution, and the top leaflet in contact with a pH neutral CaCl2 d, BaCl2 e, and MgCl2 f at 150 µM ionic strength. The images were collected with all beams P-polarized, and represent 20 × 560 ms frame averages. The scale bar (10 µm) is the same for all images. The SH image of a 150-μM KCl solution was subtracted from the images to remove the hyper-Rayleigh scattering contribution.
