Dynamical Time Scales of Aqueous Solvation at Negatively Charged Lipid/Water Interfaces

Abstract

Time-resolved second harmonic generation spectroscopy of a solvatochromic dye coumarin 314 was used to investigate how the solvation dynamics at the air/water interface are affected by lipid surfactants with anionic carboxylate and sulfate headgroups. The surfactants were chosen to mimic common functional groups and the negative charge of biological aqueous interfaces such as cell membranes and folded proteins. The diffusive component of the solvation response, associated with rearrangement of the water hydrogen bond network, exhibits a biexponential decay. The two time scales change as a function of the surface charge density, compared to the bare air/water interface solvation dynamics, τ1 = 250 ± 50 fs and τ2 = 2.0 ± 0.4 ps, which are similar to the bulk water response, τ1 = 130−250 fs and τ2 = 0.6−1.2 ps. The faster τ1 component is unaffected at low to intermediate sulfate surface coverage (270 ± 50 fs at 500 A2/molecule and 225 ± 25 fs at 250 A2/molecule) but becomes slower at high surface coverage (1...