We present measurements of rate constants for thermal-induced reactions of the 11-cis retinyl chromophore in vertebrate visual pigment rhodopsin, a process that produces noise and limits the sensitivity of vision in dim light. At temperatures of 52.0–64.6 °C, the rate constants fit well to an Arrhenius straight line with, however, an unexpectedly large activation energy of 114 ± 8 kcal/mol, which is much larger than the 60-kcal/mol photoactivation energy at 500 nm. Moreover, we obtain an unprecedentedly large prefactor of 1072±5 s−1, which is roughly 60 orders of magnitude larger than typical frequencies of molecular motions! At lower temperatures, the measured Arrhenius parameters become more normal: Ea = 22 ± 2 kcal/mol and Apref = 109±1 s−1 in the range of 37.0–44.5 °C. We present a theoretical framework and supporting calculations that attribute this unusual temperature-dependent kinetics of rhodopsin to a lowering of the reaction barrier at higher temperatures due to entropy-driven partial breakup of the rigid hydrogen-bonding network that hinders the reaction at lower temperatures.

A simple numerical model is proposed to compute the energy and momentum accommodation of molecules scattered from highly corrugated, disordered surfaces. The model is an extension of the “washboard model”, which assumes that the component of the molecule’s momentum parallel to the local surface tangent is conserved on impact and the normal component is altered by a hard, elastic collision with a moving surface “cube” with an adjustable effective mass. The surface is represented by Gaussian hills and valleys of random location and height. In contrast to the washboard model, the current model is fully three-dimensional and includes in-plane and out-of-plane scattering as well as trapping-desorption. In addition, it can be applied to highly corrugated surfaces and does not invoke a regular, periodic topography. This increased realism comes at the expense of an analytical solution; numerical simulations must be performed. We develop a very efficient procedure for carrying out the simulations. We test the model by comparing detailed angular and velocity scattering distributions for Xe scattering from a Pt(111) surface with those obtained by realistic molecular dynamics simulations of the same system. We then apply the model to the accommodation of H2, N2, CO, and CO2 on surface materials employed on vehicles in low Earth orbit. The model is capable of accurately reproducing the results of experimental measurements on these highly corrugated surfaces.