Effects of Kinetic Roughening and Liquid−Liquid Phase Transition on Lysozyme Crystal Growth Velocities

Abstract

We measured the growth velocities of the (110) face of tetragonal lysozyme, V (cm/s), at four different concentrations, c (mg/mL), as the solution temperature, T (°C or K), was reduced. For a broad range of T dependent on c, we find that the growth velocities increased as the solution temperature was reduced. The initial increase in V is well characterized by the 2D nucleation model for crystal growth, yielding the magnitude of an effective barrier for growth, γs = (1.2 ± 0.1) × 10-13 erg/molecule. Below certain temperatures, Tcr, dependent on c, however, a kinetic roughening hypothesis that considers the continuous addition of molecules anywhere on the crystal surface better describes the observed growth velocities. The application of the continuous growth model, up to the solution cloud-point temperatures, Tcl, enabled the determinations of the crossover concentration, cr, from estimated values of Tcr. For all conditions presented, we find that the crossover from growth by 2D nucleation to continuous addition occurs at a supersaturation, σc = 2.0 ± 0.1. Moreover, we find the energy barrier for the continuous addition, Ec, within the temperature range Tcl < T < Tcr to be (6 ± 1) × 10-13 erg/molecule. Further reduction of T below ∼3−4 °C of Tcl also revealed a rapid slowing of crystal growth velocities. From quasi-elastic light scattering investigations, we find that the rapid diminishment of crystal growth velocities can be accounted for by the phase behavior of lysozyme solutions. Namely, we find the reversible formation of dense fluid protodroplets comprised of lysozyme molecules to occur below Tcl. Hence, the rapid slowing of growth velocities occurs as a result of the sudden depletion of “mobile” molecules within crystal growth solutions as dense fluid protodroplets form.