GROWTH-PROCESSES OF PROTEIN CRYSTALS REVEALED BY LASER MICHELSON INTERFEROMETRY INVESTIGATION

The elementary processes of protein crystal growth were investigated b y means of laser Michelson interferometry on the example of the (101) face of tetragonal hen egg-white (HEW) lysozyme. The method allows rea l-time in situ observations of the morphology of the growing protein c rystal surface, as well as simultaneous precise measurement of growth rate and step velocity on identified growth-layer sources. At the crit ical supersaturation of 1.6 the growth mechanism was shown to transfor m from dislocation-layer generation to surface nucleation. Measurement s on different growth hillocks, with material of a different source an d at a different temperature, all indicated that for supersaturations lower than similar to 1 growth is hindered by the competitive adsorpti on of (most probably) other protein species contained in HEW, although the material is pure by most analytical methods. At supersaturations sigma less than or equal to 0.4 other impurities sometimes led to cess ation of growth. However, at sigma in the range 0.9 less than or equal to sigma less than or equal to 1.6 growth processes are determined by the kinetics of pure lysozyme. This enabled us to measure the step ki netic coefficient beta for crystallization of a protein substance for the first time: beta = 2.8 mu m s(-1). This also means that by working in this supersaturation range we can eliminate the impurity effects. Other means to reduce influence of impurities is to use, if possible, a higher crystallization temperature. It is shown that slow crystalliz ation of proteins is due primarily to impedance of the elementary act of entering the growth site and not to the low concentration of the so lution. The value of beta does not depend on temperature, indicating t he decisive role of entropy, not energy barriers, in the crystallizati on of biological macromolecules.