Pg. Vekilov et al., GROWTH-PROCESSES OF PROTEIN CRYSTALS REVEALED BY LASER MICHELSON INTERFEROMETRY INVESTIGATION, Acta crystallographica. Section D, Biological crystallography, 51, 1995, pp. 207-219
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.