Section 4.1.5.2. Techniques for studying growth mechanisms

A number of microscopies and other optical methods can be used for studying the crystal growth of macromolecules. These are time-lapse video microscopy with polarized light, schlieren and phase-contrast microscopy, Mach–Zehnder and phase-shift Mach–Zehnder interferometry, Michelson interferometry, electron microscopy (EM), and atomic force microscopy (AFM). Each of these methods provides a unique kind of data that are complementary and, in combination, have yielded answers to many relevant questions.

Time-lapse video microscopy has been used to measure growth rates (e.g. Koszelak & McPherson, 1988; Lorber & Giegé, 1992; Pusey, 1993). It was valuable in revealing unexpected phenomena, such as capture and incorporation of microcrystals by larger crystals, contact effects, consequences of sedimentation, flexibility of thin crystals, fluctuations in growth rates and initiation of twinning (Koszelak et al., 1991).

Several optical-microscopy and interferometric methods are suited to monitoring crystallization (Shlichta, 1986) and have been employed in bio-crystallogenesis (Pusey et al., 1988; Robert & Lefaucheux, 1988). Information concerning concentration gradients that appear as a consequence of incorporation of molecules into the solid state can be obtained by schlieren microscopy, Zierneke phase-contrast microscopy, or Mach–Zehnder interferometry. These methods, however, suffer from a rather shallow response dependence with respect to macromolecule concentration (Cole et al., 1995). This can be overcome by introduction of phase-shift methods and has been successfully achieved in the case of Mach–Zehnder interferometry. With this technique, gradients of macromolecular concentration, to precisions of a fraction of a mg per ml, have been mapped in the mother liquor and around growing crystals. Classical Mach–Zehnder interferometry has been used to monitor diffusion kinetics and supersaturation levels during crystallization, as was done in dialysis setups (Snell et al., 1996) or in counter-diffusion crystal-growth cells (García-Ruiz et al., 1999).

Michelson interferometry can be used for direct growth measurements on crystal surfaces (Komatsu et al., 1993). It depends on the interference of light waves from the bottom surface of a crystal growing on a reflective substrate and from the top surface, which is developing and, therefore, changes as a function of time with regard to its topological features. Because growth of a crystal surface is generally dominated by unique growth centres produced by dislocations or two-dimensional nuclei, the surfaces and the resultant interferograms change in a regular and periodic manner. Changes in the interferometric fringes with time provide accurate measures of the tangential and normal growth rates of a crystal (Vekilov et al., 1992; Kuznetsov et al., 1995; Kurihara et al., 1996). From these, physical parameters such as the surface free energy and the kinetic coefficients which underlie the crystallization process can be determined.

EM (Durbin & Feher, 1990) and especially AFM are powerful techniques for the investigation of crystallization mechanisms and their associated kinetics. The power of AFM lies in its ability to investigate crystal surfaces in situ, while they are still developing, thus permitting one to visualize directly, over time, the growth and change of a crystal face at near nanometre resolution. The method is particularly useful for delineating the growth mechanisms involved, identifying dislocations, quantifying the kinetics of the changes and directly revealing the effects of impurities on the growth of protein crystals (Durbin & Carlson, 1992; Konnert et al., 1994; Malkin et al., 1996; Nakada et al., 1999). AFM has also been applied to the visualization of growth characteristics of crystals made of RNA (Ng, Kuznetsov et al., 1997) and viruses (Malkin et al., 1995). A typical example, Fig. 4.1.5.1, shows two images of the surface of a RNA crystal with spiral growth at low supersaturation and growth by two-dimensional nucleation at higher supersaturation. A noteworthy outcome of the study was the sensitivity of growth to minor temperature changes. A variation of 2–3 °C was observed to be sufficient to transform the growth mechanism from one regime (spiral growth) to another (by dislocation).

Figure 4.1.5.1 | top | pdf | Visualization of the surface of yeast tRNAPhe crystals by AFM. (a) Spiral growth with screw dislocations occurring at lower supersaturation and (b) growth by two-dimensional nucleation occurring at higher supersaturation, showing growth and coalescence of islands and expansions of stacks. Notice that supersaturation and type of growth mechanisms are very temperature-sensitive and are modulated by temperature variation, since in (a), crystals grew at 15 °C and in (b), at 13 °C. Reproduced with permission from Ng, Kuznetsov et al. (1997). Copyright (1997) Oxford University Press.