Labile region

The effect of solution concentration on nucleation rate is shown qualitatively in Fig. 9. At low levels of supersaturation, the rate is essentially zero but, as concentration is increased, a fairly well defined critical supersaturation is reached (point 1), beyond which nucleation rate rises steeply (curve 1-2). Point 1 may be regarded as the threshold of the labile region. Data from a series of such curves at different temperatures establish the locus of points at which nucleation starts, i.e., the Miers supersolubility curve discussed in Section II. 

In terms of the phase diagram, ideal crystal growth would begin with nuclei formed in the labile region, but just beyond the metastable. There, growth would occur slowly the solution, by depletion, would return to the metastable state where no more stable nuclei could form and the few nuclei that had established themselves would continue to grow to maturity at a pace free of defect formation. Thus in growing crystals for X-ray diffraction analysis, one attempts, by either dehydration or alteration of physical conditions, to transport... 

There is another viewpoint on the stability of a supercooled liquid, according to which the region of metastable states of a one-component liquid does not pass into a labile region with decreasing temperature. A supereooled liquid has no spinodal determined by eondition (3). V. P. Skripov thought that at T = 228 K there was no divergenee of Pj., of supercooled water, but there was a sufficiently blurred and small normal maximum. The dashed line 3 (Fig. 9) corresponds to this point of view. 

Labile region—where nuclei are formed spontaneously from a clear solution. 

If the supersaturation generated during the cycle is not completely removed, the level of supersaturation attained during the following cycle is increased. This increase from cycle to cycle will continue until the supersaturation level of the solution exceeds the metastable region and enters the labile region, where spontaneous nucleation occurs. The occurrence of spontaneous nucleation means loss of control of crystal size. 

Usually the crystallization equipment is charged with a clear feed solution. As this solution is saturated, it is important to control the increase in supersaturation as the labile region is approached. This is important since the formation of an excessive number of nuclei will cause a continuous crystallizer system to have an extremely long period before desired crystal... 

Low suspension density. Since the solution cannot return to saturation before being resupersaturated in the circulation loop, the residual supersaturation builds up to the point that the solution is in the labile region. To remedy, increase the crystals in suspension to maintain the design density. 

Bacterial luciferases are exquisitely sensitive to inactivation by proteases. The protease labile region is on the a (Vh) subunit, in particular the disordered loop (Phe -Thr ). The /3 subunit, which has a shorter loop without the a sequence, is resistant to proteases. Binding of FMN, FMNH2, and/or aldehyde renders the enzyme insensitive to proteolysis. Binding of the substrates also protects the otherwise reactive SH group (aCys-106) from modification by MEM or lAA. The substrate binding apparently induces an altered conformation in which the loop region is shielded from protease attacks. 

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