Traditional theory of protein crystallization takes crystallization as a precipitation of ideally dissolved protein molecules from an ideal mother liquor in thermodynamic equilibrium. This gives good results for small organic molecules. However, large surface of protein molecules offers many different adhesion modes and some of them are mutually incompatible in a single crystal. Thus, the protein crystallization is a competitive process between these adhesion modes.
"Dynamic theory of protein crystallization (DTPC)" respects the fact that the molecularly overcrowded solution near the crystallization conditions contains variety of molecular clusters with adhesion properties different from the “naked molecules”. The crystallographer knowing the adhesion properties of clusters can control the adhesion of protein molecules to the surface of growing crystal and increase quality of these crystals simply by adding the required chemicals in the solution. The best results are quarantined by the „principle of the dominant adhesion mode (PDAM)“. In a simplified form, the criterion for successful crystal growth and increase of crystal quality is the increase of a difference between free energies of the competitive adhesion modes
ΔFcryst ~ Fdominant AM - Σ Fincompatible AM
This offers the crystallographer a rational way not only to grow quality crystals but also to choose the required crystalline form. The new approach changes the situation significantly and leads to enhancement of efficiency and accuracy of all standing crystallization methods. DTPC with PDAM are general and should be respected by any method of protein crystallization.
Homogeneous crystallization.
When the crystallographer knows the rules for formation of these temporary complexes, then he can control preferences of the adhesion modes taking part in the crystal grow. He can decide which of the mutually exclusive protein-protein adhesion modes succeeds and becomes dominant by using his knowledge of the adhesion modes using the „protein-surface-active molecules (PSAM)“. Reach source of the adhesion mode examples is the PDB offering an insight how the „protein surface shielding agents“ work in practice and how the „crystal structure forming elements“ help in finding the best crystal architecture.
Protein crystal can be regarded as a well-defined block of highly concentrated solution (20-70 % water content), where the 3D long-range periodicity is ensured by well-defined intermolecular forces for most atoms in the unit cell. Additives adhering temporally to protein surface in this solution can block some surface that may be decisive for adhesion and deposition of new protein molecules to the surface of growing crystal. The crystallization additives shielding the protein-protein adhesion modes are called “Protein Surface Shielding Agents (PSSA)”. Their cover usually a large surface on protein surface exposed to solvent protecting thus the specific adhesion modes.
The adhesion potential of good PSSA’s is lower than adhesion between protein molecules and thus PSSA’s are usually expelled from their position during the crystal growth. However, in some cases they remain built in well-defined positions on the protein surface, so that the PDB gives many examples of their function. Several thousands of examples show the most frequent binding interactions of PEG-based polymers with proteins:
Heterogeneous crystallization.
Many scientists spend a lot of effort to find crystallization initiators (bio-glass, coarsely wrinkled foils, nano-carbon materials, imprinted polymers, porous Si, hoarse hairs, properly coated nanotubes and nanostructured carbon black, etc.).The principle of dominating adhesion mode explains why these mysterious materials are work. These materials depressions with protein adhesive surface. Specific adhesion of protein molecules leads to their energetically demanding pre-orientation and restricts an access to the adhesive surfaces responsible for incompatible PPAM. The unique PPAMs in the growing crystal nuclei make the nuclei more stable because of lower number of stacking faults. They do not dissolve and can continue to grow even after their release into the seemingly under-saturated bulk solution. Contrary to former mutually contradicting explanation, he DTPC explains all experiments available by now on a unique common basis. The new insight promises better design of crystallization catalyzers promising significant advance in the structure determination by diffraction methods.