Shape change in crystallization of biological macromolecules
- PDF / 900,576 Bytes
- 6 Pages / 585 x 783 pts Page_size
- 72 Downloads / 227 Views
Introduction Intuitively, the shape of a molecule is seen to determine the structure of its crystal—we expect spheres to form closepacked face-centered-cubic or hexagonal close-packed lattices and cubes to adopt a cubic arrangement. Nevertheless, in some cases, the flow of causality is reversed, and the crystal lattice imposes a change in the molecular shape. A classic example of conformation change in crystallization is biphenyl [(C6H5)2] (see Figure 1).1 To accommodate the electron density around the four hydrogen atoms in ortho positions, the tethered benzene rings twist to a torsional angle of about 45° in the gas phase.2 In the crystal, biphenyl attains a planar structure, which offers the advantage of denser plane-to-plane packing of the individual benzene rings. To avoid overlap, the ortho hydrogens fold away from the inter-ring bond and adopt a nontypical configuration associated with increased energy.1 Thus, while the planar configuration lowers the free energy of the crystal by facilitating attractive intermolecular contacts, it increases the free energy of individual molecules. The example of biphenyl highlights the critical condition for shape change in crystallization—the gain in crystallization free energy in the modified shape should be greater than its loss due to the selection of a less favorable molecular conformation. Biphenyl also illustrates a primary question regarding
shape change in crystallization: Does crystallization select one of many configurations present in the free phase, gas, or solution, or does it enforce shape change during molecular aggregation? In this article, we discuss several scenarios in which shape change accompanies and facilitates crystallization. We focus on the realm of biological macromolecules in which the large molecular sizes ensure conformational flexibility. Proteins are emphasized, since they crystallize more readily3–5 than nucleic acids and polysaccharides, and the physical mechanisms involved in proteins are also better understood.6–8
Protein conformation in solution and in crystals Shape change in crystallization is a fundamental problem in biological crystallography. Biological macromolecules are large.9 They have hierarchical structures in which certain superatomic elements, such as α-helices and β-sheets in proteins, are relatively rigid, while the loops connecting them are flexible and permit significant conformational freedom. These molecules scatter x-rays, neutrons, and electrons weakly, and determination of their atomic structure is only possible after signal amplification by similarly aligned molecules in a regular array, such as a crystal. Therefore, the molecular structure emerging from these determinations is the one in the crystalline state. Conversely, the molecules perform their functions
Peter G. Vekilov, Department of Chemical and Biomolecular Engineering, University of Houston, USA; [email protected] Sungwook Chung, School of Chemical and Biomolecular Engineering, Pusan National University, South Korea; [email protected] Katy N. Olafson, U
Data Loading...
