Size, Shape, and Structure of Macromolecules

The phenomenon of X-ray scattering forms the basis of techniques that serve to characterize semicrystalline synthetic polymers, to elucidate the chemical structure of biopolymers (e.g., certain proteins), and to determine size, shape and state of aggregation of macromolecules in solution. X-ray scattering is also an important tool for characterizing biopolymers such as proteins and nucleic acids in the native state. 

Scattering phenomenons form the basis of techniques used for the characterization of semicrystalline synthetic polymers and the elucidation of the chemical structure of crystalline biopolymers, such as proteins. Moreover, the size, shape and state of aggregation of macromolecules in solution can be determined using X-ray scattering techniques. This subject has been covered extensively elsewhere [24-54]. 

Dendrimers and dendrons are almost perfectly monodispersed, three-dimensional macromolecules, with a well-defined tree-like globular structure and a high density of functional groups 144-461. Their size, shape, and reactivity are determined by generations [=GJ and chemical composition of the core, their degree of branching, and their surface functionalities (end groups) (Fig. 3) [44]. 

Small-angle X-ray scattering (SAXS) provides valuable information about the global structure of macromolecules, such as the size and shape of RNA (Lipfert and Doniach, 2007). Time-resolved SAXS reports the changes in the global conformation that accompany a structural transition like folding. 

Many of the methods used to extract information related to the structure of macromolecules come from studying the behavior of isolated macromolecules in solution. These techniques are based primarily on the flow behavior in a velocity gradient, the rate of Brownian motion of a particle, or osmotic effects associated with the size of individual molecules. The techniques that have been employed to study size and shape of macromolecules most extensively include viscometry, light scattering, analytical ultracentrifugation, and electron microscopy. 

The size and shape of a macromolecule can be determined by measuring the physical properties of isolated macromolecules in solution. Large rigid macromolecules that are derived from extended structures including the collagen triple helix result in rodlike rigid or semirigid structures. The size, shape, and physical parameters for macromolecules discussed in this book... 

A survey of the abundant literature shows clearly that pH, ionic strength, type of ions, protein to polysaccharide ratio, size, shape, charge density and flexibility of macromolecules are important parameters controlling the extent of phase separation as determined at equilibrium.3,4,6-10 On the contrary, out of equilibrium conditions have attracted much less attention. Some important questions such as the transition from macromolecular complexes/aggregates to the appearance of coacervates, the structure of coacervates and the phase ordering kinetics... 

The results of experiments with different colloids demonstrate some of the ways in which macromolecules of various sizes, shapes, and surface properties may interact to form, in some cases, ordered solutions similar to liquid crystals and, in other cases, three-dimensional networks with the properties of a solid and how the order or the structure present determines the mechanical properties. 

However, the inability of specifying the sizes, shapes, and spatial associations of the hypothesized macromolecules also detracts from any value that current structural models may possess. The question of size can be skirted by simply representing the models as structure elements (76, 77) (or, very loosely, monomers or unit cells) that are assumed to be variously polymerized in the coal. But with respect to molecular shape, which depends to some extent on how the macromolecules are thought to be associated, the models reflect little more than the preferences of their originators. Illustrations of this are the profound differences between structures 1 and 4 on the one hand and structures 2 and 3 on the other, as well as the incorporation in structure 1 of a triptycene moiety that endows the molecule with an extra spatial dimension. 

Dendrimers are highly branched macromolecules with precisely controlled size, shape and end-group functionality. In contrast to conventional synthetic polymers, dendrimers are unique core-shell structures consisting of three basic architectural... 

As mentioned above, concentration polarisation can be very severe in ultrafiltration because the flux through the membrane is high, the diffusivity of the macromolecules is rather low and the retention is normally very high. This implies that the solute concentration at the membrane surface attains a very high value and a maximum concentration, the gel concentration (Cg), may be reached for a number of macromolecular solutes. The gel concentration depends on the size, shape, chemical structure and degree of solvation but is independent of the bulk concentration. The two phenomena, concentration polarisation and gel formation are shown in figure VII -12. 

---