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Polymer Science and Engineering: The Shifting Research Frontiers
Biopolymers Versus Synthetic Polymers
The volume of biopolymers in the world far exceeds that of synthetic macromolecules. Biological polymers include DNA, RNA, proteins, carbohydrates, and lipids. DNA and RNA are informational polymers (encoding biological information), while globular proteins, some RNAs, and carbohydrates serve chemical functions and structural purposes. In contrast, most synthetic polymers, and fibrous proteins such as collagen (which makes up tendon and bone) and keratin (which makes up hair, nails, and feathers), are structural rather than informational or chemically functional. Structural materials are useful because of their mechanical strength, rigidity, or molecular size, properties that depend on molecular weight, distribution, and monomer type. In contrast, informational molecules derive their main properties not simply from their size, but from their ability to encode information and function. They are chains of specific sequences of different monomers. For DNA the monomers are the deoxyribonucleic acid bases; for RNA, the ribonucleic acid bases; for proteins, the amino acids; and for carbohydrates or polysaccharides, the sugars. The paradigm in biopolymers is that the sequence of monomers along the chain encodes the information that controls the structure or conformation of the molecule, and the structure encodes the function. An informational polymer is like a necklace, and the monomers are like the beads.
For RNA and DNA, there are 4 different monomers (beads of different colors). Information is encoded in the sequence of bead colors, which in turn controls the sequence of amino acids in proteins. There are 20 different types of amino acid monomers; in the necklace analogy, there are 20 different colors of beads. A globular protein folds into one specific compact structure, depending on the amino acid sequence. This balled-up shape, or structure, is what determines how the protein functions. The folding of the linear structure produces a three-dimensional shape that controls the function of the protein through shape selection.
Except in special cases, synthetic polymer science does not yet have the precision to create specific monomer sequences: polymers can be synthesized as homopolymers, chains composed of only a single type of monomer, or simple block copolymers, where the monomers repeat only in the simplest patterns, AAA BBB AAA BBB, or random sequences. But the ability to synthesize specific monomer sequences by a linear process would have extraordinary potential. For example, it is the ability to create specific monomer sequences that distinguishes biological life forms, and the corresponding complex hierarchies of structure and function, from simpler polymeric materials. Hence one of the most exciting vistas in polymer science is the prospect of creating informational polymers through control of specific monomer sequences. The present state of