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Biopolymers are a special class of polymers found in nature. Starch, proteins and peptides, DNA, and RNA are all examples of biopolymers, in which the monomer units, respectively, are sugars, amino acids, and nucleic acids.
Biopolymers vs. Polymers
A major but defining difference between polymers and biopolymers can be found in their structures. Biopolymers inherently have a well defined structure: The exact chemical composition and the sequence in which these units are arranged is called the polymer's primary structure. Many biopolymers spontaneously "fold" into characteristic shapes (also known as secondary structure and tertiary structure, which determine their biological functions and depend in a complicated way on their primary structures. Structural biology is the study of the shapes of biopolymers. In contrast most synthetic polymers have much simpler and more random or statistic structures. Another, yet very important, difference is the lack of a molecular mass distribution in most biopolymers. As their synthesis is controlled by a template directed process in most in vivo systems all biopolymers of a type (say one specific protein) are all alike: they all contain the same sequence and number of monomers and thus all have the same mass. This phenomenon is called monodispersity in contrast to the polydispersity encountered in polymers. As a result biopolymers have a polydispersity index of 1.
The convention for a protein is to list its constituent amino acid residues as they occur from the amino terminus to the carboxylic acid terminus.
The convention for a nucleic acid sequence is to list the nucleotides as they occur from the 5' end to the 3' end of the polymer chain, where 5' and 3' refer to the numbering of carbons around the ribose ring which participate in forming the phosphate diester linkages of the chain. Such a sequence is called the primary structure of the biopolymer.
There are a number of biophysical techniques for determining sequence information. Protein sequence can be determined by Edman degradation, in which the N-terminal residues are hydrolyzed from the chain one at a time, derivatized, and then identified. Mass spectrometer techniques can also be used. Nucleic acid sequence can be determined using gel electrophoresis and capillary electrophoresis. Lastly, mechanical properties of these biopolymers can often be measured using optical tweezers or atomic force microscopy.