A macromolecule is a molecule composed of a very large number of atoms. But generally the use of term is restricted to molecules of more than 100 atoms, and in particular to polymers. Many examples come from biology and in particular biochemistry. These include proteins, starches, and nucleic acids (such as DNA), which are sometimes called "biomacromolecules" or biopolymers. Synthetic examples include plastics. The integral domains of crystals and metals, while composed of very large numbers of atoms joined by molecule-like bonds, are rarely referred to as "macromolecules."
The term "macromolecule" is sometimes used to refer to aggregates of two or more macromolecules held together by intermolecular forces rather than by chemical bonds. This usage is common in particular when the individual macromolecules involved aggregate or "assemble" spontaneously and rarely exist in isolation. Such an aggregate is more properly called a macromolecular complex. In such a context, individual macromolecules are often referred to as subunits (see e.g. protein subunit).
Substances that are composed of macromolecules often have unusual physical properties. The properties of liquid crystals and such elastomers as rubber are examples. Although too small to see, individual pieces of DNA in solution can be broken in two simply by suctioning the solution through an ordinary straw. This is not true of smaller molecules. The 1964 edition of Linus Pauling's College Chemistry asserted that DNA in nature is never longer than about 5000 base pairs. This is because biochemists were inadvertently and with perfect consistency breaking their samples into pieces. In fact, the DNA of chromosomes can be tens of millions of base pairs long.
Another common macromolecular property that does not characterize smaller molecules is the need for assistance in dissolving into solution. Many require salts or particular ions to dissolve in water. Proteins will denature if the solute concentration of their solution is too high or too low.
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