Proteins are an important class of biological macromolecules present in all biological organisms,which is made up of such elements as carbon, hydrogen, nitrogen, oxygen, and sulphur. All proteins are polymers of amino acids. According to their physical size, proteins are nanoparticles (definition: 1-100 nm). The polymers, also known as polypeptides, consist of a sequence of 20 different L-α-amino acids, also referred to as residues. For chains under 40 residues the term peptide is regularly used instead of protein. To be able to perform their biological function, proteins fold into one or more specific spatial conformations, driven by a number of noncovalent interactions such as hydrogen bonding, ionic interactions, Van Der Waals forces and hydrophobic packing. To understand the functions of proteins at a molecular level, it is often necessary to determine their three dimensional structure. This is the topic of the scientific field of structural biology, that employs techniques such as X-ray crystallography, NMR spectroscopy,and Dual Polarisation Interferometry to determine the structure of proteins.
A number of residues is necessary to perform a particular biochemical function, and around 40-50 residues appears to be the lower limit for a functional domain size. Protein sizes range from this lower limit to several thousand residues in multi-functional or structural proteins. However, the current estimate for the average protein length is around 300 residues. Very large aggregates can be formed from protein subunits, for example many thousand actin molecules assemble into a microfilament.