A peptide is a natural or synthetic compound containing two or more amino acids linked by the carboxyl group of one amino acid and the amino group of another. Peptide molecules are structurally like those of proteins, but smaller. The class of peptides includes many hormones, antibiotics, and other compounds that participate in the metabolic functions of living organisms. Peptides can be obtained by partial hydrolysis of proteins. Synthetic methods to acquire peptides include: solid phase peptide synthesis and solution phase peptide synthesis.
The number of amino-acid molecules present in a peptide is indicated by a prefix: a dipeptide contains two amino acids; an octapeptide, eight; an oligopeptide, a few; a polypeptide, many. The distinction between a polypeptide and a protein is imprecise and largely academic; some authorities have adopted, as an upper limit on the molecular weight of a polypeptide, 10,000 (that of a peptide composed of about 100 amino acids).
In 1984 Bruce Merrifield, an American chemist of Rockefeller University won the Nobel Prize for his contribution to the advancement of peptide chemistry. He developed a solid phase peptide synthesis (SPPS) methodology of peptides, which uses a polymer with reactive sites (solid supports, insoluble resin supports) that chemically combine to the developing peptide chain. That solved the problem of previous peptide chemistry. Using Merrifield's technique, the problems associated with low yields due to separation and purification are avoided. The polymer can be filtered and washed without mechanical losses because the polymer is very insoluble.
Solid-phase peptide synthesis consists of three distinct sets of operations: 1) chain assembly on a resin; 2) simultaneous or sequential cleavage and deprotection of the resin-bound, fully protected chain; and 3) purification and characterisation of the target peptide. Various chemical strategies exist for the chain assembly and cleavage/deprotection operations, but purification and characterisation methods are more or less invariant to the methods used to generate the crude peptide product.
Two major chemistries for solid phase peptide synthesis are Fmoc (base labile protecting group) and t-Boc (acid labile a-amino protecting group). Each method involves fundamentally different amino acid side-chain protection and consequent cleavage/deprotection methods, and resins; t-Boc method requires use of stronger HF containing anisole alone or anisole plus other scavengers, where peptide-resins assembled by Fmoc chemistry usually cleaved by less harsh Reagents K or R. Fmoc chemistry is known for peptide synthesis of higher quality and in greater yield than t-Boc chemistry. Impurities in t-Boc-synthesized peptides mostly attributed to cleavage problems, dehydration and t-butylation. For peptide assembly HBTU/HOBt, carbodiimide-mediated coupling and PyBOP/HOBt are the most popular routines. Peptides usually purified by reversed-phase HPLC (high performance liquid chromatography) using columns such as C-18, C-8, and C-4.
For large scale synthesis of well known peptides solution or liquid phase peptide synthesis can be applied. These "classical" methods for synthesis in solution are labour, time, and skill intensive largely due to the unpredictable solubility characteristics of intermediates.