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.
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).
Solid phase peptide
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.
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.