2.Solid-phase peptide synthesis
1.Classification of peptide
Class 1. Biologically active peptides
II.Antagonists and inhibitors
Ion channel modulator
Modulator of protein-protein interaction
IV.Cellular delivery system or component
cell-specific homing sequence
Organelle targeting sequence
Class 2. Nonbiologically active sequences
Secondary structure mimetic
Protein domain mimetic
Regioselectively addressable functionalized template
2.Solid-phase peptide synthesis
Nowadays, "peptide synthesis" includes a large range of techniques and procedures that enable the preparation of materials ranging from small peptides to large proteins.The pioneering work of Bruce Merrifield,which introduced solid phase peptide synthesis(SPPS),dramatically changed the strategy of peptide synthesis and simplified the tedious and demanding steps of purification associated with solution phase synthesis.Moreover,Merrifield's SPPS also permitted the development of automation and the extensive range of robotic instrumentation now available.After defining a synthesis strategy and programming the amino acid sequence of peptides,machines can automatically perform all the synthesis steps required to prepare multiple peptide samples.SPPS has now become the method of choice to produce peptides,though solution phase synthesis can still be useful for large-scale production of a given peptide.
In SPPS,two main strategies are used: the Boc/Bzl and the Fmoc/tBu approaches for T/Pn protecting groups.The former strategy is based on the graduated acid lability of the side-chain protecting groups.In this approach,the Boc group is removed by neat TFA or TFA in dichloromethane,and the side-chain protecting groups and peptide-resin linkages are removed at the end of the synthesis by treatment with a strong acid such as anhydrous hydrofluoric acid(HF).While this method allows efficient synthesis of large peptides and small proteins,the use of highly toxic HF and the need for special polytetrafluoroethylene-lined apparatus limit the applicability of this approach to specialists only.Moreover,the use of strongly acidic conditions can produce deleterious changes in the structural integrity of peptides containing fragile sequences.
The Fmoc/tBu method is based on an orthogonal protecting group strategy.This approach uses the base-labile N-Fmoc group fro protection o the α-amino function,acid-labile side-chain protecting groups and acid-labile linkers that constitute the C-terminal amino acid protecting group.This latter strategy has the advantage that temporary and permanent orthogonal protections are removed by different mechanisms,allowing the use of milder acidic conditions for final de-protection.
For peptide-based drug design, there are several major considerations that limit clinical applications such as: (1) rapid degradation by many specific or nonspecific peptidases under physiological conditions; (2) conformational flexibility which allows a peptide to bind to more than one receptor or receptor subtype leading to undesirable side effects; (3) poor absorption and transportation because of their high molecular mass or the lack of specific delivery systems, especially for some peptides which require the passage through the blood-brain-barrier (BBB) to act in the central nervous system (CNS). In an effort to counteract these problems, peptidomimetic drug design has emerged as an important tool for both peptide chemists and medicinal chemists. This approach has evolved as an interdisciplinary scientific endeavor combining organic chemistry, biochemistry and pharmacology.
A peptidomimetic is a compound containing non-peptidic structural elements that is capable of mimicking or antagonizing the biological action of a natural parent peptide.As one of the major efforts in organic chemistry, a variety of molecules have been designed to mimic the secondary structures of peptides, such as α-helices, β-turns, and β-sheets. In order to explore the structure-activity relationships (SAR) of bioactive peptides, a number of strategies have been developed by incorporation of conformationally constrained amino acids, modification of the peptide backbone by amide bond isosteres, cyclizations, attachment of pharmacophores to a template or scaffold, and the synthesis of nonpeptide analogs. Peptidomimetic has emerged as a powerful approach in many areas of pharmceutical research,including:
Receptor-targeted drug discovery
Peptidase-targeted drug discovery
Signal transduction-targeted drug discovery