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Unnatural α-amino acids form a family of essential molecules used for, among other applications, the synthesis of modified peptides, to improve resistance to proteolytic enzyme degradation, and to modulate physico- and biochemical properties of bioactive peptides as well as chiral inducers in asymmetric synthesis. Among them, silicon-containing unnatural amino acids are becoming an interesting new class of building blocks. The replacement of carbon atoms in bioactive substances with silicon is becoming increasingly popular. Peptides containing silyl amino acids hold great promise for maintaining or reinforcing the biological activity of active compounds, while they simultaneously enhance their resistance to enzyme degradation. In addition, the lipophilicity of the silicon atom facilitates their membrane crossing and their bioavailability. Nowadays, the interest of the pharmaceutical industry in peptide- and protein-based therapies is increasing. In this respect, silicon-containing amino acids and peptides are likely to be a significant part of future innovations in this area, and more generally in the area of biomolecules. In this process, commercial availability of silicon-containing amino acids is necessary: new syntheses have been developed, and work in this area is ongoing. This review aims to be a comprehensive and general summary of the different methods used to prepare silicon-containing amino acids and their implications on conformational structures and biological applications when they are incorporated into bioactive molecules.

The analogue gamma-(dimethylsila)-proline, denoted silaproline (Sip), was synthesized in both enantiomerically pure forms by diastereoselective alkylation of a chiral glycine equivalent with use of Schöllkopf's bis-lactim ether method. The effect of replacing a proline residue in model peptides by this new proline surrogate has been examined in the crystal state by X-ray diffraction and in solution by IR absorption and NMR techniques. Silaproline and proline-containing sequences exhibit very similar conformational properties. Silaproline was also substituted for proline in a neurotensin (8-13) analogue that retained biological activity and exhibited enhanced resistance to biodegradation.

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The silylated amino acid (l)-(trimethylsilyl)alanine (TMSAla) was incorporated at the C-terminal end of the minimal biologically active neurotensin (NT) fragment, leading to the synthesis of new hexapeptide NT[8-13] analogues. Here, we assessed the ability of these new silylated NT compounds to bind to NTS1 and NTS2 receptors, promote regulation of multiple signaling pathways, induce inhibition of the ileal smooth muscle contractions, and affect distinct physiological variables, including blood pressure and pain sensation. Among the C-terminal modified analogues, compound 6 (JMV2007) carrying a TMSAla residue in position 13 exhibits a higher affinity toward NT receptors than the NT native peptide. We also found that compound 6 is effective in reversing carbachol-induced contraction in the isolated strip preparation assay and at inducing a drop in blood pressure. Finally, compound 6 produces potent analgesia in experimental models of acute and persistent pain.

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The results presented here show that elementary design enhancements have led to a 20-fold increase in the cellular uptake properties of a Pro-rich cell-penetrating peptide. These results are relevant not only due to the increasing interest in using CPPs as molecular shuttles for intracellular drug delivery but also because they illustrate the power of combining conformational analysis with rational design to modulate the behavior of biologically active compounds.

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