Synthesis of Novel Amino Acids and Use of Peptides & Peptidomimetics Containing Unnatural Amino Acids for the Development of Selective Melanocortin Peptide Antagonists and for the Study of Melanocortin Receptor Signaling

Abstract

Unnatural amino acids are indispensible tools, not only for the elucidation of molecular mechanisms during the study of the complicated biological system, but also for the development of novel peptide and protein drugs with better efficacy and lower toxicity. Beta-substituted gamma,delta-unsaturated amino acids have been shown to be an important type of novel amino acid because of the terminal double bond which can be converted to many other functionalities. The methodology for the synthesis of syn-beta-substituted gamma,delta-unsaturated amino acids has been developed. However, there is no satisfactory general method for the synthesis of anti-beta-substituted gamma,delta-unsaturated amino acids. Therefore, a general methodology was developed by using the Eschenmoser-Claisen rearrangement for the synthesis of both racemic and optically active anti-beta-substituted gamma,delta-unsaturated amino acids. This rearrangement is highly diastereoselective and good asymmetric induction was obtained with a relatively small C2-symmetric chiral auxiliary (2R,5R)-dimethylpyrrolidine. In an effort to design peptide antagonists that are selective for human melanocortin 4 receptor, highly constrained trans and cis 4-guanidinium proline derivatives were synthesized and incorporated in various melanotropin analogues designed to mimic the endogenous hMC1,4R selective antagonist hASIP (Agouti Signaling Protein) central loop. Biological assays show that some of these analogues are highly selective for hMC1R and/or hMC4R with partial agonist or antagonist activities due to a new beta-turn structure induced by the presence of the constrained amino acids. According to molecular modeling studies, the lowest energy conformations of these selective analogues resemble the NMR solution structure of the hASIP central loop. Therefore, a new template was developed for the rational design of novel selective melanotropin analogues that may have therapeutic potential. To further understand the molecular mechanisms of hMC4R signaling upon agonist activation, an hMC4R selective nonpeptide agonist THIQ and its fluorescent dye labeled derivatives were needed to compare to peptide agonist MTII with regard to receptor phosphorylation, internalization, etc. An improved synthetic method was developed for the efficient synthesis of THIQ. A method for the synthesis of TRITC labeled THIQ derivatives was also developed.