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Thesis Format

Integrated Article

Degree

Master of Science

Program

Chemistry

Collaborative Specialization

Molecular Imaging

Supervisor

Luyt, Leonard G.

Abstract

Constrained peptides have been successful as therapeutics owing to the rigidity in their framework, allowing them to have enhanced pharmacological properties. Apart from covalent bond based cyclization, constraints can be induced by cyclization using metal coordination. The metals Re and 99mTc are one of the few metals which have not been studied in great detail for their ability to cyclize linear peptides. A tri-alanine sequence was employed in the peptide backbone due to its ability to form stable helices. In addition, another tripeptide sequence RGD, which is recognized by the integrin avb3, was also used. These tripeptide sequences flanked by unnatural amino acid and synthetic chelating agent on the terminal ends, were cyclized using [Re(CO)3(OH2)3]+ to form a 2+1 chelation complex. This involved chelation to the metal through a bidentate ligand, pyridyl triazole acetic acid or 2,2’-bipyridine-4-carboxylic acid (synthetic chelating ligand), and a monodentate ligand 3-Pal (unnatural amino acid) from N- and C- terminal respectively. NMR spectroscopy was used to compare the changes in chemical shift between linear and cyclic peptides. Specific changes in the NMR of cyclic peptide confirmed the formation of 2+1 chelation system. Circular dichroism spectroscopy was able to confirm the formation of a turn into the linear peptide on cyclization. Variable temperature NMR spectroscopy suggested the formation of a secondary structure by detecting intramolecular hydrogen bonding in the cyclic peptide. Lastly, the linear peptide pyta-Ala-Ala-Ala-3Pal was successfully radiolabeled using [99mTc(CO)3(OH2)3]+. Linear peptides were successfully cyclized through 99mTc/Re(CO)3, thus creating metal foldamers where the metal acts as the central core around which the turn occurs.

Summary for Lay Audience

Peptides have increasingly garnered attention as a targeting component for molecular imaging probes due to cumulative factors such as low molecular weight, small size and their ease to synthesize. They display good tumour to background ratio and showcase the ability of having a variety of bifunctional ligands attached to its C and N terminus. However, there are number of disadvantages associated with having linear peptides as the targeting entity, such as their poor in vivo stability and reduced specific binding.

Structural constraints, through cyclization of peptide backbone, can resolve some of the difficulties seen with short linear peptides. Cyclic peptides have improved stability and targeting affinity compared to their linear peptide counterparts. Owing to a locked conformation, cyclic peptides maintain a desirable secondary structure which results in higher binding affinity. Apart from covalent bond based cyclization, cyclization can also be induced through metal coordination. Metal complexes are known to stabilize peptide sequences. A turn can be induced within a peptide backbone by metal coordination where the metal acts as a central core around which, a turn occurs.

99mTc is used in 80 % of all nuclear medicine procedures worldwide. However, 99mTc is a short-lived radioisotope which is used in sub-microgram scale. This makes standard spectroscopic characterization quite difficult. Rhenium and technetium belong to the same group of the periodic table and therefore have similar physical and chemical properties. Hence, rhenium is used as a non-radioactive analogue.

The research focuses on cyclization of linear peptides by 99mTc/ Re(CO)3+ coordination through the use of a mono and a bidentate ligand. Pyta and 2,2’-bipyridine-4-carboxylic acid were used as bidentate ligand along with 3-pyridyl alanine as a monodentate ligand. The cyclic and linear peptides were characterised by various spectroscopic methods, which suggested a change in conformation upon cyclization. This was especially observed for peptides with pyta as a ligand, suggesting formation of a secondary structure. Thus, an innovative approach of introducing structural constraints through metal based cyclization is described. This approach would provide a new direction for creating imaging agents, where a medically relevant metal is incorporated into a peptide structure.

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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