Luminescent platinum(II) probes for mismatched DNA sensing, cell types differentiation and intramolecular variations under environmental changes

Abstract

Platinum(II) complexes showing attractive photophysical and biological properties have been developed for decades; their emissions can be tuned between metal-to-ligand charge-transfer (3MLCT) / intraligand (3IL) and metal-metal-to-ligand charge-transfer (3MMLCT) excited states, allowing applications including sensing of pH, proteins and DNA or biological imaging. Development of a method for selectively detecting DNA mismatched base pairs is highly important to early cancer diagnosis, as mutations or epigenetic alterations in MMR genes lead to accumulation of DNA mismatched base pairs, eventually resulting in genetic aberrations that can drive carcinogenesis. This thesis, with syntheses and characterization of several types of new platinum(II) complexes, describes (i) a new design strategy for luminescent sensor, which features emission colour changes and can be employed to differentiate mismatched DNA and matched DNA, (ii) sensing of organic solvent, temperature, protein and DNA by dinuclear platinum(II) complexes which undergo dynamic conformational changes, and (iii) differential cell labeling by luminescent platinum(II) complex, which has potential application in distinguishing cancer cells from normal cells.

Complexes [PtII(C^N)(NHC)(X)], including PtCN2 (HC^N = benzo[h]quinolone, NHC = N-heterocyclic carbene, X = Cl), and [PtII(C^N)(NHC)(C≡NR)]+ were prepared. PtCN2 showed distinct emission colour changes in the presence of mismatched DNA, due to switching from aggregate emission to the binding monomer emission. This complex not only recognized CC mismatched DNA, but also selectively interacted with various mismatched base pairs, leading to drastic emission colour change from orange-yellow to blue-green.

A series of [(C^N^N)PtII(LL)PtII(C^N^N)]2+ complexes bearing cyclometallated 6-phenyl-2,2′-bipyridines and bridging di-NHC or di-phosphine ligand, including Pt-4 and Pt-5 (LL = bis(N-n-butyl)- and bis(N-benzyl)-NHC-CH2-NHC, respectively; HC^N^N = 4,4′-di(tert-butyl)-6-phenyl-2,2′-bipyridine for both), were synthesized, and their responses to stimuli in solution, such as solvent, temperature, ionic strength, viscosity, protein, and DNA, were studied. The emission profiles of these complexes changed with environment due to varying intramolecular Pt-Pt distance and the resulting emissive excited states. The distinct emission spectra serve as fingerprint for G-quadruplex and double-stranded DNA. The emission measurements can monitor the protein aggregation of native insulin. Pt-5 also showed characteristic mitochondrial localization at HCT116 cells; however, it is located mainly on cell membrane in other cell lines such as HepG2 and Hela. The differential cell imaging property may reflect distinct properties of different cell lines.

5.4 was found to label preferentially the hepatocellular carcinoma cell lines (PLC and HepG2) compared to non-tumorigenic liver cell line (L02). There was a higher level of cellular uptake of 5.4 in PLC and HepG2 cells compared to L02 cells, in agreement with the higher emission intensity observed in PLC and HepG2 cells using fluorescence microscopy. Aggregation effect of 5.4 in the presence of thiol containing amino acid and peptide was observed. The higher cellular uptake of 5.4 in PLC cells also resulted in a yellow coloration of cell pellets, consistent with UV-vis spectral changes in the presence of thiols amino acid or peptide. 5.4 showed no significant cytotoxicity towards normal cell lines (L02), having the potential to be a molecular probe for discriminating liver cancer cells from the normal cells.