Design and synthesis of luminescent alkynylplatinum (II) terpyridine complexes and their function as optical probes for biomolecules and ions : from self-aggregarion, supramolecular assembly to host-guest chemistry

Abstract

A number of cationic water-soluble alkynylplatinum(II) terpyridine complexes, [Pt(tpy){C≡CCH2OC6H3-(CH2OH)2-3,5}]OTf, [Pt(tBu3tpy){C≡CCH2OC6H3- (CH2OH)2-3,5}]Cl, [Pt(tpy){C≡CCH2OC6H4-(CH2OH)-4}]OTf, [Pt(tpy){C≡CC6H3- (CH2OH)2-3,5}]OTf, [Pt(tpy){C≡CC6H4-(CH2OH)-4}]OTf, [Pt(tpy)(C≡C－C≡CH)]OTf and [Pt(tpy){C≡CC6H4(CH2NMe3)-4}](OTf)2, have been synthesized. Phosphate anion derivatives of polyanionic nature, e.g. adenosine triphosphate and tyrosine-containing phosphopeptide, were employed to investigate the induced supramolecular helical assembly behaviors of several of the water-soluble platinum(II) complexes through the study of their UV-vis absorption, emission and circular dichroism (CD) spectroscopies. The extent of such induced self-aggregation properties has been shown to be dependent on the concentration of substrates and capable of screening their respective structurally similar analogues. Two of the complexes were further explored for their use as label-free detection probes for real-time monitoring of the conversion of these phosphate anion derivatives to their metabolic products catalyzed by several biologically important enzymes. Michaelis-Menten analyses have been performed to determine the kinetic parameters.

Supramolecular self-assembly and disassembly behaviors of water-soluble platinum(II) complexes have also been explored for their sensing applications for biomacromolecules with quantitative analyses based on UV-vis, emission and CD spectral titration studies. Making use of the selective binding affinity of aptamers towards their target substrates, the extent of self-assembly of the platinum(II) complexes in the presence of lysozyme and thrombin, which arose from their electrostatic interactions with the negatively charged aptamers, could be modulated for the quantification of these substrates. The potential applicability of this probing strategy in real biological environment has been assessed. Other than that, CD spectroscopic studies revealed the formation of supramolecular helical assembly of the platinum(II) complexes upon electrostatic interaction with heparin. Such helical conformation of the heparin–complex mixture together with the distribution of the negatively charged functionalities on the heparin have endowed the complex with high differentiating power for screening possible interfering analogues. Insulin has also been employed as a template for inducing the self-assembly of the platinum(II) complex. Changes in negative surface charge distribution on insulin, brought about by its conformational changes upon conversion to insulin amyloid under destabilizing conditions, have resulted in different extent of induced self-assembly of the metal complex. Spectral changes of the platinum(II) complex has provided a convenient monitoring of insulin amyloid formation, with kinetic parameters determined.

A number of platinum(II) complexes with nucleobase-functionalized alkynyl ligands, [Pt(tpy)(C≡CCH2-T-1)]OTf, [Pt(tBu3tpy)(C≡CCH2T-1)]OTf, [Pt(tpy)(C≡C– CH2-A-9)]OTf, [Pt(tpy)(C≡CCH2-C-1)]OTf and [Pt(tpy)(C≡CCH2-G-9)]OTf (T = thymine; A = adenine; C = cytosine; and G = guanine), and amide-functionalized terpyridine ligands, [Pt{tpy–(CONHPh)2}(C≡CPh)]PF6 and [Pt{tpy–(CONHC6H13)2} (C≡CPh)]PF6, have been synthesized. Cation-binding properties of [Pt(tpy)(C≡CCH2-T-1)]OTf have been examined using UV-vis and emission spectroscopies, and ESI-MS. Pronounced selectivity towards Hg2+ ions was obtained due to the formation of thymine–Hg2+–thymine base pair, through which the Pt… Pt interactions were turned on. Such binding mechanism has been confirmed with the use of a control complex [Pt(tBu3tpy)(C≡CCH2T-1)]OTf and resonance light-scattering experiments. Anion-binding properties of the complexes with amide-functionalized terpyridine moieties towards spherical and non-spherical anions have been examined with UV-vis and emission spectral titration studies. Drastic color changes were observed for [Pt{tpy–(CONHC6H13)2} (C≡CPh)]PF6 in acetone upon F– ion addition, which was attributed to the F– ion-induced deprotonation of the amide functionalities.