Induced self-assembly of platinum(II) alkynyl complexes through specific interactions between citrate and guanidinium for proof-of-principle detection of citrate and an assay of citrate lyase

Water-soluble alkynylplatinum(II) terpyridine complexes appended with guanidinium moieties, [Pt(tpy)(C≡C-Ar)][OTf]2 (tpy=terpyridine; OTf=trifluoromethanesulfonate; Ar=C6 H4-{NHC(=NH2(+))(NH2)}-4 (1), C6 H4-{CH2 NHC(=NH2(+))(NH2)}-4 (2)), and [Pt(tBu3 tpy)(C≡CC6H4-{NHC(=NH2(+))(NH2)}-4)][OTf]2 (3; tBu3 tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine), have been synthesized and characterized. The photophysical properties of the complexes have been studied. Based on the results of UV/Vis absorption, resonance light scattering, and dynamic light scattering experiments, in aqueous buffer solutions complexes 1 and 2 undergo aggregation in the presence of citrate through strong and specific electrostatic and hydrogen-bonding interactions with citrate. The emergence of a triplet metal-metal-to-ligand charge transfer ((3 MMLCT) emission in the near-infrared (NIR) region brought on by the induced self-assembly of complex 1 has been demonstrated for proof-of-principle detection of citrate with good sensitivity and selectivity over other mono- and dicarboxylate substrates in the tricarboxylic acid (TCA) cycle as well as phosphate and lactate anions. Such a good selectivity toward citrate has been rationalized by the high charge density of citrate under physiological conditions and specific interactions between the guanidinium moiety on complex 1 and citrate. Extension of the work to citrate detection in fetal bovine serum and real-time monitoring of the activity of citrate lyase by the NIR emission of complex 1 have also been demonstrated.