A chiral lactate reporter based on total and circularly polarized Tb(iii) luminescence

Abstract

The coordination features and signaling of a L-lactate ion by a [Tb(bpcd)]⁺ (bpcd = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N′-diacetate) complex have been investigated by means of a combination of techniques, including total luminescence, calorimetry and circularly polarized luminescence. The L-lactate/[Tb(bpcd)]⁺ association constant, determined by both luminescence titration and isothermal titration calorimetry, indicates a weak interaction (log K = 1.3–1.45) between the analyte and both enantiomers of the complex. The theoretical DFT calculations suggest that the most likely coordination of L-lactate to the possible stereoisomers of the [Tb(S,S-bpcd)]⁺ complex (trans-O,O or trans-N_py,N_py) is one involving a hydroxyl group. The results of [Tb(rac-bpcd)]⁺ as a chiroptical luminescent probe of L-lactate underline the peculiar role of the chiral 1,2-diaminocyclohexane (DACH) backbone. Indeed, the target anion is capable of inducing CPL activity in the racemic mixture of Tb complexes containing DACH-based ligands. The same is not observed for the achiral analogue [Tb(bped)]⁺ (bped = N,N′-bis(2-pyridylmethyl)ethylenediamine-N,N′-diacetate) complex, likely because of the flexibility of the ethylenic group which allows an interconversion between different isomers which produces a null net CPL activity. Thanks to the differential quantum yield of the two diastereomeric species (R,R)-L and (S,S)-L, one can use the racemic complex to reveal L-lactate by measuring the induced CPL spectrum. Interestingly, this has been demonstrated in a commercial complex solution for medical use, containing several electrolytes, namely Ringer's lactate.