Molecular diversity in several pyridyl based Cu(ii) complexes: biophysical interaction and redox triggered fluorescence switch

Abstract

Variation of pyridyl based receptors around a Cu(II) center results in structural diversity allowing redox triggered Cu(II)/Cu(I) fluorescence switch for intracellular Cu(I) imaging. Single crystal X-ray structures of 2-hydroxy-6-methylnicotinic acid (L1), its mono nuclear Cu(II) complex, viz. [Cu(L1)₂(H₂O)(SO₄)]·2H₂O (C1a), polymeric Cu(II) complex (C1b) and a dinuclear Cu(II) complex of 2-(naphthalen-2-ylmethylsulfanyl)-nicotinic acid (L4), viz. [Cu₂(L4)₄(H₂O)₂]·1.12H₂O (C4) with a Cu–Cu distance of 2.6028(7) Å are reported. Moreover, C1a interacts with calf thymus DNA, which is useful for disruption of normal DNA activity with plausible applications in biology (binding constant, K′ = 1.56 × 10⁴). Chemical or electrochemical reduction of C1a generates a fluorescent Cu(I) complex. The lowest detection limit of L1 for Cu(I) is 50 μM. The association constant of L1 for Cu⁺ is 7.09 × 10². Quenching constants of Cu(II) for L1, L2, L3 and L4 are 4 × 10⁵, 4.2 × 10⁶, 8.9 × 10⁵ and 4.4 × 10⁶, respectively. The magnetic behavior of C1b indicates a ferromagnetic one-dimensional solid.