Divergence in Cu(II)- and Zn(II)-Based Coordination Polymers: The Sensing of Melatonin and Tryptophan, and Their Anticancer Activity

Accurate and selective recognition of ions and molecules is crucial in medical and diagnostic research. Cu(II)- and Zn(II)-based coordination polymers (CPs) have been designed in this work to detect trace levels of melatonin and tryptophan and evaluate their anticancer activity. The [Cu₂(4-bph)₂(adc)₄]_<i>n</i> <b>(CP1)</b> (4-bph = (1E,2E)-1,2-bis(pyridin-4-ylmethylene) hydrazine; Hadc = 1-adamantanecarboxylic acid) structure shows that 4-bph serves as a bridging pyridyl-N ligand and adc^- is a chelating and binuclear bridging ligand, forming an eight-membered Cu(μ-COO)₂Cu motif. In Zn(II)-CP, 4-bph is a bridging ligand, while adc^- is monodentate, yielding [Zn(4-bph)(adc)₂]_<i>n</i> (<b>CP2</b>). In <b>CP1</b>, π-π stacking (∼3.875 Å) and hydrogen bonding generate a 3D supramolecular network while <b>CP2</b> forms pyridyl-N bridging zigzag 1D CP. The BET analysis measures higher pore volume of <b>CP1</b> (0.06 cm³ g^-1) than <b>CP2</b> (0.018 cm³ g^-1). The <b>CP1</b> is weakly emissive, and upon irradiation at 312 nm, it emits at 392 nm which has been enhanced by the addition of tryptophan (Trp) (LOD, 44.65 nM), in the presence of 19 other amino acids. The <b>CP1</b> senses melatonin (MEL) (LOD, 38 nM) also in the presence of various proteins, enzymes, and neuroactive metal ions. Blood serum is used for the measurement of melatonin in blood serum (pH 7.4) and also tryptophan measurement in milk. The <b>CP2</b> is inactive toward sensing performance. DFT computation using crystallographic parameters reveals a stronger binding of <b>CP1</b> with Trp (-221 kcal mol^-1) than that of <b>CP2</b> (-38.22 kcal mol^-1). Anticancer assays show that <b>CP1</b> is more potent than <b>CP2</b> against MCF-7 breast cancer cells, IC₅₀ values are 196.8 ± 2.31 nM (<b>CP1</b>) and 258.2 ± 2.08 nM (<b>CP2</b>). Both CPs exhibit minimal toxicity toward normal PBMCs at these doses. Theoretical evaluation has also been used to explain the luminescence and selective sensing behavior to Trp and MEL.