Hydrogen Bonding Interaction between Active Methylene Hydrogen Atoms and an Anion as a Binding Motif for Anion Recognition: Experimental Studies and Theoretical Rationalization

Two new reagents, having similar spatial arrangements for hydrogen atoms of the active methylene functionalities, were synthesized and interactions of such reagents with different anionic analytes were studied using electronic spectroscopy as well as by using ¹H and ³¹P NMR spectroscopic methods. Experimental studies revealed that these two reagents showed preference for binding to F^– and OAc^–. Detailed theoretical studies along with the above-mentioned spectroscopic studies were carried out to understand the contribution of the positively charged phosphonium ion, along with methylene functionality, in achieving the observed preference of these two receptors for binding to F^– and OAc^–. Observed differences in the binding affinities of these two reagents toward fluoride and acetate ions also reflected the role of acidity of such methylene hydrogen atoms in controlling the efficiencies of the hydrogen bonding in anion–H_methylene interactions. Hydrogen bonding interactions at lower concentrations of these two anionic analytes and deprotonation equilibrium at higher concentration were observed with associated electronic spectral changes as well as visually detectable change in solution color, an observation that is generally common for other strong hydrogen bond donor functionalities like urea and thiourea. DFT calculations performed with the M06/6-31+G**//M05-2X/6-31G* level of theory showed that F^– binds more strongly than OAc^– with the reagent molecules. The deprotonation of methylene hydrogen atom of receptors with F^– ion was observed computationally. The metal complex as reagent showed even stronger binding energies with these analytes, which corroborated the experimental results

Role of Metal Ion in Specific Recognition of Pyrophosphate Ion under Physiological Conditions and Hydrolysis of the Phosphoester Linkage by Alkaline Phosphatase

Complexes synthesized from Zn­(II), Cu­(II), and Cd­(II), using a dipicolyl amine derivative (<b>L</b>), showed unique specificity toward pyrophosphate ion (PPi or P₄O₇^4–) among all other common anionic analytes, including different biologically significant phosphate ion (PO₄^3–, H₂PO₄^2–) or phosphate-ion-based nucleotides, such as AMP, ADP, ATP, and CTP. However, the relative affinities of PPi toward these three metal complexes were found to vary and follow the order <i>K</i>_a^{<b>L.Zn</b>–PPi} > are given in units of _a^{<b>L.Cu</b>–PPi} ≥ <i>K</i>_a^{<b>L.Cd</b>–PPi}. Luminescence responses of the receptor <b>L</b> were substantial on binding to Zn²⁺ and Cd²⁺, while relatively a much smaller luminescence response was observed in the presence of Cu²⁺. Luminescence responses of <b>L.M</b>–PPi (<b>M</b> is Zn²⁺, Cd²⁺, and Cu²⁺) were further modified on binding to the PPi ion. This could be utilized for quantitative detection of PPi in physiological condition as well as for developing a real time “turn-on” (for <b>L.Zn</b> and <b>L.Cu</b>) and “turn-off” (for <b>L.Cd</b>) fluorescence assay for evaluating the enzymatic activity of alkaline phosphatase (ALP). Experimental results revealed how the subtle differences in the binding affinities between PPi and M in <b>L.M</b> (<b>M</b> is Zn²⁺, Cd²⁺, and Cu²⁺), could influence the cleavage of the phosphoester linkage in PPi by ALP. The DFT calculations further revealed that the hydrolytic cleavage of the metal ion coordinated phosphoester bond is kinetically faster than that for free PPi and thus, rationalized the observed difference in the cleavage of the phosphoester bond by an important mammalian enzyme such as ALP in the presence of different metal complexes

A Switch-On NIR Probe for Specific Detection of Hg²⁺ Ion in Aqueous Medium and in Mitochondria

A new 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based probe molecule (<b>L</b>) is synthesized for specific binding to Hg²⁺ ion in physiological condition with an associated <i>luminescence ON</i> response in the near-IR region of the spectrum. Appropriate functionalization in the 5-position of each of two pyrrole moieties with styryl functionality in a BODIPY core helped us in achieving the extended conjugation and a facile intramolecular charge transfer transition with a narrow energy gap for frontier orbitals. This accounted for a poor emission quantum yield for the probe molecule <b>L</b>. Binding to Hg²⁺ helped in interrupting the facile intramolecular charge transfer (ICT) process that was initially operational for <b>L</b>. This resulted in a hypsochromic shift of absorption band and a <i>turn-on</i> luminescence response with λ_Max^Ems of 650 nm on specific binding to Hg²⁺. Observed spectral changes are rationalized based on quantum chemical calculations. Interestingly, this reagent is found to be localized preferentially in the mitochondria of the live human colon cancer (Hct116) cells. Mitochondria is one of the major targets for localization of Hg²⁺, which actually decreases the mitochondrial membrane potential and modifies various proteins having sulfudryl functionality­(ies) to cause cell apoptosis. Considering these, ability of the present reagent to specifically recognize Hg²⁺ in the mitochondrial region of the live Hct116 cells has significance