Tuning the Spectroscopic Properties of Ratiometric Fluorescent Metal Indicators: Experimental and Computational Studies on Mag-fura-2 and Analogues

Abstract

In this joint theoretical and experimental work, we investigate the properties of Mag-fura-2 and seven structurally related fluorescent sensors designed for the ratiometric detection of Mg cations. The synthesis of three new compounds is described, and the absorption and emission spectra of all of the sensors in both their free and metal-bound forms are reported. A time-dependent density functional theory approach accounting for hydration effects using a hybrid implicit/explicit model is employed to calculate the absorption and fluorescence emission wavelengths, study the origins of the hypsochromic shift caused by metal binding for all of the sensors in this family, and investigate the auxochromic effects of various modifications of the "fura" core. The metal-free forms of the sensors are shown to undergo a strong intramolecular charge transfer upon light absorption, which is largely suppressed by metal complexation, resulting in predominantly locally excited states upon excitation of the metal complexes. Our computational protocol might aid in the design of new generations of fluorescent sensors with low-energy excitation and enhanced properties for ratiometric imaging of metal cations in biological samples.

Tuning Mg(II) Selectivity: Comparative Analysis of the Photophysical Properties of Four Fluorescent Probes with an Alkynyl‐Naphthalene Fluorophore

“…The excitation-based ratiometric behaviours een here has been reported previously for an umber of different cation sensors with Mg 2 + and Ca 2 + , [2,[17][18][19][20][21] including Mag-Fura-2, where a3 5nmh ypsochromic shift was observed in the metal-bound state. [35] Dissociation constants of 4.5(2) mm and3 9.6(5) mm were calculated following addition of Mg 2 + and Ca 2 + respectively (Table 3), consistentw ith typicala ffinities observed for APTRA analogues in the literature. [35] Dissociation constants of 4.5(2) mm and3 9.6(5) mm were calculated following addition of Mg 2 + and Ca 2 + respectively (Table 3), consistentw ith typicala ffinities observed for APTRA analogues in the literature.…”

“…[21] TD-DFT has been used to model the effect of Mg 2 + binding on the excited state of Mag-Fura-2, revealing that the ICT state is perturbedf ollowing Mg 2 + binding, with elimination of ac harge transfer process involving the aniline nitrogen lone pair.T he excited state insteadp ossesses localised p-p*c haracter in its metal-bound form, resultingi na drastic change in the spectrum on binding. [35] Dissociation constants of 4.5(2) mm and3 9.6(5) mm were calculated following addition of Mg 2 + and Ca 2 + respectively ( Table 3), consistentw ith typicala ffinities observed for APTRA analogues in the literature. [17][18][19][20][21] Comparable values were calculated from fluorescence emission spectralc hanges, with excitation at both the isosbestic point and the wavelength maximum in the absorption spectrum (SupportingI nformation).…”

“…Calculated pK a values for L 1 -L 4 all fall in the range of 5.2 to 6.2 ( Table 2), comparable to those reported for Mag-Fura-2, for which ap K a value of 5.0w as reported by London et al [21] The fluorescenceo fe ach ligand was insensitive to pH from 6.5 to 7.5, indicating that at physiological pH the aniline nitrogen and carboxylate/phosphinate ligatingg roups are fully de-protonateda nd availablet ocoordinate to metal ions. [2,21,35] Fluorescence metal ion binding studies with L 1 -L 4…”

Role of Water in the Reaction Mechanism and endo/exo Selectivity of 1,3‐Dipolar Cycloadditions Elucidated by Quantum Chemistry and Machine Learning

“…The above results strongly suggest that the deviation between the computed and experimental values mainly stems from the fact that not only the bulk properties of the solvent, but also special interactions between solvent and solute within the first solvent shell play a significant role in determining the activation free‐energy barrier, the overall free‐energy change, and the endo / exo selectivity. In fact, a combination of implicit and explicit solvation has been suggested to provide a general improvement for calculations of p K a , nuclear shielding constant, stability of surface protons, the absorption and fluorescence emission wavelengths, the hydrated structures of the metal ions, the reaction mechanism, and so on. These results suggest the possibility that the inclusion of more explicit water molecules at the rim may provide a reliable way to explore the mechanism, reactivity, and selectivity of the 1,3‐dipolar cycloadditions in Scheme .…”

DFT/MRCI assessment of the excited‐state interplay in a coumarin‐schiff Mg2+ fluorescent sensor

“…On the other hand, chemical sensors designed for the detection of Mg 2+ and Ca 2+ containing other atoms besides of O and N as chelating groups have been synthetized and characterized. Recently, the replacement of oxygen atom in the chelating group by either sulfur or selenium was evaluated in a series of sensors containing Mag‐fura‐2 analogues . Both experimental and theoretical calculations were carried out to investigate the structural, absorption and emission properties.…”