Determining Metal Ion Complexation Kinetics with Fluorescent Ligands by Using Fluorescence Correlation Spectroscopy

Inter-molecular Interaction Kinetics: Tale of Photon Anti-bunching and Bunching in Fluorescence Correlation Spectroscopy (FCS)

Molecular interactions are fundamental to any chemical or biological processes, and their rates define the operational sequence and control for any desirable product. Here, we deliberate on a recently developed novel fluorescence spectroscopic method, which combines fluorescence photon anti-bunching, photon bunching, time-correlated single-photon counting (TCSPC), and steady-state fluorescence spectroscopy, to study composite chemical reactions with single molecule sensitivity. The proposed method captures the full picture of the multifaceted quenching kinetics, which involves static quenching by ground state complexation and collisional quenching in the excited state under dynamic exchange of fluorophore in a heterogeneous media, and which cannot be seen by steady-state or lifetime measurements alone. Photon correlation in fluorescence correlation spectroscopy (FCS) provides access to interrogate interaction dynamics from picosecond to seconds, stitching all possible stages of dye-quencher interaction in a micellar media. This is not possible with the limited time window available to conventional ensemble techniques like TCSPC, flash photolysis, transient absorption, stop-flow, etc. The basic premises of such unified global analysis and sanctity of extracted parameters critically depends on the minimum but precise description of reaction scheme, for which careful inspection of ensemble spectroscopy data for photo-physical behaviour is very important. Though in this contribution we discussed and demonstrated the merits of photon antibunching and bunching spectroscopy for dye-quencher interaction in cationic cetyltrimethylammonium bromide (CTAB) micellar solution by photo-induced electron transfer mechanism and the influence of micellar charge and microenvironment on the interaction kinetics, but in principal similar arguments are equally applicable to any other interaction mechanisms which alter fluorescence photon correlations, like Förster resonance energy transfer (FRET), proton transfer, isomerisation, etc.

Binding Constant Determined from the Angstrom-Scale Change in Hydrodynamic Radius of Transferrin upon Binding with Europium Using Dual-Focus Fluorescence Correlation Spectroscopy

Monitoring the binding of a large fluorescently tagged molecule to a small solute by fluorescence correlation spectroscopy (FCS) is rather uncommon because the binding-related change in diffusion coefficient is very small. Here, we use a high-precision variant of FCS, namely, dual-focus FCS (2fFCS), for measuring the angstrom-scale change of the hydrodynamic radius of the bilobal metal transport protein transferrin (Tf) upon binding europium ions. Applying a sequential 1:2 complexation model, we use these measurements for determining the binding constants (K). Our results show a 0.7 Å change of the protein's hydrodynamic radius upon 1:1 Tf-Eu complex formation and a second change of 1.8 Å upon subsequent binding of a second europium ion. More than one unit variation in logK indicates an intrinsic dissimilarity in metal affinity of the C- and N-lobes of Tf, which agrees well with earlier reported ensemble spectroscopy results.