Protein-Mediated Efficient Synergistic “Antenna Effect” in a Ternary System in D₂O Medium

A ternary system consisting of a protein, catechin (either + or – epimer), and Tb­(III) in suitable aqueous buffer medium at physiological pH (= 6.8) has been shown to exhibit highly efficient “antenna effect”. Steady state and time-resolved emission studies of each component in the binary complexes (protein with Tb­(III) and (+)- or (−)-catechin with Tb­(III)) and the ternary systems along with the molecular docking studies reveal that the efficient sensitization could be ascribed to the effective shielding of microenvironment of Tb­(III) from O–H oscillator and increased Tb–C (+/−) interaction in the ternary systems in aqueous medium. The ternary system exhibits protein-mediated efficient antenna effect in D₂O medium due to synergistic ET from both the lowest ππ* triplet state of Trp residue in protein and that of catechin apart from protection of the Tb­(III) environment from matrix vibration. The simple system consisting of (+)- or (−)-catechin and Tb­(III) in D₂O buffer at pH 6.8 has been prescribed to be a useful biosensor

Pentanuclear 3d–4f Heterometal Complexes of M^II₃Ln^III₂ (M = Ni, Cu, Zn and Ln = Nd, Gd, Tb) Combinations: Syntheses, Structures, Magnetism, and Photoluminescence Properties

A new family of pentanuclear 3d–4f heterometal complexes of general composition [Ln^III₂(M^IIL)₃(μ₃-O)₃H]­(ClO₄)·<i>x</i>H₂O (<b>1</b>–<b>5</b>) [Ln = Nd, M = Zn, <b>1</b>; Nd, Ni, <b>2</b>; Nd, Cu, <b>3</b>; Gd, Cu, <b>4</b>; Tb, Cu, <b>5</b>] have been synthesized in moderate yields (50–60%) following a self-assembly reaction involving the hexadentate phenol-based ligand, viz., <i>N</i>,<i>N</i>-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)-<i>N</i>^′,<i>N</i>^′-diethylethylenediamine (H₂L). Single-crystal X-ray diffraction analyses have been used to characterize these complexes. The compounds are all isostructural, having a 3-fold axis of symmetry that passes through the 4f metal centers. The [M^IIL] units in these complexes are acting as bis-bidentate metalloligands and, together with μ₃-oxido bridging ligands, complete the slightly distorted monocapped square antiprismatic nine-coordination environment around the 4f metal centers. The cationic complexes also contain a H⁺ ion that occupies the central position at the 3-fold axis. Magnetic properties of the copper­(II) complexes (<b>3</b>–<b>5</b>) show a changeover from antiferromagnetic in <b>3</b> to ferromagnetic 3d–4f interactions in <b>4</b> and <b>5</b>. For the isotropic Cu^II–Gd^III compound <b>4</b>, the simulation of magnetic data provides very weak Cu–Gd (<i>J</i>₁ = 0.57 cm^–1) and Gd–Gd exchange constants (<i>J</i>₂ = 0.14 cm^–1). Compound <b>4</b> is the only member of this triad, showing a tail of an out-of-phase signal in the ac susceptibility measurement. A large-spin ground state (<i>S</i> = 17/2) and a negative value of <i>D</i> (−0.12 cm^–1) result in a very small barrier (8 cm^–1) for this compound. Among the three Nd^III₂M^II₃ (M = Zn^II, Ni^II, and Cu^II) complexes, only the Zn^II analogue (<b>1</b>) displays an NIR luminescence due to the ⁴F_3/2 → ⁴I_11/2 transition in Nd^III when excited at 290 nm. The rest of the compounds do not show such Nd^III/Tb^III-based emission. The paramagnetic Cu^II and Ni^II ions quench the fluorescence in <b>2</b>–<b>5</b> and thereby lower the population of the triplet state

Steady State and Time Resolved Emission of TC in Various Solvents.

<p>(<b>A</b>) Fluorescence spectra of TC (25 µM) at 298 K in (1) water, (2) ethanol (EtOH), (3) isopropanol (iPrOH), (4) ethylene glycol (EG), (5) dimethylformamide (DMF), (6) dimethyl sulphoxide (DMSO); λ_exc = 370 nm; excitation and emission band pass = 10 nm and 5 nm respectively. (<b>B</b>) Fluorescence decay of TC (25 µM) at 298 K in (B) EtOH, iPr-OH, EG, DMF, DMSO; λexc = 370 nm; excitation and emission bandpass = 10 nm each.</p

Plot of φ (A) and <τ> (B) against (I) dielectric constant (ε), (II) solvent polarizability parameter (π*), (III) Hydrogen bond donating ability (α) of different solvents, (1) DMSO, (2) DMF, (3) EG, (4) EtOH, (5) i-PrOH.

<p>Plot of φ (A) and <τ> (B) against (I) dielectric constant (ε), (II) solvent polarizability parameter (π*), (III) Hydrogen bond donating ability (α) of different solvents, (1) DMSO, (2) DMF, (3) EG, (4) EtOH, (5) i-PrOH.</p

The Changes in Accessible Surface Area (ΔASA)^a of the Residues of Serum Albumins after Complexed with Tetracycline.

a<p>ΔASA_i = ASAⁱ_HSA/BSA – ASAⁱ_{HSA/BSA – TC complex.}</p

Fluorescence anisotropy decays of TC (25 µM) at 298 K in pure EG.

<p>I_VV and I_VH represent decays of emission of TC with excitation polarizer at vertical position and emission polarizer at vertical and horizontal position, respectively. λ_exc = 370 nm; excitation and emission band pass = 10 nm each.</p

Variation of (I) fluorescence quantum yield (φ).

<p>(II) fluorescence anisotropy (r); (III) singlet state average lifetime (τ) of TC (25 µM) in aqueous buffer with increasing concentration of serum albumins. (A) for BSA, (B) for HSA.</p

The plot of (A) (I) [F_∞ − F₀]/[F_x − F₀] against [BSA]⁻¹(II) (r−r_f)/R(r_b−r) against [BSA]; (B) (I) and (II) similar plot for HSA.

<p>The plot of (A) (I) [F_∞ − F₀]/[F_x − F₀] against [BSA]⁻¹(II) (r−r_f)/R(r_b−r) against [BSA]; (B) (I) and (II) similar plot for HSA.</p

The Energy Transfer Efficiency and The Rate Constant of Protein-TC Complexes at 298 K.

a<p>Ref <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060940#pone.0060940-Anand1" target="_blank">[66]</a>.</p>b<p>Considering the distance from indole N of Trp to O attached with 10-C of TC.</p

Distances and of Tryptophan Residues of Different Proteins from TC Changes in Accessible Surface Area (ΔASA) of Tryptophan Obtained by the Docking Studies.

<p>Distances and of Tryptophan Residues of Different Proteins from TC Changes in Accessible Surface Area (ΔASA) of Tryptophan Obtained by the Docking Studies.</p