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

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

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

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

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

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

Docked poses of serum albumin-TC complexes.

<p>(A) The surrounding amino acid residues of (I) BSA (II) HSA within 5 Å from TC. (B) Distances (in Å) obtained from docked poses of different Trp residue/s of (I) BSA (II) HSA from TC.</p

Phosphorescence Data for Wild-Type Serum Albumins and its Complex with TC in a 40% Ethylene Glycol Matrix at 77 K (λ_exc = 280 nm).

<p>Phosphorescence Data for Wild-Type Serum Albumins and its Complex with TC in a 40% Ethylene Glycol Matrix at 77 K (λ_exc = 280 nm).</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