Physicochemical Peculiarities of Iodine-Dimethylsulfoxide-H 2 O Solutions and Eﬀect on Ion Binding to Bovine Serum Albumin

e interaction of iodine with bovine serum albumin (BSA) in dimethylsulfoxide (DMSO) aqueous solutions was studied by means of �uorescence and UV/Vis absorption spectroscopy methods. Physicochemical peculiarities of these solutions were revealed. e results showed that the tri-iodide ion formed in the 1DMSO:2H 2 O solution caused the �uorescence quenching of BSA. e modi�ed Stern-Volmer quenching constant 𝐾𝐾 𝑎𝑎 and corresponding thermodynamic parameters, the free energy change ( Δ𝐺𝐺 ), enthalpy change ( Δ𝐻𝐻 ), and entropy change ( Δ𝑆𝑆 ), at diﬀerent temperatures (293, 298, and 303K) were calculated, which indicated that the hydrophobic and electrostatic interactions were the predominant operating forces. e binding locality distance r between BSA and tri-iodide ion at diﬀerent temperatures was determined based on F�rster nonradiation �uorescence energy transfer theory.


Introduction
Iodine is an element that is needed for the production of the most important organoiodine compounds for human health thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Without sufficient iodine, body is unable to synthesize these hormones and play a role in virtually all physiological functions. T4 is transported in blood, being protein-bound, principally to globulin, transthyretin, and serum albumin [1]. e absorption of inorganic-iodine, such as − , is certainly bound to serum albumin. Serum albumins, human serum albumin (HSA), and bovine serum albumin (BSA) play an important role in the transport and disposition of a wide variety of substances like metals, fatty acids, amino acids, hormones, and drugs [2][3][4][5][6]. Competitive interactions in the aqueous solutions containing DMSO and ions were a subject of numerous studies [7,8]. In this paper, the interaction of iodine with BSA in DMSO aqueous solutions was studied at different temperatures by using �uorescence and UV/Vis spectroscopy methods. e binding constants and the binding locality distance are calculated, and the thermodynamic parameters of the process are proposed.

Methodology
e materials used are as follows. BSA and DMSO were purchased from Sigma Chemical Company (USA). Iodine was puri�ed by sublimation. Doubly distilled water was used for the preparation of binary mixtures of 1DMSO : 2H 2 O. BSA concentration was 0.4 mg/mL, determined by electron absorption spectra in the UV region using a molar absorption coefficient [9]. e methods applied are the following. To perform the UV/Vis absorption measurements, we apply the following. e absorption spectra of iodine in 1DMSO : 2H 2 O solutions were recorded in the range of 250-500 nm, using 10 mm path length quartz cell. e UV/Vis measurements were performed on a spectrophotometer Specord 50PC (Germany). Ultraviolet spectra of these solutions are characterized by absorption peaks at 289 and 354 nm, indicating the formation of − [10]. In DMSO-H 2 O solutions with low content of DMSO ( 1), iodine behaves as if it was in pure aqueous solution [11,12].
To perform the �uorescence measurements, we apply the following. Fluorescence spectra were recorded on a Varian (Australia) �uorescence spectrophotometer equipped with a circulating water bath (Lauda 100). e �uorescence spectra were scanned under the following conditions: entrance slit and exit slit widths were adjusted at 5 nm, the excitation wavelength for BSA was adjusted at 280 nm, and the emission spectra were recorded in the range of 290-500 nm. e quenching experiments at different temperatures (293, 298, and 303 K) were carried out by keeping the BSA concentration constant (0.4 mg/mL) while varying the concentration of 2 (5.0 × 10 −6 −2.5 × 10 −5 M). 1DMSO : 2H 2 O (v/v) binary mixture was used. Each experiment was performed triply, and the average data were used for the analysis. Energy transfer between tri-iodide ion and BSA was determined according to the Förster nonradiative resonance energy transfer theory. e overlap of the UV/Vis absorption spectra of iodine in 1DMSO : 2H 2 O with the �uorescence emission spectrum of BSA using Matlab 6.5 soware was determined, and the energy transfer efficiency as per Förster nonradiation �uorescence energy transfer theory was calculated as well. e intrinsic �uorescence of BSA is obtained at 345 nm being excited at 280 nm. e �uorescence intensities of Trp residues of BSA in the presence of tri-iodide ion are given in Dataset Item 1 (Table). Fluorescence quenching can occur via different mechanisms, usually classi�ed as dynamic quenching and static quenching. In order to elucidate the �uorescence quenching mechanism, the �uorescence quenching data at different temperatures (293, 298, and 303 K) using classical Stern-Volmer equation were analyzed [13]: where 0 and are the �uorescence intensities of BSA in the absence and presence of quencher, respectively, [ ] is the concentration of quencher, is the Stern-Volmer quenching constant, and is the quenching rate constant of the biological macromolecule. Figure 1 shows the Stern-Volmer plots of BSA �uorescence quenching by iodine at different temperatures (▾) 293, (■) 298 K, and (•) 303 K, constructed by the data given in Dataset Item 2 (Table). e Stern-Volmer quenching constant was determined by linear regression of a plot of 0 / against [ ] (linear region). 0 is the average lifetime of the molecule without any quencher. When the �uorescence lifetime of the biopolymer was 10 −8 s [14], the quenching rate constant at different temperatures was then calculated. e results are summarized in Dataset Item 3 (Table).
ermodynamic parameters and the nature of binding forces are as follows. To get the binding constant, quenching data were analyzed using the modi�ed Stern-Volmer equation: where 0 is the total �uorescence in the absence of quencher, Δ is the difference of intensities in the absence and in the presence of quencher, is the Stern-Volmer quenching constant of the accessible fraction, [ ] is the concentration of quencher, and is the fraction of the initial �uorescence that is accessible to quencher. e modi�ed Stern-Volmer plots for BSA quenching by iodine at different temperatures 293 K (■), 298 K (•), and 303 K (▾) are shown in Figure 2, constructed by the data given in Dataset Item 4 (Table).
e thermodynamic parameters, enthalpy change (Δ ), entropy change (Δ ), and free energy change (Δ ), are the main quantities to make conclusions about the binding mode. If the temperature does not vary signi�cantly, the enthalpy change can be regarded as a constant and the value of enthalpy change and entropy change can be estimated from the van't Hoff equation: where the associative binding constant is analogous to the effective quenching constant at the corresponding temperature. e free energy change (Δ ) can be estimated from the following relationship: e thermodynamic parameters were calculated from van't Hoff plot for BSA-iodine system shown in Figure 3 (Dataset Item 5 (Table)).
en we have the energy transfer between tri-iodide and BSA. e distance between the donor and the acceptor can be calculated according to Förster nonradiation �uorescence energy transfer theory [15]. e efficiency of energy transfer, , is calculated by using where 0 and are the �uorescence intensities of BSA in the absence and presence of iodine, respectively, is the distance between the donor and acceptor, and 0 is the critical distance where 2 is the space factor of orientation, is the refractive index of the medium, Φ is the �uorescence quantum yield of the donor, and is the overlap integral of the �uorescence emission spectrum of the donor and the absorption spectrum of the acceptor. erefore, where ( ) is the �uorescence intensity of the donor at the wavelength and ( ) is the molar absorption coefficient of the acceptor at the wavelength . e data for the overlap integrals, Förster radius ( 0 ), distance between the donor and acceptor ( ), and efficiency of energy transfer ( ) for BSA-iodine system are given in Dataset Item 6 ( Table), when 2 = 2/3 = 1.336, and Φ = 0.15 [15].

Dataset Description
e dataset associated with this Dataset Paper consists of 7 items which are described as follows.
Dataset Item 1 (Table). Fluorescence intensities of BSA at the presence of iodine at different temperatures (293, 298, and 303 K). In the �rst column is given the concentration of the quencher [ (10 5 mol L −1 ). In the second, third, and fourth columns are collected the �uorescence intensities of BSA at 293, 298, and 303 K temperatures (arbitrary units (a.u.)).  (Table). Collected data for 0 / of BSA quenching at the presence of iodine at different temperatures (293, 298, and 303 K). In the �rst column is given the concentration of the quencher [ (10 5 mol L −1 ). In the second, third, and fourth columns are collected the data for 0 / at 293, 298, and 303 K temperatures.  (Table). Temperature effect on Stern-Volmer quenching constants for the interaction of iodine with BSA. In the �rst column is given the temperature; in the second, Stern-Volmer quenching constants (×10 4 L mol −1 ); in the acceptor ( , and efficiency of energy transfer ( for BSAiodine system. In the �rst column are given the temperatures. In the second column are given the overlap integrals (10 15 cm 3 L mol −1 ). In the third column are collected Förster radii, 0 (nm); in the fourth, distances between the donor and acceptor, (nm); in the ��h, the data for the efficiencies of energy transfer .

Concluding Remarks
Tri-iodide ion is formed in 1DMSO : 2H 2 O solution, which causes the �uorescence quenching of BSA. Interactions in this system are driven mainly by hydrophobic and ionic interactions.
Dataset Availability e dataset associated with this Dataset Paper is dedicated to the public domain using the CC0 waiver and is available at http://dx.doi.org/10.7167/2013/534328/dataset.