Quantitative Evaluation of the Ability of Ionic Liquids to Offset the Cold- Induced Unfolding of Proteins

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1 Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics. This journal is the Owner Societies 2014 Supplimentary informations Quantitative Evaluation of the Ability of Ionic Liquids to Offset the Cold- Induced Unfolding of Proteins Awanish Kumar a, Anjeeta Rani a, Pannuru Venkatesu a * and Anil Kumar b a Department of Chemistry, University of Delhi, Delhi , India. Fax: ; Tel: ; venkatesup@hotmail.com; pvenkatesu@chemistry.du.ac.in b Physical Chemistry Division, National Chemical Laboratory, Pune , India a.kumar@ncl.res.in Materials and Method 1. Materials Salt free Myoglobin (Mb) and α-chymotrypsin (CT), 1-butyl-3-methylimidazolium thiocyanate (0.7% water), 1-butyl-3-methylimidazolium hydrogensulfate ( 1.0% water), 1-butyl-3- methylimidazolium chloride ( 0.2% water), 1-butyl-3-methylimidazolium bromide ( 0.2 % water), 1-butyl-3-methylimidazolium acetate ( 0.5% water) and 1-butyl-3-methylimidazolium iodide ( 0.5% water), were purchased from Sigma Aldrich Chemical Company (USA). High purity and anhydrous sodium salts of anions such as chloride, bromide, iodide, sulfate, acetate and thiocyanate were purchased from Thomas Baker Chemicals Pvt. Ltd. (India). All materials were used without further purification. Tris-HCl buffer solution (10 mm) of ph 7.0 was prepared using distilled deionized water at 18.3 MΩ. The concentration of the buffer was kept low in order to minimize the concentration effects of buffer on the stability of proteins. On the other hand, hydrochloric acid (HCl) was used to attain the low ph 2.0. All mixture samples were prepared gravimetrically using a Mettler Toledo balance with a precision of g. The protein concentration of 8.0 mg/ml was used for fluorescence measurements.the proteins aggregated and then precipitated out from the solution in the presence of 1-butyl-3- methylimidazolium iodide, hence, the data is not provided here. 2. Method Fluorescence Spectroscopy Cary Eclipse spectrofluorimeter from Varian optical spectroscopy instruments, Mulgrave, Victoria, Australia was used to monitor the fluorescence emission spectra of CT that was equipped with thermostated cell holders. The steady-state fluorescence measurements were conducted at a constant temperature using a circulating water bath controlled by a UV1007M192 1

2 Peltier device attached to the sample holder of the fluorimeter. The excitation wavelength was set at 295 nm in order to calculate the contribution of the tryptophan (Trp) residues to the overall fluorescence emission. The experiments were carried out in the range between 20 to C by using a 1.0 cm sealed cell and both excitation and emission slit width were set at 5 nm. The fluorescence intensity at the emission maximum for the native enzyme (~332 nm) was continuously recorded as the temperature was decreased from 20 to C at an approximate rate of C/min. This allowed the samples to attain thermal equilibrium before the measurements. Both the changes in the fluorescence intensity and the shift in fluorescence maximum wavelength were recorded in order to monitor the unfolding transitions in the protein structure. The fluorescence intensity was not observed for CT in the presence of 1 M of sodium salts at ph 7.0 and is not provided in here. Thermodynamic analysis of proteins from the fluorescence curves The unfolding of proteins has been observed to approach closely a two-state folding mechanism experimentally such as that shown in equation 1. Folded Unfolded (1) Experimentally, the fraction of unfolded molecules is measured by the intensity of the emission. The fraction unfolded is determined as (2) α = [U] ([F] + [U]) α = (I f - I) (I f - I u ) In an equation, α is the fraction of unfolded molecules, I is the measured intensity at a given temperature, I f is the measured intensity of the folded state, and I u is the intensity of the completely unfolded state. The cold transition temperature of the protein (T cd ) is the temperature at which α = 0.5. The importance as well as the details of obtaining the thermodynamic parameters such as free energy change of unfolding (ΔG u ), enthalpy changes (ΔH), and heat capacity change (ΔC p ) through fluorescence thermal analysis is recently elucidated elsewhere. 1-3 The equilibrium constant (K eq ) between the native and the denatured states at a given temperature was obtained using the following equation (3) 2

3 K eq = α 1 α (4) The difference in free energy between the denatured and the native conformations (ΔG) can then be calculated using the equation as below, ΔG = RTln K eq (5) Where, R is the gas constant (1.987 Cal mol -1 ) and T is the absolute temperature. The entropy changes (ΔS) were calculated using the laws of thermodynamics; we have a relation as shown through a relation as below, ΔS = [ ( G) T ]P (6) And therefore, ΔS (at T cd ) = ΔS cd = the slope of ΔG vs T. ΔH is needed at only a single temperature, and the best temperature to use is T c, the midpoint of the thermal unfolding curve where K eq = 1.Therefore, at T cd the free energy change can be termed as (ΔG(T cd )) and can be expressed as below, ΔG(T cd ) = 0 = ΔH c T c ΔS c (7) Now the ΔH cd = (T cd in K) (slope at T cd ). Additionally, the heat capacity change was obtained as shown below, S = C P ln (T cd /T s ) (8) Where, T s is referred to as the standard temperature at which the stability of the protein is maximum. The obtained values were used to calculate the ΔG of unfolding at any temperature T, using Gibbs Helmholtz equation as shown below, G (T) = H m[ 1 ( T T cd )] C P[(T cd T) + T ln ( Circular Dichroism Spectroscopy T T ) cd ] All CD spectroscopic studies were performed using a J715 Spectropolarimeter from JASCO, Europe, equipped with a Peltier system for temperature control. CD calibration was performed (9) 3

4 using (1S)-(+)-10-camphorsulphonic acid (Aldrich, Milwaukee, WI), which exhibits a 34.5 M/cm molar extinction coefficient at 285 nm, and 2.36 M/cm molar ellipticity (θ) at 295nm. The sample was pre-equilibrated at the desired temperature for 15 min and the scan speed was fixed for adaptative sampling (error ) with a response time of 1 sec and 1nm bandwidth. Each spectrum was collected by averaging six spectra. Each sample spectrum was obtained by subtracting appropriate blank media containing no protein from the experimental spectrum. The CD spectrum for the Mb and CT in the presence of ILs was not observed. However, we observed strong absorbance bands for the proteins in the 240 nm region in the presence of ILs. This may be due to the high absorbance of the imidazolium cation or might be due to the formation of protein-in which interferes in the Far-UV region. References: 1. P. L. Privalov, Crit. Rev. Biochem. Mol. Biol., 1990, 25, P. L. Privalov, Y. V. Griko, S. Y. Venyaminov, 1986, 190, A. Kumar, P. Venkatesu, Chem. Rev. 2012, 112,

5 Supplementary Figure Captions Figure 1S. Variations in Far-UV CD of (a) Mb in [Bmim][CH 3 COO] and (b) CT in [Bmim][ CH 3 COO] at ph 2.0 at different temperature. The CD for the proteins in rest of the ILs follow similar pattern and is not provided. Figure 2S. Fraction unfolded of Mb (black) in cold (a-f) and thermal (a'- f') denaturation in the presence of 1 M of ionic liquids (ILs) at ph 2.0. The ILs include, [Bmim][SCN] (red), [Bmim][HSO 4 ] (green), [Bmim][Cl] (blue), [Bmim][Br] (cyan) and [Bmim][CH 3 COO] (magenta). Figure 3S. Fraction unfolded of Mb (black) in cold (a-f) and thermal (a'- f') denaturation in the presence of 1 M of sodium salts at ph 2.0. The salts include, NaSCN (red), Na 2 SO 4 (green), NaCl (blue), NaBr (cyan), NaCH 3 COO (magenta) and NaI (dark yellow). Figure 4S. Fraction unfolded of Mb (black) in cold (a-f) and thermal (a'- f') denaturation in the presence of 1 M of ionic liquids (ILs) at ph 7.0. The ILs include, [Bmim][SCN] (red), [Bmim][HSO 4 ] (green), [Bmim][Cl] (blue), [Bmim][Br] (cyan) and [Bmim][CH 3 COO] (magenta). Figure 5S. Fraction unfolded of Mb (black) in thermal (a- f) denaturation in the presence of 1 M of sodium salts at ph 7.0. The salts include, NaSCN (red), Na 2 SO 4 (green), NaCl (blue), NaBr (cyan), NaCH 3 COO (magenta) and NaI (dark yellow). Figure 6S. Fraction unfolded of CT (black) in cold (a-e) and thermal (a'- e') denaturation in the presence of 1 M of ionic liquids (ILs) at ph 2.0. The ILs include, [Bmim][HSO 4 ] (green), [Bmim][Cl] (blue), [Bmim][Br] (cyan) and [Bmim][CH 3 COO] (magenta). The protein (CT) precipitated out in the presence of 1M of [Bmim][SCN] IL. Figure 7S. Fraction unfolded of CT in cold (a-d) and thermal (a'-d') denaturation in the presence of 1 M of sodium salts at ph 7.0. The salts include, Na 2 SO 4 (green), NaCl (blue), NaBr (cyan), 5

6 NaCH 3 COO (magenta). The protein (CT) precipitated out in the presence of 1 M of NaSCN and NaI. Figure 8S. Fraction unfolded of CT in cold (a-d) and thermal (a'- d') denaturation in the presence of 1 M of ionic liquids (ILs) at ph 7.0. The ILs include, [Bmim][Cl] (blue), [Bmim][Br] (cyan) and [Bmim][CH 3 COO] (magenta). The protein (CT) precipitated out in the presence of 1M of [Bmim][SCN], [Bmim][HSO 4 ] IL. 6

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15 Table 1S. Cold (T c ) and thermal (T m ) transition temperature, enthalpy change (ΔH) and heat capacity change (ΔC p ) determined by fluorescence spectroscopy and calculated Gibbs free energy changes (ΔG u ) in unfolding state at 25 0 C for the myoglobin (Mb) in 1 M concentrations of ionic liquids (ILs) and sodium salts. Sample T c ( 0 C) ΔH k Cal mol -1 ΔG u k Cal mol -1 Cold denaturation of Mb ph = 2.0 Pure Mb M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] ΔC p k Cal mol -1 K -1 1 M NaBr M NaCl M Na 2 SO M NaSCN M Na CH 3 COO M NaI ph = 7.0 Pure Mb M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] Thermal denaturation of Mb Sample T m ( 0 C) ΔH k Cal mol -1 ΔG u k Cal mol -1 ΔC p k Cal mol -1 K -1 ph = 2.0 Pure Mb M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] M NaBr M NaCl

16 1 M Na 2 SO M NaSCN M Na CH 3 COO M NaI ph = 7.0 Pure Mb M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] M NaBr M NaCl M Na 2 SO M NaSCN M Na CH 3 COO M NaI Note: the dashed line indicate the aggregation of protein for which the spectra was not obtained 16

17 Table 2S. Cold (T c ) and thermal (T m ) transition temperature, enthalpy change (ΔH) and heat capacity change (ΔC p ) determined by fluorescence spectroscopy and calculated Gibbs free energy changes (ΔG u ) in unfolding state at 25 0 C for the α-chymotrypsin (CT) in 1 M concentrations of ionic liquids (ILs) and sodium salts. Sample T c ( 0 C) ΔH k Cal mol -1 ΔG u k Cal mol -1 Cold denaturation of CT ph = 2.0 Pure CT M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] ΔC p k Cal mol -1 K -1 1 M NaBr M NaCl M Na 2 SO M NaSCN M Na CH 3 COO M NaI ph = 7.0 Pure CT M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] Thermal denaturation of CT Sample T m ( 0 C) ΔH k Cal mol -1 ΔG u k Cal mol -1 ΔC p k Cal mol -1 K -1 ph = 2.0 Pure CT M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] M NaBr M NaCl

18 1 M Na 2 SO M NaSCN M Na CH 3 COO M NaI ph = 7.0 Pure CT M [Bmim][Br] M [Bmim][Cl] M [Bmim][HSO 4 ] M [Bmim][SCN] M [Bmim][CH 3 COO] M [Bmim][I] Note: the dashed line indicate the aggregation of protein for which the spectra was not obtained 18