Probing Ion/Molecule Interactions in Aqueous Solutions with Vibrational
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Article pubs.acs.org/JPCB Probing Ion/Molecule Interactions in Aqueous Solutions with Vibrational Energy Transfer Jiebo Li, Hongtao Bian, Xiewen Wen, Hailong Chen, Kaijun Yuan, and Junrong Zheng* Department of Chemistry, Rice University, Houston, Texas 77005, United States *S Supporting Information ABSTRACT: Interactions between model molecules representing building blocks of proteins and the thiocyanate anion, a strong protein denaturant agent, were investigated in aqueous solutions with intermolecular vibrational energy exchange methods. It was found that thiocyanate anions are able to bind to the charged ammonium groups of amino acids in aqueous solutions. The interactions between thiocyanate anions and the amide groups were also observed. The binding affinity between the thiocyanate anion and the charged amino acid residues is about 20 times larger than that between water molecules and the amino acids and about 5−10 times larger than that between the thiocyanate anion and the neutral backbone amide groups. The series of experiments also demonstrates that the chemical nature, rather than the macroscopic dielectric constant, of the ions and molecules plays a critical role in ion/ molecule interactions in aqueous solutions. 1. INTRODUCTION The situation that many different opinions on the mechanism The effects of salts on protein solubility and conformational of salt/protein interactions coexist partially results from the complexity of the systems investigated. A protein has many stability have been of great research interest for decades. functional groups that can potentially interact with ions in Interestingly, despite their importance in biochemistry, the aqueous solutions. In a protein solution, these interactions can molecular level mechanism of salt/protein specific interactions simultaneously exist, leading to experimental difficulties in is yet to be fully elucidated.1 There has been a long discussion determining which interactions are more important than others about how salts can change the properties of proteins, via for changing the conformation of the protein. Instead of directly interacting with proteins2,3 or indirectly through 4,5 studying proteins directly, in this work we have investigated modifying water structures. It was originally believed that model compounds representing different building blocks of a the disruption of water structures by the presence of ions plays 6 protein to avoid the copresence of many interactions in one a dominant role in biological processes. However, some recent measurement. One model compound represents one functional studies showed that ions only have short-range effects on the − fi group which can potentially bind to SCN , a strong denaturant. structure and dynamics of the rst hydration shell but no long- We measured the relative strength of interactions between fl 7,8 − range in uence on the water hydrogen bonding network. these model compounds and SCN using vibrational energy − Recently, the direct ion biological molecule interactions began exchange methods.15,16 From these measurements, we were to be recognized as a very important factor for the protein able to determine the relative priorities of these model 2 ff − stability and solubility. From X-ray di raction (XRD) studies compounds interacting with SCN . Our results show that in on protein structures and molecular dynamics (MD) aqueous solutions, besides the charged amino acids, other simulations, anions were found to be able to bind to the molecules representing the neutral functional groups of a 9−13 positive charged groups of a protein. It was suggested that protein can also interact with ions with different strengths. this direct interaction could be responsible for the solubility of the protein. Because the backbone of a protein is not charged, it 2. EXPERIMENTS was believed that interactions between the amide groups of the backbone and anions were weak and therefore played a very Materials. Unless specified, chemicals were purchased from 14 − 13 15 minor role in the salt-induced denaturation. However, an Sigma Aldrich and used as received. KS C N was purchased opposite opinion argued that the direct interaction between from Cambridge Isotope Laboratory. Formamide-ND2 was anions and the amide groups could be responsible for the obtained by deuterating formamide in methanol-od. protein denaturation since the positively charged groups always bind to the anions in both natural and denatured states, Received: June 28, 2012 resulting in no net contribution to the conformational changes Revised: August 2, 2012 of the proteins.3 Published: September 17, 2012 © 2012 American Chemical Society 12284 dx.doi.org/10.1021/jp306369w | J. Phys. Chem. B 2012, 116, 12284−12294 The Journal of Physical Chemistry B Article Figure 1. (A) Model compounds of protein building blocks. (B) The time dependence of the 2D IR spectrum of an aqueous solution with 13 15 ff Pro:KSCN:KS C N:D2O = 0.4:1:1:20 (molar ratio). As waiting time increases, the o -diagonal peaks (5, 6, 7, 8) grow in because of energy exchange between SCN− and S13C15N−. 15,16 Methods. The optical setup was described previously. was purged with clean air free of CO2 and H2O. All Briefly, a picosecond amplifier and a femtosecond amplifier are measurements were carried out at room temperature (22 °C). synchronized with the same seed pulse from an oscillator which produces ∼0.6 W 800 nm output with a bandwidth ∼35 nm 3. RESULTS AND DISCUSSION and a repetition rate ∼77 MHz. The picosecond amplifier 3.1. Adding Amino Acids into KSCN Aqueous pumps an OPA to produce ∼1 ps mid-IR pulses with a Solutions Can Significantly Slow down the Vibrational − −1 Energy Exchange among SCN− Anions. Recently, thio- bandwidth of 10 35 cm in a tunable frequency range from − 500 to 4000 cm−1 with energy 1−40 μJ/pulse at 1 kHz. The cyanate (SCN ) anions were found to be able to form femtosecond amplifier pumps another OPA to produce 110− substantial amounts of ion clusters in 1−10 M alkaline metal 15 150 fs mid-IR pulses with a bandwidth of 200−400 cm−1 in a thiocyanate aqueous solutions. The discovery suggests that in tunable frequency range from 500 to 4000 cm−1 with energy the solutions water molecules and ions tend to separate. The 1−40 μJ/pulse at 1 kHz. In experiments, the picosecond IR result also implies a relatively weak water/thiocyanate pulse is the pump beam. The femtosecond IR pulse is the probe interaction presumably because of the highly delocalized charge beam. Its wavelength is measured by a spectrograph, yielding on the anion. More experiments on NH4SCN solutions and mixed salt (KSCN + KF, KI, K CO ,KHPO , or KOD) the probe axis (ω ) of the 2D IR spectrum. Scanning the pump 2 3 2 4 3 further support the ion clustering phenomenon observed and frequency yields the other axis (ω ) of the spectrum. Two 1 suggest that ion paring, clustering, and segregations are the polarizers are inserted into the probe beam path to selectively results of chemical equilibriums among ion/ion, ion/water, and measure the parallel or perpendicular polarized signal relative to water/water interactions which depends on the chemical nature the pump beam. Vibrational lifetimes are obtained from the 17,18 − × of the ions. Following the series of experiments, in this rotation-free 1 2 transition signal Plife = P∥ +2 P⊥, where P∥, work we applied a similar approach to investigate ion/molecule fi P⊥ are data from the parallel and perpendicular con gurations, interactions in aqueous solutions by measuring vibrational respectively. Rotational relaxation times are acquired from the energy exchanges among isotopically labeled thiocyanate definition τ =(P∥ − P⊥)/(P∥ +2× P⊥). Samples were anions. contained in sample cells composed of two CaF2 windows We utilized the phenomenon that thiocyanate anions in the separated by a plastic spacer. In experiments, the thickness of ion clusters of KSCN aqueous solutions can exchange the spacer was adjusted from 0.5 to 250 μm, depending upon vibrational energy15,16 to determine the relative interaction the samples’ optical densities. The experimental IR optical path strength between a model compound and SCN−: adding a 12285 dx.doi.org/10.1021/jp306369w | J. Phys. Chem. B 2012, 116, 12284−12294 The Journal of Physical Chemistry B Article Figure 2. (A) 2D IR spectra at 30 ps of proline-containing solutions compared to KSCN solution without proline. Solution A no proline: 13 15 13 15 13 15 (KSCN:KS C N:D2O = 1:1:20); Solution B (Pro:KSCN:KS C N:D2O = 1:1:1:20); and Solution C (Pro:KSCN:KS C N:D2O = 2:1:1:20). ff 13 15 (B) 2D IR spectra at 30 ps for solutions containing di erent amino acids: Solution D (Gly:KSCN:KS C N:D2O = 1:1:1:20); Solution E 13 15 13 15 (Cys:KSCN:KS C N:D2O = 1:1:1:20); and Solution F (Lys:KSCN:KS C N:D2O = 1:1:1:20). Red rings highlight peak 6. The intensity of peak 6 decreases with the addition of all amino acids. The growth of peaks 5 and 7 with the addition of amino acids is caused by the faster vibrational relaxation induced by the direct interactions of the amino acids with the thiocyanate. model compound into a salt aqueous solution reduces the anions can exchange vibrational energy much more efficiently concentration of water, thus if there is no specific interaction than those separated anions because the energy transfer rate is between the ions and the model compound molecules, one inversely proportional to the sixth power of the donor/acceptor would expect that the ion clusters would grow. This will result distance under the general assumption of dipole/dipole in a faster observed vibrational energy transfer among the ions interaction.19 The energy exchange between SCN− and as observed in the mixed salt solutions.18 If the ions interact S13C15N− in the clusters produces off-diagonal cross peak strongly with the model compound molecules, ions might be pairs (peaks 5 and 6 and 7 and 8) in the 2D IR spectra.