Impact of Ionic Liquids on the Structure and Dynamics of Collagen

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Impact of Ionic Liquids on the Structure and Dynamics of Collagen Aafiya Tarannum, Alina Adams, Bernhard Blümich, and Nishter Nishad Fathima J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b09626 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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The Journal of Physical Chemistry

Impact of Ionic Liquids on the Structure and Dynamics of Collagen Aafiya Tarannum,1 Alina Adams, Bernhard Blümich,2 Nishter Nishad Fathima1* 1

Inorganic and Physical Chemistry Laboratory, CSIR-Central Leather Research Institute,

Chennai 600020, India 2

Institut für Technische and Makromolekulare Chemie, RWTH Aachen University,

Templergraben 55, D-52056 Aachen, Germany

ACS Paragon Plus Environment

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ABSTRACT. The changes in the structure and dynamics of collagen treated with two different classes of ionic liquids, bis-choline sulphate (CS) and 1-butyl-3-methyl imidazolium dimethyl phosphate (IDP) have been studied at the molecular and fibrillar levels. At molecular level, circular dichroic studies revealed an increase in molar ellipticity values for CS when compared to native collagen indicating crosslinking, albeit pronounced conformational changes for IDP were witnessed indicating denaturation. The impedance was analysed to correlate the conformational changes with the hydration dynamics of protein. Changes in the dielectric properties of collagen observed upon treatment with CS and IDP reported molecular reorientation in the surrounding water milieu suggesting compactness or destabilization of the collagen. This was further confirmed by proton transverse NMR relaxation times measurements, which demonstrated that the water mobility changes in the presence of the ILs. At fibrillar level, DSC thermograms for rat tail tendon collagen fibres treated with CS show a 5 ˚C increase in denaturation temperature suggesting imparted stability. On the contrary, a significant temperature decrease was noticed for IDP indicating the destabilization of collagen fibres. The obtained results clearly indicate that the changes in the secondary structure of protein are due to the changes in the hydration dynamics of collagen upon interaction with ILs. Thus, this study on the interaction of collagen with ionic liquids unfolds the propensity of ILs to stabilize or destabilize collagen depending on the changes invoked at molecular level in terms of structure and dynamics of protein, which also got manifested at fibrillar level.


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1. INTRODUCTION Ionic liquids (ILs) have been the focus of many recent investigations since they can be tuned for specific applications. As “greener” and “designer” solvents, ILs has been used for a myriad of applications in various synthetic reactions, in separation and extraction studies, in electrochemical and biotechnological processes1-2. Studies related to protein-IL interaction are gaining considerable interest due to the potential applications of ILs3-4. Collagen, a significant structural protein has many commercial applications in the fields of food, cosmetics and medicine. Excellent biocompatibility, superior biodegradability and weak antigenicity make collagen the primary source in biomedical applications. Collagen has been used as shields in ophthalmology, as sponges for burns/wounds and mini pellets for protein delivery. It has been used as grafts, bone substitutes and artificial blood vessels and valves5-6. The dissolution of collagen fibers by 1-butyl-3-methylimidazolium chloride and utilizing different precipitants to regenerate collagen films has been proposed by Meng et al7.The effect of imidazolium IL on the different hierarchical ordering of collagen has been reported, where imidazolium based ILs has brought about significant changes at the higher structural hierarchical level of collagen8. In addition, imidazolium chloride demonstrated fibre opening of the skin matrix, which has opened avenues for cleaner and greener processing systems9. Phosphonium and ammonium ionic liquids with variable anions also have a destabilizing effect on collagen due to the chaotropicity of the anions10-11. On the other hand, choline dihydrogen phosphate, a biocompatible ionic liquid was ascertained to stabilize collagen by exerting an electrostatic force on collagen, thus making it a potential biocompatible crosslinker12. Choline salts have been used as crosslinkers for preparation of collagen based biomaterials, wherein it has exhibited good cell viability and adhesion properties as required for biomedical implant applications13.All these


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studies have shown that ILs have potential to stabilize or destabilize collagen at different hierarchical levels of order depending on the nature and type of cation and anion. Ions are classified as kosmotropic and chaotropic depending on their abilities to interact with water. The choice of ions was based on kosmotropic and chaotropic behavior as described in Hofmeister series. It is well known that a chaotropic cation and kosmotropic anion is said to stabilize the proteins, whereas a kosmotropic cation and chaotropic anion is said to destabilize the proteins. A kosmotropic anion is said to exhibit stronger interaction with water than water itself, which tends to stabilize the protein structure as it promotes an irreversible H-bonding. Inferring the behavior of Hofmeister series of ionic liquid on stability of protein elucidates the interaction of anions with proteins in aqueous medium. Thus, the choice of ions plays a key role in stabilization and destabilization of proteins14-16. Therefore, it is important to understand the changes brought by the ionic liquids at the molecular and fibrillar levels. Correlating the conformational and thermal stability with the structure and dynamics of collagen for biomedical applications field with different types of ILs has not been reported yet. The hydration dynamics of protein can be studied using Nuclear Magnetic Resonance (NMR) and dielectric spectroscopy. NMR methods play a key role in investigating the structure and dynamics of water in collagenous tissues. They exploit among other multiple-quantum coherences, self-diffusion coefficients, and relaxation times17-20. In particular, the transverse (T2) relaxation time provides valuable information regarding the dynamics of water molecules present in the collagen matrix21. Using such measurements, deeper insights into the changes in hydration structure of collagen after crosslinking have been gained22. Dielectric measurement offers a valuable tool for studying hydration behavior of water molecules. Pietrucha et al., and Marzec et al., have studied the effects of water and electric field


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frequencies on the dielectric properties of constituent phases of unmodified collagen, thus providing insights into the dynamics of collagen23-25. In this work, we have attempted to examine the changes brought about by two different classes of ILs (one an imidazolium cation based and another a choline cation based) on the conformation of collagen and correlate these changes with the hydration dynamics of collagen using NMR and impedance measurements. Further, the effect of these ILs on collagen has been studied by DSC at higher order i.e., at the fibrillar level using rat tail tendon collagen fibres to investigate if the molecular level changes affect higher order stability. Thus, the present study is aimed at gaining new insights in the quest of unraveling the propensity of ILs to stabilize or destabilize collagen, where the molecular level changes are manifested at higher order as well.

2. MATERIALS AND METHODS 2.1 Materials. Ionic liquid, 1-butyl-3-methyl imidazolium dimethyl phosphate and bis-choline sulphate was purchased from Iolitec, GmBH, Germany and used further without purification. Millipore water was used for all experiments. 2.2 Isolation of Type I Collagen. Acid soluble type I collagen was obtained by teasing tendons from six month old albino rats (Wistar strain). The teased tendons were washed with 0.9% sodium chloride. They are solubilized in 0.5 M acetic acid for overnight under stirring at 4 ˚C. After solubilisation, they were centrifuged at 15,000 rpm for 15 min, followed by salt precipitation. The obtained precipitate was again centrifuged for 30 minutes and the supernatant was collected. It was stirred overnight at 4 ˚C, followed by dialysis with the repetitive changing of phosphate buffers until the white precipitate was obtained. The material was centrifuged at 15,000 rpm for 30 minutes. The second dialysis was carried out with 0.05M acetic acid, and


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finally the collagen solution was obtained and stored at 4 ˚C. The collagen concentration was estimated with a hydroxyproline assay following Woessner26. 2.3 Preparation of Collagen-IL Samples. For solution studies, type I collagen was diluted in 50 mM acetate buffer at pH 4.0. 10% of bis-choline sulphate (CS), and 1-butyl-3-methyl imidazolium dimethyl phosphate (IDP) was added drop wise to collagen and solution was mixed homogenously under stirring for three hours. The temperature was maintained at 4 ˚C throughout the preparation using acooling system, to prevent the denaturation of collagen and to avoid heat generated during mixing. The solution was stored at 4 ˚C for overnight for completion of reaction. The working concentration of collagen used was 1.33 µM at pH 4.0. For thermal studies, the rat tail tendon (RTT) collagen fibres were incubated for 24 hours at 25 ˚C. 2.4 Circular Dichroic Studies. The circular dichroic spectra of native collagen and IL-treated collagen were detected under nitrogen atmosphere in the far UV region from 190-260 nm using aJasco-815 Circular Dichroism Spectropolarimeter. Approximately, 400 µl of sample was required for analysis. It was scanned with 0.2 nm intervals with averaged three scans for each sample with a path length of 1 mm. The data obtained in milli degrees should be further converted to molar ellipticity (deg. cm2. dmol-1). Molar ellipticity was plotted against wavelength in nanometres (nm). 2.5 Impedance Measurements. The impedance for native collagen and IL-treated collagen was investigated using a CH-model 660 B electrochemical analyzer from CH Instrumental (USA). The effect of ILs on the resultant dipole of the collagen responding to an alternating electric field was measured using the three classical electrode systems, wherein, the glassy carbon, a platinum and saturated calomel electrode served as a working electrode, counter electrode and as reference electrode, respectively.


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Dielectric data can be described in terms of the imaginary part Y″ of the complex admittance in units of Ω-1, ∗ = + "

(1)

Where the real component Y′ describes the energy stored and Y″ relates to the energy dissipated by the system. 2.6 NMR Measurements. The NMR measurements were performed under static conditions at room temperature on a Bruker DSX200 spectrometer working at a proton frequency of 199.56 MHz and using a 4 mm solid-state probe-body without background. Prior to the measurements, the excess of water from the surface of the fully hydrated IL-treated and non-treated collagen fibres was gently removed with tissue paper. Several of these collagen fibres were then packed with random orientation in a 4 mm rotor closed with a tight cap to avoid water loss during the measurement. The proton transverse relaxation were measured with a CPMG (Carr, Purcell, Meiboom, Gill) pulse sequence 90°-τ-(180°-2τ)n with τ = 0.4 ms. The length of the 90° radiofrequency pulse and the recycle delay were 2 µs and 5 s, respectively. The obtained relaxation decays were analysed best by fitting three exponential functions to extract the relaxation times T2short, T2intermediate, and T2long. The average values of the relaxation times from two measurements performed for each type of sample are reported. 2.7 Thermal Stability of RTT Collagen Fibers Using DSC. The denaturation temperatures (Td) for native RTT and RTT treated with ILs were determined using a differential scanning calorimeter (Q200, TA instruments) in the temperature range 20-100 ˚C at the scan rate of 5 ˚C/min. 2.8 Dimensional Stability of RTT Collagen Fibers Using Optical Microscopy. Optical micrographs of native RTT and RTT treated with ILs for 24 h were visualized using Aven Inc.,


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Digital Mighty scope, 1.3 M (Product code: 48708-25) of 10x resolution. The changes occurring in the dimensions were monitored.

3. RESULTS AND DISCUSSION 3.1 Analysis with Circular Dichroism. In order to understand the influence of bis-choline sulphate and 1-butyl-3-methyl imidazolium dimethyl phosphate on the secondary structure of collagen, circular dichroic (CD) spectral studies were carried out. Neat IDP and CS ionic liquids do not show any CD signal in the range 190-260 nm indicating that the observed CD spectrum is due to collagen. At 222 nm, there was an increase in molar ellipticity values for C-CS when compared to native collagen indicating stability of the collagen. Albeit, for C-IDP there was a complete degradation of the collagen with the disappearance of peaks at 197 and 222 nm, which are usually the signature peaks for collagen, authenticating the typical polyproline II conformation27. Choline ILs are said to be biocompatible and the best illustration so far is given by studies on choline dihydrogen phosphate (CDHP), which have shown that CDHP stabilizes collagen by exerting electrostatic force that results in attachment of phosphate anion to functional amino acid side chains of collagen12. For imidazolium dimethyl phosphate (C-IDP), it was observed that there was denaturation of collagen though the anion was identical. For C-CS, the combination of choline, a chaotropic cation and sulphate, a kosmotropic anion has shown stabilizing effect on collagen as the combination of chaotropic cation and kosmotropic anion is favorable to stabilize the protein14. For C-IDP, the kind of interaction of ions to the functional groups of collagen had led to the reorientation in H-bonding helices and its surrounding water milieu thereby leading to the destabilization of collagen.


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Figure 1. CD spectra for native, C-CS and C-IDP-10% treated collagen and the molar ellipticity at 222 nm for native and C-CS-10% treated collagen (inset) 3.2 Dielectric Analysis


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Figure 2. Impedance measurements for native, CS-10%, IDP-10% treated collagen and neat CS and IDP (A) Nyquist Plot, (B) Cole-Cole Plot and (C) Bode plot Dielectric measurement is a profound sensitive tool to investigate the dynamics of water surrounding proteins, which elucidates the structural stability of protein. Thedielectric behavior of native and different ILs treated collagen is compared in Figure 2. Dielectric plots for native collagen and ILs treated collagen are shown in Figure 2. Figure 2A and B exhibits the Nyquist plot (Y′ vs Y″) for studying the admittance of biomacromolecules, and Cole-Cole plot (Z′ vs Z″) ascertains the total impedance. It can be seen from the Nyquist and Cole-Cole plot that CS and IDP treated collagen have the highest permittivity and native collagen has lowest permittivity. For IL treated collagen samples permittivities were also found to vary with concentration. Changes in dipole moment indicate the mobility in the functional groups, which further points out the structural stability of collagen. This is due to the fact, that collagen is a protein with various functional groups, which led to its charged behavior. In case of CS, they may form intraand intermolecular cross-links by H-bonds, H-bound water, dispersion forces, and hydrophobic forces, which are understood to enhance the stability of triple-helix structure of collagen28. Bode plots (Figure 2C) for native collagen, collagen treated with ILs (C-CS/IDP-10%), and neat ILs CS and IDP show a phase angle at high frequency of about 103 Hz. When treated with ILs, phase angle is shifted to higher frequencies (103 to 105 Hz). The change in phase angle with frequency suggests electrostatic interactions of functional groups from collagen with CS, resulting in chemical crosslinks. The increased permittivity for C-CS-10% could possibly be due to dielectric behavior, resulting in movement of the charges between the functional groups of collagen and sulphate anion. This attributes to the electrostatic interaction of sulphate group with the functional groups of collagen resulting in formation of H-bonds leading to the alteration in the


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hydration shell surrounding collagen, albeit such changes were not observed with C-IDP-10% when compared to C-CS-10% in the Bode plot, thus suggesting the differential behavior of two ILs with regards to dielectric effect brought on collagen. This is suggestive of reorientation in water milieu surrounding collagen. 3.3 Analysis of the Water Dynamics. In order to correlate the changes brought about by the two different ionic liquids on the conformational stability of the collagen, NMR relaxation measurements were carried out. The CPMG decays of the IL-treated and non-treated collagen samples depicted in Figure 3 indicate that the type of IL affects the relaxation behavior. An improved understanding of the observed changes can be gained by the analysis of the relaxation decays. The employed echo-time precludes the detection of the protein itself17 and therefore the three relaxation times T2short, T2intermediate and T2long describes the behavior of the water in different states corresponding to the bound water, weakly bound water, and free (lattice) water, respectively18. The detection of three relaxation times for water is in agreement with earlier studies although the shorter relaxation times of the native collagen are shorter than those previously reported, probably due to different levels of hydration and orientation of the collagen fibers with respect to the static magnetic field B0 during the measurements19. Interactions of collagen with the two ILs influence the mobility of water significantly as seen from the changes in T2 summarized in Table 1. The effect is the strongest for the short relaxation time, which is much lower in CS treated RTT than in the native collagen and the highest in IDP treated RTT. The same effect, but to a lesser extent, can be observed also for the relaxation time of the intermediate component. The long relaxation component does not show the same trend but it is the longest for the IDP treated collagen. The detected changes in the water mobility relate to structural changes of the collagen. In particular, it has been recently reported that increased


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crosslink density in collagen, as it happens during aging, decreases the rotational and translational mobility of water29-31. Therefore, it is likely that the decrease in the short and intermediate relaxation times for the CS-treated collagen is due the formation of additional crosslinks induced by this IL. Contrary to this, a fragmentation of the collagen matrix and the loss of tissue anisotropy are reported to result from higher water uptake than in the non-damaged collagen-based tissue32. An increased amount of water results in higher collagen and water mobility. In this case, the shortest relaxation time for the IDP-treated collagen is even higher than the intermediate relaxation time for the native collagen. Thus, the increase in all three relaxation times is a clear demonstration that IDP destabilizes the collagen. Table 1. 1H T2 Relaxation Times of Water in Native, Choline sulphate (CS) and Imidazolium dimethyl phosphate (IDP) Treated Collagen Fibers Sample T2short[ms] T2intermediate[ms] T2long[ms] Native 3.95 17.09 66.63 CS treated 0.82 15.43 70.10 IDP treated 19.90 44.39 174.92

Normalized amplitude (a. u.)

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1.0 native CS treated IDP treated

0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

600

Time (ms)

Figure 3. 1H T2 relaxation decays of native and choline sulphate (CS) and imidazolium dimethyl phosphate (IDP) treated collagen fibers. The continuous lines are the fit results.


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3.4 Thermal Stability of Collagen Fibers from Rat Tail Tendon using DSC. To understand if the molecular level changes brought about by ILs affects higher order at the fibrillar level, the effect of ILs on the thermal stability of native RTT collagen fibers and RTT treated with CS and IDP has been assessed using differential scanning calorimetry (DSC). The denaturation temperature (Td) for RTT collagen fibers is around 63 ˚C33. The denaturation temperature of RTT treated with CS and IDP was found to be 66 and 44 ˚C, respectively (Figure 4). A decrease in thermal stability for IDP-10% indicates the destabilization effect on RTT collagen fibres, whereas, in the case of CS treated RTT collagen fibers, a 5 ˚C increase in thermal stability was observed indicating stability imparted by CS to the RTT collagen fibres. This was in accordance with the molecular level changes observed, where an increase in molar ellipticity values was noticed for CS and a structural deformation was noticed for IDP. Also, the changes brought about in dielectric properties of collagen when treated with IDP and CS are indicative of how differently these two ILs interacted with collagen, wherein one stabilizes collagen and the other destabilizes it. This impact on the collagen stability is invoked by changes in the microenvironment of the collagen and a reorientation in the surrounding water milieu leading to the compacting or destabilization of collagen as evidenced by the NMR studies.


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Figure 4. DSC Thermograms of Native RTT and 10% CS and IDP Treated RTT Collagen Fibers 3.5 Dimensional Stability of Collagen Fibers from Rat Tail Tendon Using Optical Microscopy. Rat tail tendons are composed of type I collagen fibrils, which has the characteristic banding pattern in aqueous solution, revealing the helicity of fibrils34. Figure 5 connotes treatment of RTT with IDP-10%, explicating the huge impact on dimensional stability resulting in distorted wave pattern. This could be due to the chaotropicity of ions resulting in agglomeration, whereas for CS-10%, no swelling was observed indicating negligible changes in RTT when treated with choline sulphate. This indicates that CS imparts stability to the RTT collagen fibres as against IDP.

Figure 5. Optical Micrographs of Native RTT and 10% CS and IDP Treated RTT Collagen Fibers.


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4. CONCLUSIONS In summary, the interactions of two ILs with collagen studied at both molecular and fibrillar levels clearly indicate that the ILs exert a stabilising or destabilising impact though changes in the structure and hydration dynamics of the protein. Impedance measurements indicative of dielectric changes in the protein were in congruence with NMR measurements, wherein the mobility of water molecules surrounding collagen clearly depends on the nature of ionic liquid. Also, the molecular level changes get manifested at higher order on the fibrillar level, wherein the negative effect on the conformational stability of collagen in the solution state also decreases the thermal stability at the fibrillar level and vice versa depending on the type of IL.Understanding and unraveling the changes brought about in the hydration dynamics of collagen after interaction with ILs will give new insights into how ILs stabilize or destabilize collagen for various applications ranging from biomedical to tanning.

AUTHOR INFORMATION Corresponding Author Tel.: +91 44 24437188, E-mail addresses: [email protected], [email protected] (N.N. Fathima) ACKNOWLEDGMENT One of the authors (NNF) thanks INSA-DFG fellowship under which the NMR studies were carried out in Germany. CLRI Communication code: 1254


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33. Bigi, A.; Cojazzi, G.; Roveri, N.; Koch, M. H. J. Differential Scanning Calorimetry and X-Ray Diffraction Study of Tendon Collagen Thermal Denaturation. Int. J. Biol. Macromol. 1987, 9, 363–367. 34. Mehta, A.; Rao, J. R.; Fathima, N. N. Can Green Solvents be Alternatives for Thermal Stabilization of Collagen? Int. J. Biol. Macromol. 2014, 4346, 1-8.


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The changes in the molecular and fibrillar level are manifested in T2 measurements 223x115mm (96 x 96 DPI)


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Impact of Ionic Liquids on the Structure and Dynamics of Collagen

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