Notable Stabilization of α-Chymotrypsin by the Protic Ionic Additive, [ch][dhp]: Calorimetric Evidence for a Fine Enthalpy/Entropy Balance

An impact of 0.5 to 3 M choline dihydrogen phosphate, [ch][dhp], the biotechnologically relevant ionic substance, on the thermal stability of a model globular protein, α-chymotrypsin (α-CT), has been studied exploiting the highly sensitive differential scanning calorimetry (DSC) technique. The notable overall stabilizing effect of 11 ± 2 K regarding the thermal transition (melting) temperature, T m, has been detected. For this kind of series, for the first time, the calorimetric melting enthalpy (ΔH cal) and transition entropy (ΔS m) parameters have been determined simultaneously throughout. The first analysis indicated a two-phase impact implying (a) the initial, dramatic drop in both ΔH cal and ΔS m, obviously connected to specific, direct interaction between the [ch][dhp] components and α-CT's charged groups (within 0 to 1 mol/L [ch][dhp]), leading to the essential rearrangement of the interfacial hydrogen-bonded (HB) network; and (b) the follow-up (within 1 to 3.0 mol/L [ch][dhp]), modest changes in ΔH cal and lack of changes in ΔS m, seemingly connected with a subsequent steady strengthening of already reformed HB network, respectively. These changes, presumably, are primarily facilitated by Coulombic interactions between the [dhp] anions and solvent-exposed positively charged amino groups of α-CT.


Introduction
Currently, there is a great interest in the multipurpose application of recently revived substances, organic salts possessing relatively low melting points (also known as "roomtemperature ionic liquids, " RTILs, or "plastic ionic crystals"). These substances, when mixed with water, are able to form a huge amount of unique liquid or semisolid compositions apt for the novel nanotechnological and bionanotechnological developments [1][2][3][4]. Because of virtually unlimited possibilities for the variation of an intrinsic molecular balance between the charged and uncharged (hydrophobic) groups on both positive and negative ionic components [1], these 2 International Scholarly Research Notices calorimetry (DSC) [4, 5, 8-11, 13-17, 23-26]. Specifically, the DSC technique detects the temperature-induced unfolding (melting) process (associated with a global transition along the protein's collective conformational coordinate) as a variation of the protein's partial heat capacity versus temperature and, in uncomplicated cases, in a model-free way (directly) determines the unique "transition temperature" (temperature of thermal melting), , the unfolding (melting) calorimetric enthalpy, Δ cal (i.e., the heat absorbed through the unfolding transformation around ), and calorimetric entropy, Δ cal [27][28][29].
Numerous papers on DSC studies for the stabilizing/destabilizing impact of various organic additives [4, 5, 8-11, 13-17, 23-26, 30], including RTILs [4,5,[8][9][10][11][13][14][15][16][17], on the proteins' global stability have been published. However, because of a complex, dualistic character of the RTIL impact (due to the involvement of oppositely charged surfaceactive ions), notwithstanding of several serious attempts [16,17,21], no conclusive rationalization of the existing data was drawn so far. Obviously, more systematic and careful work with the application of the DSC combined with other methodologies is required. Indeed, in many cases of serial DSC studies implying the RTIL impact, typically, only the parameter , as a measure of stability, has been considered; the easily calculable parameter Δ cal has been addressed in very few cases (see, e.g., [5,8,9]; for other similar studies involving uncharged additives, see, e.g., [4,23,26,30]). It is instructive that another easily calculable parameter, Δ cal (vide infra), has never been implicated for this kind of series. An entropic contribution is equally important for the understanding of the protein stability phenomenon, as its enthalpic counterpart [29,[31][32][33]. In fact, stability of the native state results from the fine balance between a multitude of the enthalpic and entropic contributing factors (vide infra) [4,27,30,31,33]. Among other model proteins, in which thermal stability has been extensively explored by the DSC technique, -chymotrypsin ( -CT, EC 3.4.21.1; = 26,000), a hydrolytic enzyme of biotechnological importance, should be mentioned [4,5,23,24,26,30]. However, detailed studies regarding the impact of RTILs on -CT, using highly sensitive DSC technique (implying determination and analysis of all the key parameters, , Δ cal , and Δ cal ), are still lacking. In this communication, we report preliminary data and principal thermodynamic analysis on the stabilizing impact of biotechnologically important concentrations of a protic ionic salt, choline dihydrogen phosphate, [ch][dhp], on the thermal stability of model protein, -CT. Determination of both enthalpic and entropic components contributing to the protein melting process allowed for a conjecturing of fine interfacial effects behind the notable stabilization impact.

Experiment
Highly purified and lyophilized -CT was purchased from Fluka and [ch][dhp] was from Iolitech, GmbH, and used without further purification. Other chemicals were of highest purity available. The -CT samples for the DSC experiments were prepared at concentrations of 1.5 mg/mL, by dissolving in phosphate 10 −2 M buffer solutions (pH 4.2). Microcalorimetric measurements for temperatureinduced melting (denaturation) of -CT in buffered [ch][dhp] aqueous solutions were performed with a DASM-4A adiabatic scanning calorimeter (Biopribor, Russia) integrated with a PCI-DAS1001 (Measurement Computing Corporation) interface unit, providing direct PC access [30]. The heating rate was 2 K/min. The methodological aspects of the DSC technique and data processing have been described earlier [23,27,28,30].    results of thermodynamic analysis of these melting curves.

Results and Discussion
The values for a melting enthalpy, Δ cal , were determined as areas under the calorimetric curves of Figure 1 [23,[27][28][29][30]: where is the absolute temperature and 1 and 2 are the temperatures that correspond to the start and completion of heat absorption due to the protein's thermal melting ( 1 < < 2 ). The values of the counterpart parameter, melting entropy, Δ cal , can be determined as follows [29]: . ( Under the assumption that a two-state model for the protein melting process is operative, that is, (a) at concentrations of the native and melted (unfolded) protein are equal and (b) virtually the entire entropy change takes place at this temperature (therefore, Δ = Δ cal − Δ = 0), (2) is simplified as follows: However, one may question whether (1) to (3) and related analysis (vide infra) are valid for the case of -CT, known as a protein in which thermal unfolding process, in general, deviates from a simple two-state pattern (see, e.g., [24,30]). Rather detailed analysis of this issue is given in our previous paper [30] (see also [17,34]). In brief, in the course of a thermal melting of most globular proteins of modest dimensions, including -CT, the native and "normally" unfolded states can be considered as yet dominantly populated states against the relatively minor, thermally induced molten globule (predenatured) [25,34], "intermediate" (metastable transitional) [16,34], and misfolded/selfaggregated (postdenatured) states [16,29,35]. Importantly, the DSC curves retained symmetrical, nearly Gaussian shape (half widths were within 6.7 ± 0.2 K throughout) upon the addition of 0.5 to 3 mol/l [ch][dhp] (the fact that favors the abovementioned outline, yet, as a crude approximation [30]), whilst their maximum position ( ) was gradually shifted to higher temperatures (for ca. 11 K, in overall), pointing to the notable stabilizing impact of [ch][dhp] ( Table 1). The values of Δ cal (attained for selected cases) and Δ , as determined through (2) and (3), coincided within the limits of experimental error, that is, an additional support for the practical applicability of (3). The slope for the first change of versus the [ch][dhp]) content (from 0 to 1 mol/l [ch][dhp]) was a little bit larger than one for the rest of the concentration range (Figure 2). This fact could be ascribable to the experimental scatter of experimental points if there is no remarkable and drastic mutually compensating change of both Δ cal and Δ resulting only in a modest shift of (Figures 2 to 4 Figure  2 of [11]). However, as can be judged based on the incomplete Figure 1 of [14], the increase in was accompanied by the substantial increase in Δ cal . Unfortunately, the whole trend for Δ cal (and, even more so, for Δ ) is not reported in [11]. In another recent work, Rodrigues et al. [15] reported on ca. 11 K increase in a melting temperature of the third model protein, lysozyme, upon the adding of 2.5 mol/L [ch] [dhp]. The overall effect is absolutely similar to ours (implying the interpolated value of ≈ 74 ∘ C at 2.5 mol/L [ch][dhp] for our system); however, the overall dependence of on the [ch][dhp] content for lysozyme, in contrast to our case (Figure 1), exhibited well-pronounced minimum at 0.5 mol/L [ch] [dhp]. Unfortunately, again, the authors of [15] did not report the values for Δ cal and Δ cal (Δ ) at any concentration of the additive. The lacking information would allow one to see the variable balance between Δ cal and Δ behind the resultant behavior of in these studies, hence, to compare the complete data for all three model proteins and gain better insight into atomistic mechanisms of stabilization.
Nonetheless, the first analysis of our results together with comparable published data [11,15] permits presuming that the mechanisms behind the initial drop in values of both Δ cal and Δ and further slight increase with and virtual independence of , respectively, are obviously different. When going from zero to 0.5 ÷ 1 mol/L additive concentration, seemingly, strong association between the [ch][dhp] components and proteins' charged groups takes place that should be rather specific to the protein entity and its ionization state (pH); note different patterns of for -CT and ribonuclease A on the one hand and lysozyme on the other hand (note also different Δ cal patterns for -CT and ribonuclease A, mentioned above). Around this point of the additive concentration, the specific interactions, probably, arrive at the saturation level, after which more gradual/monotonic changes of (in all three cases) and very small/no variation of Δ cal and Δ (as detected in our case) can be observed. The small changes in Δ cal , seemingly, are connected with the formation of additional contacts and some redistribution of existing ones between the protein and the [ch][dhp] components. Development of the latter process seems to favor proteins' gradual stabilization via the Δ cal rather than the Δ parameter. According to the analysis of [16] (the most systematic one, so far), the choline cation is deemed to act as a slightly destabilizing component whereas dihydrogen phosphate anion is known as an efficient stabilizer. In addition, this pattern turned to be dependent on the additive concentration and has to be also essentially pH dependent [16]. Hence, around pH 4.2, where the exposed amino groups are 100% positively charged, the [dhp] anions, able to effectively interact with these groups through the Coulombic attraction, may eventually replace the existing hydrogen-bond networks originally formed by solvating water and contribute to the protein stabilization. The enthalpic rather entropic nature of the stabilization impact of [ch][dhp] above 1 mol/L concentration definitely points to a prevailing role of the hydrogen-bond network strengthening rather than that of additional strengthening of electrostatic interactions (between any kind of the "point charges" involved). At this pH, the surface exposed carboxylate groups of -CT are for only ca. 50% negatively charged; therefore, respective Coulombic and associated hydrogenbonding interactions may play relatively minor role. Further systematic work is in progress aiming at more comprehensive insights at the atomistic level.

Conclusions
(a) We applied the very sensitive DSC technique to study an impact of 0.5 to 3 mol/L additives of the biotechnologically relevant ionic substance [ch][dhp] on the thermal stability of a model globular protein -CT and found the notable stabilizing effect of 11 ± 2 K regarding the thermal unfolding (melting) temperature, , that varied monotonically with the additive concentration.
(b) The calorimetric melting enthalpy (Δ cal ) and transition entropy (Δ ) parameters were determined simultaneously throughout the series, and the trend analysis indicated a two-phase pattern including the low-concentration (0 to 1 mol/L [ch][dhp]) and high-concentration (1 to 3 mol/L [ch][dhp])) regions, connected to the initial dramatic and the follow-up minor or no changes of Δ cal and Δ , respectively.
(c) The atomistic mechanism behind the observed overall stabilizing effect, presumably, is guided by the ion pair formation between the protein's charged groups and the [ch][dhp] components, with a prevailing role of restructured hydrogen-bonded networks facilitated mostly by Coulombic interaction between the [dhp] anions and solvent-exposed positively charged amino groups of -CT.
(d) A need for the determination of parameters Δ cal and Δ cal (Δ ) throughout this kind of series, for the proper understanding of atomistic mechanisms behind the stabilizing/destabilizing impact of various additives on proteins, is clearly demonstrated.