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  • To understand the behavior of

    2019-07-11

    To understand the behavior of enzymes in ILs at a molecular level, it is first desirable to dissolve enzymes in the solvents. For soluble catalysts, there are many relatively simple spectroscopic and other techniques for obtaining accurate information at a molecular level. Parker et al. [9] reported that cation solvating power of solvents is well measured by solvent donor properties (donor number, DN), while anion solvating power of solvents is well measured by solvent acceptor properties (acceptor number, AN). Water is a strongly amphoteric solvent with large DN and AN values (DN=33.0, AN =54.8) [10]. Enzymes are polyelectrolytes and can hence readily dissolve in water. And we might expect that active enzymes will tend to dissolve in water-like solvents with a hydroxyl-functionality. Walker and Bruce [11], [12] were the first to describe the design of an IL in which the cation contained a hydroxyalkyl group to stabilize the dissolved enzyme. The incorporation of a hydroxyl-functionality in the solvents for dissolving enzymes with modest to good catalytic activity was also AdipoRon structure demonstrated by several groups [13], [14], [15] including our own [16], [17], [18]. ILs based on other functional groups such as NO3−, lactate, EtSO4−, and CH3COO− may also dissolve enzymes, however, most of them cause severe enzyme deactivation [3], [19], [20], [21]. What causes activation or deactivation of enzyme dissolved in ILs? The behavior of enzymes is strongly dependent on the protonation state of their ionizable groups [17], [22], [23], [24], [25], [26], [27]. Enzymes catalyze reactions using a variety of ionizable groups functioning as electrophiles, nucleophiles, or general acid/base catalysts. For an ionizable group (like COOH), the ionization process can be broken conceptually into two steps (Fig. 1). In the first step, solvent molecules act as a Lewis base (electron pair donor) to the H atom and as a Lewis AdipoRon structure (electron pair acceptor) to the O atom, and ionize the H and O atoms, resulting in the breaking of the OH covalent bond and the formation of an ion pair (O−,H+)solv in which both ions now interact with solvent. The ionization step is a function of the DN and AN of the solvent. Higher values mean that solvent has higher ability to ionize polar OH covalent bond and to stabilize the formed ions. In other words, strongly amphoteric solvents are good ionizing solvents. In the second step, the ion pair (O−,H+)solv dissociates into free ions. The dissociation process is easy if the solvent has a high relative permittivity (dielectric constant, εr). According to the above analysis, solvents need to have water-like ionizing–dissociating abilities in order to dissolve active enzymes (In this paper we will refer to such solvents as “biocompatible”). Recently, we have proposed basic principles for biocompatible organic solvent design [18], which involve the introduction of hydroxyl groups into high dielectric constant compounds. The main objective of the solvent design is to improve ionizing–dissociating abilities of the studied solvent. The great success in designing biocompatible organic solvents in our previous studies [17], [18] inspired us to design and synthesize biocompatible ILs. The basic principles for biocompatible IL design are: 1. The design process starts with a known high dielectric compound used as a lead, so that the new IL has strong dissociating ability. Among common organic compounds, we found three classes of compounds that meet the demand, which can be used as lead compounds for construction of ILs. They are represented by dimethyl sulfone (εr=47.4 at 383K), triethanolamine (εr=29.4), and imidazole (εr=23.0) [28]. 2. Starting from the structure of the selected lead, the IL is designed to have strong ionizing ability. This is the functionalization stage, and a functional group with large values of both DN and AN is built into the molecular structure of the lead compound. Hydroxyl group possesses high DN and AN values, therefore, hydroxyl-functionalization of the lead with high dielectric constant value may be advantageous for ionizing and dissociating enzyme functional groups. The incorporation of a hydroxyl-functionality in the high dielectric organic solvents for ionizing and dissociating enzymes with high catalytic activity was demonstrated by our group [17], [18].