Choline Salts Research Articles

Binary aqueous solutions of choline salts: Determination and modelling of liquid density (298.15 or 313.15 K) and Vapour Pressure Osmometry (313.15 K)

Salts composed of the cholinium cation (such as choline hydroxide and choline chloride) have become one of the main reactants for the synthesis of greener ionic liquids, but, for a proper description of the aqueous solutions of these strong electrolytes by thermodynamic modelling, reliable data on liquid density (ρ) and vapour pressure (p) are needed, which are often unavailable. So, in this work, the liquid density (ρ) of binary aqueous solutions of choline bicarbonate ([Ch][Bic]), choline (2R,3R)-bitartrate ([Ch][Bit]), choline chloride ([Ch]Cl), choline dihydrogen citrate ([Ch][H2Cit]) and choline hydroxide ([Ch]OH) were measured at 298.15 or 313.15 K and 0.1 MPa and correlated using second-degree polynomials with salt molality (m), obtaining determination coefficients (R2) higher than 0.9974. Furthermore, the osmotic coefficients (ϕ) of these binaries were determined using vapour pressure osmometry (VPO), at 313.15 K and 0.1 MPa, being satisfactorily described by the Extended Pitzer Model of Archer, which yielded low values of standard deviation (3.58<σϕ·103<12.86). Then, the mean molal activity coefficients (γ±) and excess Gibbs free energies (GE/RT) were calculated, following the order [Ch][H2Cit] > [Ch][Bit] > [Ch][Bic] > [Ch]OH > [Ch]Cl, which corresponds to the decreasing polarity of the choline salts and agrees with the empirical law of matching water affinities (LMWA).

Novel Green Aqueous Two-Phase Systems (ATPSs) based on choline salts and application to the removal of pharmaceutical pollutants from water

Aqueous Two-Phase Systems (ATPSs) constitute extractive media with applicability for the removal of a wide range of solutes, such as antioxidants or pharmaceuticals, and can be designed in a non-toxic and environmentally friendly manner. Nevertheless, prior to their general use at large scale, their liquid-liquid equilibria (solubility curves and tie-line compositions) need to be thoroughly determined.In this work, 6 solubility curves were obtained for novel aqueous ternary systems containing choline salts (choline chloride, choline bicarbonate, choline (2R,3R)-bitartrate and choline di-hydrogen citrate) and either ethyl lactate (EL) or polyethylene glycol (PEG), at 298.15 K and 0.1 MPa. Moreover, 3 tie-line compositions were successfully determined for the system {ethyl lactate (1) + choline (2R,3R)-bitartrate (2) + water (3)}. Then, this novel ATPS was applied in the removal of three common pharmaceutical pollutants from aqueous media (acetaminophen, amoxicillin and salicylic acid). In these partition studies, salicylic acid and acetaminophen were effectively extracted to the top phase, with the highest partition coefficients (K=9±2) being obtained for salicylic acid in the longest tie-line (length of 0.8816). Acetaminophen achieved the most promising results (K=2.2±0.1) in the shortest tie-line (length of 0.6192).

Equilibrium phase behavior of ternary systems formed by polyethylene glycol + choline salts + water

The economic and environmental potentialities of the application of choline salts in aqueous two-phase systems (ATPS), as well as the incipiency of works of these ionic liquids (ILs) combined with polyethylene glycol (PEG), are the reasons for the characterization of new “green” ATPS formed by PEG (400 and 1500 g.mol−1) combined with choline-based ILs (choline chloride, choline bitartrate, and choline dihydrogen citrate). Choline chloride was preferentially concentrated in the top phase, while the lower phase was rich in PEG. The opposite behavior was observed for bitartrate and dihydrogen citrate. ATPS formed by PEG1500 exhibits a significant increase in the biphasic area as the temperature increases; while for systems formed by PEG400, this same trend is slight, demonstrating that phase segregation has an endothermic character. The salting-out effect and the hydrophobicity of the polymer explain the increase of the biphasic area with increasing the charge density of the anion and increasing polymer molar mass, respectively. The data were fitted with good correlation using the Merchuk equation for the binodal curves and Guan equation for the tie-lines.

Effective Recognition of Lithium Salt in (Choline Chloride: Glycerol) Deep Eutectic Solvent by Reichardt’s Betaine Dye 33

Deep eutectic solvents (DESs) have emerged as novel alternatives to common solvents and VOCs. Their employment as electrolytes in batteries has been an area of intense research. In this context, understanding changes in the physicochemical properties of DESs in the presence of Li salts becomes of utmost importance. Solvatochromic probes have the potential to gauge such changes. It is reported herein that one such UV–vis molecular absorbance probe, Reichardt’s betaine dye 33, effectively manifests changes taking place in a DES Glyceline composed of H-bond accepting salt choline chloride and H-bond donor glycerol in a 1:2 molar ratio, as salt LiCl is added. The lowest energy intramolecular charge–transfer absorbance band of this dye exhibits a 17 nm hypsochromic shift as up to 3.0 molal LiCl is added to Glyceline. The estimated ETN parameter shows a linear increase with the LiCl mole fraction. Spectroscopic responses of betaine dye 33, N,N-diethyl-4-nitroaniline and 4-nitroaniline are used to assess empirical Kamlet–Taft parameters of dipolarity/polarizability (π*), H-bond-donating acidity (α) and H-bond-accepting basicity (β) as a function of LiCl concentration in Glyceline. LiCl addition to Glyceline results in an increase in α and no change in π* and β. It is proposed that the added lithium interacts with the oxygen of the –OH functionalities on the glycerol rendering of the solvent with increased H-bond-donating acidity. It is observed that pyrene, a popular fluorescence probe of solvent polarity, does respond to the addition of LiCl to Glyceline, however, the change in pyrene response starts to become noticeable only at higher LiCl concentrations (mLiCl ≥ 1.5 m). Reichardt’s betaine dye is found to be highly sensitive and versatile in gauging the physicochemical properties of DESs in the presence of LiCl.

The regulation of protein crystallization using choline chloride precipitant and its mechanism

Inorganic salts are the most commonly used precipitants in protein crystallization. However, due to their strong ability to destroy the hydration shell and diffuse electric double layer of proteins, the use of inorganic salts can easily cause protein precipitation. This work studied protein crystallization using organic salt choline chloride (cc) as the precipitant. Compared with NaCl, crystallization, it was found that lysozyme could nucleate at a lower supersaturation and the single-crystal area was significantly enlarged with cc. Even at a high supersaturation of nearly 30, lysozyme crystals could still maintain a complete tetragonal morphology with no urchin-like crystals in cc solution. Furthermore, the average crystal size in cc solution was larger than that in NaCl solution even at a lower supersaturation. In addition, the quantitative efficiency of yield was about equal in cc and NaCl system. Combining Zeta potential, fluorescence spectrum and MD simulation, it was found that cc could interact with lysozyme through hydrogen bond, which weakened the damage of Cl− to diffuse electric double layer of proteins and increased the stability of lysozyme. DLS revealed that the presence of cc could promote the formation of large protein aggregates, which was favorable for crystal growth. In addition, CD spectra and lysozyme activity showed that the presence of cc did not affect the secondary structure and activity of lysozyme. This work proved that cc could be used as a substitute precipitant to grow protein crystals with large size and improve the stability of protein crystallization.

Organic salt composition of pressure sensitive adhesives produced by spiders

Natural glues offer great potential as bio-inspired solutions to problems associated with the performance of synthetic adhesives. Spider viscous glues are elastic pressure sensitive adhesives (PSAs) that physically adhere to surfaces on contact across a range of environmental conditions. Extracting useful components from these secretions remains a challenge that can be met by the comparative analyses of functional analogues. Here we used 1H NMR spectroscopy and mass spectrometry to ascertain the organic salt compositions of the PSAs of four different species of Australian spiders belonging to two lineages that independently acquired aqueous gluey secretions: the St Andrew’s cross (Argiope keyserlingi), the redback (Latrodectus hasselti), the false widow (Steatoda grossa), and the daddy long-legs spider (Pholcus phalangiodes). The PSAs from each of these spiders contained similar organic salts, albeit in variable concentrations. The adhesives of the false widow and daddy long-legs spider had mixtures of only a few components, of which betaine predominated, while the PSAs of the other spiders predominantly contained small organic acids such as GABA/GABA-amide, isethionate, and choline salts. Our results suggest that the PSA composition of spiders is likely to be influenced more by environmental factors than evolutionary history and are guided by common principles. Our findings could be valuable for facilitating the design of more sustainable synthetic glues.

Formation of choline salts and dipolar ions for CO2 reactive eutectic solvents.

Choline-based sorbents derived from imidazole (ImH), phenol (PhOH), pyrrole-2-carbonitrile (CNpyrH), and 1,2,4-triazole (TrzH) are developed for CO2 capture to enable alternative regeneration approaches over aqueous amines. During synthesis, the equilibrium between [Ch]+[OH]- and Ch± dipolar in water shifts to support the formation of Ch±ImH and Ch±PhOH in the presence of ImH and PhOH upon drying. In contrast, salts of [Ch]+[CNpyr]- and [Ch]+[Trz]- were obtained with CNpyrH and TrzH, as confirmed by NMR and FTIR spectroscopy. Density functional theory (DFT) calculations support a spontaneous proton transfer from CNpyrH and TrzH to Ch±, while they show an energy barrier in the case of ImH. These sorbents formed eutectic solvents upon mixing with ethylene glycol (EG) where deprotonation of EG and subsequent binding of CO2 contributed to capacities up to 3.56 mol CO2 kg-1 at 25 °C and 1 bar of CO2. The regenerability of the eutectic solvents was demonstrated by dielectric heating via microwaves (MWs) in support of renewable energy utilization. This study shows the impact of proton sharing on the CO2 capacity and regenerability of eutectic sorbents as molecular design guidance.

Why Does Ethaline Apparently Behave as an Ideal Binary Mixture?

Phase diagram mixtures of the salt choline chloride (ChCl) with ethylene glycol (EG) surprisingly seem to behave as ideal binary nonelectrolyte mixtures [Agieienko, V.; Buchner, R. Phys. Chem. Chem. Phys.2022, 24, 5265]. To shed some light on this conundrum, results of broad-band dielectric relaxation spectroscopy (DRS) and quantum-chemical calculations are reported for solutions of ChCl, choline iodide (ChI), and chlorocholine chloride (ClChCl), in EG up to saturation at 298.15 K. The data revealed that all three solutes are only weakly solvated in the sense that on average per equivalent of solute only one EG OH-group is dynamically affected. While contact ion pairs are significant for solute concentrations of ≲1 M, free cation concentrations are rather low. Instead, over the entire concentration range a large fraction of the dipolar cations could not be detected by DRS. We argue that the latter are embedded in large solute aggregates, explaining thus the phase diagram of ChCl + EG and the very low ionicity of all systems.

(Invited) 20 Years of CO2 Capture By Ionic Liquids, Molecular Liquids, and Liquids in-between

In 2002 we reported the first synthesis and use of a functionalized ("task-specific") ionic liquid for the reactive but reversible capture of CO2. Our original application target was the use of these salts in natural gas purification. However, in succeeding years considerable fervor grew up around us focused on accomplishing large-scale CO2 capture due to concerns about its contribution to global climate change, and that has since supplanted the former application as a major driver of our research. Recognizing that our early materials were likely prohibitively expensive for large scale use, we then spent five years devising new ones which could be made inexpensively from bulk commodity chemicals. In the time since, work with that class of salts, including the edible-in-principle salt choline taurate, has continued apace. These efforts have now resulted in systems which are being actively assessed by the Navy and NASA for closed-system removal of CO2 from breathing air, and which are being tested at pilot plant scale, in collaboration with Norton Engineering, for industrial emissions gas CO2 capture, as well as CO2 capture from ambient air. A brief discussion of the timeline of these developments will be presented, as will details about the ionic liquids currently being used in these tests and our results to date. In addition, we will briefly discuss the potential for using in-situ electrochemical reduction of CO2 in the 'loaded' ILs as a means to convert them into value-added products. Finally, we will also briefly comment on terminology as it relates to ionic liquids, molecular liquids, and liquids "in-between." Figure 1

Effect of 2,4-D formulation on volatility under field conditions

AbstractAuxinic herbicides have been commonly used in production systems for broadleaf weed control for many years. One potential negative aspect to their use is their propensity to volatilize and move away from the treated area after application. This research examined three herbicide formulations and their relative amounts of vaporization following application under field conditions in Knoxville, TN, in 2017, 2018, and 2019. Herbicide treatments evaluated included 2,4-D choline, 2,4-D amine, and the diglycolamine (DGA) salt of dicamba. Ten field studies were conducted with major parameters including air sampler height (0.3 and 1.3 m) and applied surface condition (dry wheat stubble or green-plant vegetation). The relative volatility indicated by the study was that dicamba &gt; 2,4-D choline = 2,4-D amine. Detected herbicide concentrations were numerically higher at the 0.3-m sampling height and in the green-plant surface condition. These results confirm that dicamba is more volatile than 2,4-D and that there was no difference in vapor emissions between the amine and choline salts of 2,4-D under field conditions.

Investigating the effect of systematically modifying the molar ratio of hydrogen bond donor and acceptor on solvation characteristics of deep eutectic solvents formed using choline chloride salt and polyalcohols

Choline chloride-based deep eutectic solvents (DESs) are immensely popular in organic synthesis, catalysis, electrochemistry, and separation science. A popular choice of hydrogen bond donor (HBD) among these DESs consists of both straight-chain and branched polyols that can incorporate additional functional groups, such as ether linkages. Previous studies have shown that the extraction efficiency is significantly altered when the molar ratio of HBD in choline chloride-based DES systems is varied, but no study has been able to relate it to their solvation characteristics. This is largely due to the limited sensitivity of existing solvatochromic dye techniques to detect minor changes in solvation interactions when the DES composition is varied. In this study, inverse gas chromatography was employed for the first time to investigate the variation in solvation properties for DESs comprised of choline salts as hydrogen bond acceptors (HBAs) and polyols as HBDs when their HBA/HBD ratio is systematically altered. Unlike many organic solvents, DES systems investigated in this work possessed a significant hydrogen bond character. It was observed that the hydrogen bond basicity generally plateaued at higher molar ratios of HBD while the hydrogen bond acidity was observed to be the highest at HBA/HBD ratios of 1:10 in all DESs. Amongst all solvents, neat HBDs (triethylene glycol and 1,8-octane diol) possessed the weakest hydrogen bond basicity since they lack the chloride anion that acts as the primary hydrogen bond acceptor. Results from this study demonstrate that the solvation characteristics of DESs are largely different from their starting materials while the HBA/HBD ratio further influences their solvation interactions that can in turn impact important parameters such as extraction yields.

Vibrational spectroscopy and molecular dynamics simulation of choline oxyanions salts

The structure of choline salts containing the anions acetate, [Chol][Ac], and dihydrogen phosphate, [Chol][DHP], were investigated by infrared, Raman, and inelastic neutron scattering (INS). The chosen systems allow for the comparison of structural effects related to the bond acceptor characteristic of [Ac] and the simultaneous acceptor and donor characteristics of [DHP] in forming hydrogen bonds (H-bond) in salts of [Chol], which is itself prone to forming H-bonds. Different computational tools were used for the analysis of different spectral ranges. The calculation of the low-frequency range of Raman and INS spectra of the crystalline phases at low-temperatures by solid state DFPT (density functional perturbation theory) unveils the coupling between vibrations of the H-bonds and intramolecular modes. Changes observed in the spectral pattern of lattice and [DHP] modes upon heating crystalline [Chol][DHP] are analogous to the ferroelectric–paraelectric phase transition known in the potassium salt of [DHP]. The fingerprint region of the vibrational spectra provides information concerning the [Chol] conformation in the solid phase (gauche in [Chol][Ac] and anti in [Chol][DHP]) and in aqueous solution. DFT calculations of ionic pairs and ionic clusters unveil the interplay between [Chol] conformation and the [DHP] ability to form H-bonded dimers of anions. The high-frequency spectral range and the structures driven by H-bonds are discussed using classical molecular dynamics (MD) simulations. The MD simulations of aqueous solutions highlight the strong anion-cation H-bond in [Chol][Ac], in contrast to the strong anion–anion H-bond in [Chol][DHP] due to occurrence of dimers and larger clusters of [DHP].

Mixtures of 2,4-D and dicamba with other pesticides and their influence on application parameters

Due to the evolution of resistant weeds to glyphosate, the new technologies Xtend® and Enlist® cropping systems are expected to boost the use of auxin herbicides dicamba and 2,4-D. This study examined the effects of two formulations of 2,4-D and dicamba in tank-mixtures with other pesticides on selected physico-chemical properties of the spray solution, such as stability and pH, as well as other characteristics, such as size in dropcard and surface tension. The research included two factors, factor A comprised the herbicides dicamba and 2,4-D amine and choline salts and a auxin herbicide-free treatment, while factor B comprised 12 pesticides commonly sprayed in Brazil (i.e., five formulations of glyphosate, clethodim, haloxyfop-p-methyl, trifloxystrobin + prothioconazole, azoxystrobin + benzovindiflupyr, mancozeb, chlorantraniliprole, and thiamethoxam + lambda-cyhalothrin). Stability and pH, surface tension, and droplet parameters of the tank-mixtures were analyzed. No incompatibility was observed in the tank mixtures tested. Mixing dicamba with clethodim and haloxyfop-p-methyl resulted in pH values below 4.5. Auxin herbicides did not show pH buffering capacity and did not prevent pH changes when the others pesticides were added to the tank-mixtures. All mixtures affected surface tension of herbicides relative to the single solutions. The results support the use of auxin herbicides with other pesticides, given that the use of auxin herbicides will probably increase, following the adoption of new dicamba and 2,4-D technologies.

Comparison of Serum Choline and Trimethylamine N-oxide after ingestion of Alpha Glyceryl Phosphoryl Choline and Choline Salts.

Background: Choline supplements may provide potential improvements to health outcomes and exercise performance, yet the bioavailability of choline supplements is poorly understood. Objective: The objective of the present investigation was to examine the levels of serum Choline and Trimethylamine N-oxide (TMAO) after two dose of Alpha Glyceryl Phosphoryl Choline (A-GPC) in comparison to common choline salts. Methods: High-dose and low-dose A-GPC along with choline salt supplements were administered to participants over the course of four weeks in a randomized double-blind fashion. Serum levels of choline and TMAO, the gut byproduct of choline were examined acutely over the course of four hours and again following one week and four weeks of supplementation. Results: High-dose A-GPC and the choline chloride supplement yielded significantly higher choline levels compared to low-dose A-GPC and choline bitartate (F=31.31, p&lt;0.01) though the effect for time was not significant. TMAO levels were not significantly different between supplements (F=1.96. p=0.1361) or across time (F=0.0795, p=0.7795). Conclusion: A-GPC increases serum choline similar to that seen with high-dose choline chloride ingestion without a concomitant increase in TMAO levels and therefore, may be a desirable option as a dietary supplement.

Choline-based eutectic mixtures as catalysts for effective synthesis of cyclic carbonates from epoxides and CO2

• Choline salts have been mixed with hydrogen bond donors to form eutectic mixtures. • Choline-based eutectic mixtures catalyze CO 2 conversion to cyclic carbonates. • Carbonatation run in mild condition of temperature (80 °C) and pressure (0.1–0.4 MPa). • Good to very good yields are obtained on various terminal epoxides. • The eutectic catalysts are bio-based, non-toxic, recyclable and easily synthesized. CO 2 is a renewable, abundant and cheap C1-feedstock and its conversion to cyclic carbonates starting from epoxides has been widely explored in the last years. Nevertheless, conducting this reaction under mild and sustainable conditions is still a challenging task. Herein we present the use of choline-salt based eutectic mixtures as catalysts for the reaction of CO 2 with epoxides to give cyclic carbonates. Choline chloride and choline iodide have been coupled with various hydrogen bond donors (HBDs), mainly cheap and bio-based carboxylic acids and polyols, to form two classes of eutectic mixtures. Very good yields were achieved under mild conditions (80 °C in 7–22 h) for various terminal epoxides, with both classes of catalysts. While a pressure of 0.4 MPa of CO 2 is required to obtain appreciable conversions using choline chloride-based mixture, atmospheric pressure of CO 2 (balloon) has been successfully used with choline iodide-based mixtures. Furthermore, the catalysts could be recycled without appreciable loss of the catalytic activity. The improved catalytic performance of both choline-based eutectic mixtures is attributed to the synergistic activity of the halide, responsible for the opening of the epoxy-ring, and the HBD that has a role in the stabilization of the alkoxide intermediate.

Preparation of cellulose nanofibrils from okara by high pressure homogenization method using deep eutectic solvents

The interest in the application of deep eutectic solvents (DES) as a green solvent has increased rapidly. The application of DES in agro-industrial okara treatment can be developed into industrialization. In order to simplify the degreasing, deproteinization and cellulose extraction steps in the process of pretreating okara cellulose, a new approach to cellulose extraction from okara via one-pot DES treatment is established. Three different complexing agents, choline salts in oxalic acid, in glycerol and in urea are studied in this work. The structure and properties of cellulose nanofibers (CNFs) prepared by high pressure homogenization following pretreatment of okara with anionic systhesized DES are investigated. The effects of different systems on the degree of microfibrillation of okara are analyzed using techniques such as SEM, FTIR, TD and DTG. The results indicate that by pretreating okara with choline chloride–oxalic acid type solvent, CNFs with average diameter of 27 nm are obtained using high pressure homogenization.

Natural Choline from Egg Yolk Phospholipids Is More Efficiently Absorbed Compared with Choline Bitartrate; Outcomes of A Randomized Trial in Healthy Adults.

Choline is a vitamin-like essential nutrient, important throughout one’s lifespan. Therefore, choline salts are added to infant formula, supplements and functional foods. However, if choline is present in a natural form, e.g. bound to phospholipids, it may be more efficiently absorbed. The study’s aim was to evaluate if choline uptake is improved after consumption of an egg yolk phospholipid drink, containing 3 g of phospholipid bound choline, compared to a control drink with 3 g of choline bitartrate. We performed a randomized, double blind, cross-over trial with 18 participants. Plasma choline, betaine and dimethylglycine concentrations were determined before and up to six hours after consumption of the drinks. The plasma choline response, as determined by the incremental area under the curve, was four times higher after consumption of the egg yolk phospholipid drink compared with the control drink (p < 0.01). Similar outcomes were also observed for choline’s main metabolites, betaine (p < 0.01) and dimethylglycine (p = 0.01). Consumption of natural choline from egg yolk phospholipids improved choline absorption compared to consumption of chemically produced choline bitartrate. This information is of relevance for the food industry, instead of adding choline-salts, adding choline from egg yolk phospholipids can improve choline uptake and positively impact health.

Starch treatment with deep eutectic solvents, ionic liquids and glycerol. A comparative study

In this work changes in potato starch during processing (dissolution and plasticization via thermocompression) in the presence of three types of media: deep eutectic solvents (DES) with different anions in choline salts and components molar ratios, ionic liquids (IL) and glycerol (G) were investigated. Urea-based DES and IL exhibited dissolving capacity towards starch (confirmed with microscopic observations and DSC). Plasticizing ability was studied using rheological characteristics of starch/media premixtures as well as mechanical tests, DMA, XRD and FTIR analysis of thermoplastic starch (TPS). Additionally, influence of concentration of two types of DES: dissolving and non-dissolving on final properties was investigated. Results revealed that viscosity of premixtures with DES and IL was lower that with G. Dissolving DES and IL led to highly amorphous materials with lower tensile strength and much higher elongation at break than TPS with non-dissolving DES and G. Imidazolium IL significantly decreased thermal stability of TPS.

Norharmane prototropism in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are gaining widespread attention as novel and alternate solvents in chemical and biological sciences. Prototropic equilibria are known to be governed by the solubilizing milieu, and thus, it is imperative to study them within DESs. Prototropic behavior of β-carboline commonly named as norharmane (9H-pyrido[3,4-b]indole) is investigated in eight different DESs constituted of the salt choline chloride with eight different H-bond donors (HBDs) to ascertain their role in controlling prototropism within DESs. UV–vis molecular absorbance is used to obtain information on ground-state prototropism whereas steady-state and time-resolved fluorescence are used to investigate prototropic forms in the excited-state. In the ground-state, DESs with HBDs glycerol and ethylene glycol support both neutral and cationic forms of norharmane, however, within DESs constituted of HBDs urea, 1,4-butane-diol, and acetamide, respectively, only the neutral form exists. Within the remaining three DESs with HBDs tetraethylene glycol, 3-phenylpropionic acid, and malonic acid, respectively, only cationic form is supported. As the cationic form is preferred more over the neutral form of norharmane in the excited-state, DESs with HBDs tetraethylene glycol, 3-phenylpropionic acid, and malonic acid, respectively, support only cationic form in the excited-state. DES with glycerol also supports only the cationic form in the excited-state, DES with ethylene glycol, acetamide, and 1,4-butane-diol support both cationic and neutral forms. The DES with urea as the HBD is the only DES that supports only the neutral form in the exited-state. Excited-state intensity decay data confirms the presence of different prototropic forms of norharmane within investigated DESs with distinct decay times for the neutral and cationic forms. Correlation between the common empirical solvent polarity parameter, ETN, along with Kamlet-Taft parameters, H-bond donating acidity (α) and H-bond accepting basicity (β), of the DESs and the relative presence of the norharmane prototropic forms is established.

Photophysical Behavior and Fluorescence Quenching of l-Tryptophan in Choline Chloride-Based Deep Eutectic Solvents.

Intrinsic fluorescence from l-tryptophan (l-Trp) is routinely used to obtain insight into the structural features and dynamics of proteins and enzymes. In contrast to aqueous enzymology, different parameters that control and influence the behavior of proteins and enzymes in nonaqueous media depend heavily on the solvent. Detailed analysis of the intrinsic fluorescence from l-Trp dissolved in two deep eutectic solvents (DESs), reline and glyceline, prepared by mixing salt choline chloride with H-bond donors urea and glycerol, respectively, in a 1:2 molar ratio within 298.15-358.15 K temperature range, is presented. Fluorescence emission maxima of l-Trp dissolved in DESs show bathochromic shift with increasing temperature. In comparison to water and several organic solvents, the fluorescence quantum yields of l-Trp in both DESs are significantly higher. While the rates of nonradiative decay in the two DESs are comparable and increase with increasing temperature, radiative decay rates are independent of temperature and are higher in glyceline than in reline, resulting in a higher fluorescence quantum yield of l-Trp in glyceline. Excited-state emission intensity decays of l-Trp fit best to a double exponential model irrespective of the identity of the DES and temperature. Average lifetime decreases with increasing temperature due to increased thermal deactivation; however, this decrease is much slower in DESs as compared to that in water. Both steady-state fluorescence anisotropy and rotational reorientation times for l-Trp are governed by the inherent complexity of the DESs as solubilizing milieu resulting in noncompliance to simple hydrodynamic treatment. Fluorescence quenching of l-Trp by acrylamide in reline is purely dynamic in nature. This is in contrast to the aqueous media where the decrease in fluorescence is a combined result of both dynamic and static quenching. The quenching within reline is fairly efficient considering the high viscosity of the medium. Significantly lower activation energy of the bimolecular quenching process as compared to the activation energy of the viscous flow indicates facilitation of the electron/charge transfer quenching of l-Trp by acrylamide within the ionic environment offered by reline. The effect of high viscosity is partly overcome by the strongly ionic environment of reline during the electron/charge transfer between l-Trp and acrylamide. The results highlight the structural complexity of these DESs especially within the cybotactic region of the probe, which is absent in common molecular solvents of similar high viscosity.