R. Parthasarathi

Efficient biomass pretreatment using ionic liquids derived from lignin and hemicellulose

Significance Ionic liquids (ILs) have unique properties applicable to a variety of industrial processes. Nearly universal solvating capabilities, low vapor pressures, and high thermal stabilities make these compounds ideal substitutes for a wide range of organic solvents. To date, the best performing ILs are derived from nonrenewable sources such as petroleum or natural gas. Due to their potential for large-scale deployment, ILs derived from inexpensive, renewable reagents are highly desirable. Herein, we describe a process for synthesizing ILs from materials derived from lignin and hemicellulose, major components of terrestrial plant biomass. With respect to overall sugar yield, experimental evaluation of these compounds showed that they perform comparably to traditional ILs in biomass pretreatment. Ionic liquids (ILs), solvents composed entirely of paired ions, have been used in a variety of process chemistry and renewable energy applications. Imidazolium-based ILs effectively dissolve biomass and represent a remarkable platform for biomass pretreatment. Although efficient, imidazolium cations are expensive and thus limited in their large-scale industrial deployment. To replace imidazolium-based ILs with those derived from renewable sources, we synthesized a series of tertiary amine-based ILs from aromatic aldehydes derived from lignin and hemicellulose, the major by-products of lignocellulosic biofuel production. Compositional analysis of switchgrass pretreated with ILs derived from vanillin, p-anisaldehyde, and furfural confirmed their efficacy. Enzymatic hydrolysis of pretreated switchgrass allowed for direct comparison of sugar yields and lignin removal between biomass-derived ILs and 1-ethyl-3-methylimidazolium acetate. Although the rate of cellulose hydrolysis for switchgrass pretreated with biomass-derived ILs was slightly slower than that of 1-ethyl-3-methylimidazolium acetate, 90–95% glucose and 70–75% xylose yields were obtained for these samples after 72-h incubation. Molecular modeling was used to compare IL solvent parameters with experimentally obtained compositional analysis data. Effective pretreatment of lignocellulose was further investigated by powder X-ray diffraction and glycome profiling of switchgrass cell walls. These studies showed different cellulose structural changes and differences in hemicellulose epitopes between switchgrass pretreatments with the aforementioned ILs. Our concept of deriving ILs from lignocellulosic biomass shows significant potential for the realization of a “closed-loop” process for future lignocellulosic biorefineries and has far-reaching economic impacts for other IL-based process technology currently using ILs synthesized from petroleum sources.

Insights into Hydrogen Bonding and Stacking Interactions in Cellulose

In this quantum chemical study, we explore hydrogen bonding (H-bonding) and stacking interactions in different crystalline cellulose allomorphs; namely, cellulose I(β) and cellulose III(I). We consider a model system representing a cellulose crystalline core made from six cellobiose units arranged in three layers with two chains per layer. We calculate the contributions of intrasheet and intersheet interactions to the structure and stability in both cellulose I(β) and cellulose III(I) crystalline cores. Reference structures for this study were generated from molecular dynamics simulations of water-solvated cellulose I(β) and III(I) fibrils. A systematic analysis of various conformations describing different mutual orientations of cellobiose units is performed using the hybrid density functional theory with the M06-2X with 6-31+G(d,p) basis sets. We dissect the nature of the forces that stabilize the cellulose I(β) and cellulose III(I) crystalline cores and quantify the relative strength of H-bonding and stacking interactions. Our calculations demonstrate that individual H-bonding interactions are stronger in cellulose I(β) than in cellulose III(I); however, the total H-bonding contribution to stabilization is larger in cellulose III(I) because of the highly cooperative nature of the H-bonding network. In addition, we observe a significant contribution from cooperative stacking interactions to the stabilization of cellulose I(β). The theory of atoms-in-molecules (AIM) has been employed to characterize and quantify these intermolecular interactions. AIM analyses highlight the role of nonconventional CH···O H-bonding in the cellulose assemblies. Finally, we calculate molecular electrostatic potential maps for the cellulose allomorphs that capture the differences in chemical reactivity of the systems considered in our study.

Understanding the role of water during ionic liquid pretreatment of lignocellulose: co-solvent or anti-solvent?

Biomass pretreatment with certain ionic liquids (IL) can be highly effective at generating a substrate that can be easily saccharified into fermentable sugars with high yields. In order to improve overall process economics, using mixtures of these ILs with water are more favored over the use of anhydrous IL; however, the solvent property of IL–water mixtures and correlations between cellulose digestibility, cellulose solvation and lignin depolymerization during IL–water pretreatment of lignocellulosic biomass are not well understood. We investigated pretreatment of switchgrass with mixtures of 1-ethyl-3-methylimidazolium acetate, [C2mim][OAc], and water at 160 °C. Results indicate that the chemical composition and crystallinity of the pretreated biomass, and the corresponding lignin dissolution and depolymerization, were dependent on [C2mim][OAc] concentration that correlated strongly with cellulose digestibility. In addition, the hydrogen bond basicity of the [C2mim][OAc]–water mixtures was found to be a good indicator of cellulose dissolution, lignin depolymerization, and sugar yields. Molecular dynamics simulations provided molecular level explanations on cellulose Iβ dissolution at different [C2mim][OAc]–water loadings. The knowledge gained from this study provides a better understanding of the duality of water as a co-solvent/anti-solvent in dissolving cellulose and serves as a design basis for the targeted design of IL–water mixtures that are effective at biomass pretreatment.

Analyzing toxicity through electrophilicity.

SummaryThe toxicological structure-activity relationships are investigated using conceptual DFT based descriptors like global and local electrophilicities. In the present work the usefulness of electrophilicity in predicting toxicity of several polyaromatic hydrocarbons (PAH) is assessed. The toxicity is expressed through biological activity data (pIC50) defined as molar concentration of those chemicals necessary to displace 50% of radiolabeled tetrachlorodibenzo-p-dioxin (TCDD) from the arylhydrocarbon (Ah) receptor. The experimental toxicity values (pIC50) for the electron acceptor toxin like polychlorinated dibenzofurans (PCDF) are taken as dependent variables and the DFT based global descriptor electrophilicity index (ω) is taken as independent variable in the training set. The same model is then tested on a test set of polychlorinated biphenyls (PCB). A good correlation is obtained which vindicates the importance of these descriptors in the QSAR studies on toxins. These toxins act as electron acceptors in the presence of biomolecules whereas aliphatic amines behave as electron donors some of which are also taken into account for the present work. The toxicity values of the aliphatic amines in terms of the 50% inhibitory growth concentration (IGC50) towards ciliate fresh-water protozoa Tetrahymena pyriformis are considered. Since there is no global nucleophilicity we apply local nucleophilicity (ωmax+) as the descriptor in this case of training set. The same regression model is then applied to a test set of amino alcohols. Although the correlation is very good the statistical analysis reflects some cross validation problem. As a further check the amines and amino alcohols are used together to form both the training and the test sets to provide good correlation. It is demonstrated that the toxicity of several toxins (both electron donors and acceptors) in the gas and solution phases can be adequately explained in terms of global and local electrophilicities. Amount of charge transfer between the toxin and the biosystem, simulated as nucleic acid bases and DNA base pairs, indicates the importance of charge transfer in the observed toxicity. The major strength of the present analysis vis-à-vis the existing ones rests on the fact that it requires only one descriptor having a direct relationship with toxicity to provide a better correlation. Importance of using the information from both the toxin and the biosystem is also analyzed.

Nature and kinetic analysis of carbon-carbon bond fragmentation reactions of cation radicals derived from SET-oxidation of lignin model compounds.

Features of the oxidative cleavage reactions of diastereomers of dimeric lignin model compounds, which are models of the major types of structural units found in the lignin backbone, were examined. Cation radicals of these substances were generated by using SET-sensitized photochemical and Ce(IV) and lignin peroxidase promoted oxidative processes, and the nature and kinetics of their C-C bond cleavage reactions were determined. The results show that significant differences exist between the rates of cation radical C1-C2 bond cleavage reactions of 1,2-diaryl-(β-1) and 1-aryl-2-aryloxy-(β-O-4) propan-1,3-diol structural units found in lignins. Specifically, under all conditions C1-C2 bond cleavage reactions of cation radicals of the β-1 models take place more rapidly than those of the β-O-4 counterparts. The results of DFT calculations on cation radicals of the model compounds show that the C1-C2 bond dissociation energies of the β-1 lignin model compounds are significantly lower than those of the β-O-4 models, providing clear evidence for the source of the rate differences.

Understanding pretreatment efficacy of four cholinium and imidazolium ionic liquids by chemistry and computation

Certain ionic liquids (ILs) offer a potentially more sustainable and environmentally responsible alternative to organic solvents for many industrial applications, including biorefineries, where they are used to pretreat lignocellulose. To gain a more robust understanding of the roles of cations and anions in the process, we monitored the impact of the respective ILs on Panicum virgatum (switchgrass) in terms of lignin content, cellulose crystallinity, and enzymatic digestibility. The behaviors of four ILs, based on one of two cations, 1-ethyl-3-methylimidazolium ([C2mim]+) and cholinium ([Ch]+), and one of two anions, acetate ([OAc]−) and lysinate ([Lys]−), were compared. While all four ILs were effective in pretreating switchgrass, ILs containing [Lys]− anions provided greater delignification (70–80% vs. 16–50%) after addition of water as an anti-solvent and higher glucose yields (78–96% vs. 56–90%) compared to those obtained by the use of ILs containing [OAc]− anions. Measurements of the Kamlet–Taft parameters using a series of dyes indicated a greater hydrogen bond basicity for the ILs with [Lys]− anions as compared to acetate ILs. To understand the effective delignification ability of lysinate-based ILs, interaction energies of individual ions and ion pairs with a model dilignol substrate were determined by quantum chemical calculations. The results show that the addition of antisolvent significantly influenced the interaction energies governing lignin removal during the process.

New Insights into the Vacuum UV Photodissociation of Peptides

Vacuum ultraviolet laser photodissociation (UVPD) of peptide ions leads to unusual dissociation channels involving backbone C-C bond breaking. However, the molecular basis for the observed behavior is not clearly understood. We now report theoretical investigations using ab initio/density functional theory (DFT) techniques on neutral and protonated dipeptides undergoing vacuum ultraviolet (VUV) induced fragmentation via a Rydberg excitation (and/or electron detachment) and subsequent C-C bond cleavage. New experimental results on VUV photodissociation of dipeptides (protonated Ala_Arg and Arg_Ala) provide strong support for our proposed model. Our mechanism also provides a natural explanation for the presence of immonium ions that are sometimes observed in such experiments.

Relationship between electrophilicity index, Hammett constant and nucleus-independent chemical shift

Inter-relationships between the electrophilicity index (Ω), Hammett constant (óp@#@) and nucleusindependent chemical shift (NICS (1) — NICS value one ångstrom above the ring centre) have been investigated for a series of meta- and para-substituted benzoic acids. Good linear relationships between Hammett constant vs electrophilicity and Hammett constant vs NICS (1) values have been observed. However, the variation of NICS (1) against CO shows only a low correlation coefficient.

Structure and stability of water chains (H2O)n, n = 5-20.

The structure and stability of linear (helical) water chains (H2O)n, n = 5-20 as obtained from ab initio/DFT calculations are reported along with an atoms-in-molecules (AIM) analysis of hydrogen bond critical points and their characteristics. The resulting helical chain arrangement is one of the predominant motifs in different host environments; although they may not be the most stable, it is shown that these linear water chain clusters could exist in their own right.

A conceptual DFT approach towards analysing toxicity

The applicability of DFT-based descriptors for the development of toxicological structure-activity relationships is assessed. Emphasis in the present study is on the quality of DFT-based descriptors for the development of toxicological QSARs and, more specifically, on the potential of the electrophilicity concept in predicting toxicity of benzidine derivatives and the series of polyaromatic hydrocarbons (PAH) expressed in terms of their biological activity data (pIC50). First, two benzidine derivatives, which act as electron-donating agents in their interactions with biomolecules are considered. Overall toxicity in general and the most probable site of reactivity in particular are effectively described by the global and local electrophilicity parameters respectively. Interaction of two benzidine derivatives with nucleic acid (NA) bases/selected base pairs is determined using Parr’s charge transfer formula. The experimental biological activity data (pIC50) for the family of PAH, namely polychlorinated dibenzofurans (PCDF), poly-halogenated dibenzo-p-dioxins (PHDD) and polychlorinated biphenyls (PCB) are taken as dependent variables and the HF energy (E), along with DFT-based global and local descriptors, viz., electrophilicity index (Ω) and local electrophilic power (Ω+) respectively are taken as independent variables. Fairly good correlation is obtained showing the significance of the selected descriptors in the QSAR on toxins that act as electron acceptors in the presence of biomolecules. Effects of population analysis schemes in the calculation of Fukui functions as well as that of solvation are probed. Similarly, some electron-donor aliphatic amines are studied in the present work. We see that global and local electrophilicities along with the HF energy are adequate in explaining the toxicity of several substances, both electron donors or acceptors when they interact with biosystems, in gas as well as solution phases.