Utilization of Lignin-Derived Small Molecules: Epoxy Polymers from Lignin Oxidation Products.

Much effort has been invested in developing methods for producing small molecules from lignin as a way to source feedstock chemicals from renewable sources. Significant progress is being made, and methods for deconstructing lignin are producing good yields of small, mononuclear aromatic products-sufficient amounts to enable studies of the potential use of these compounds as replacements for compounds currently produced from petroleum. To investigate the use of lignin products in epoxies, we begin with aromatic acids that can be produced from lignin, treat them with epichlorohydrin to make glycidyl ethers, and investigate the thermal and mechanical properties of cured mixtures of these compounds with a commercial epoxy resin (EPON 826) and an anhydride curing agent (NMA). While most of the lignin-modified epoxy polymers exhibit good physical and thermal properties, the polymer prepared from p-hydroxybenzoic acid (compound 6) has a higher glass-transition temperature (Tg = 159 °C) than do thermosets made with other lignin-derived materials, such as vanillic acid diglycidyl ether (compound 4) and matches the Tg of cured samples of the commercial EPON-826/NMA epoxy system. This is significant, as p-hydroxybenzoic acid is readily available by simple hydrolysis of several different lignins and functions as a drop-in replacement for 50% of the BPA-based material in this commercial system without significant degradation of material properties. The use of lignin-derived small molecules in high-value systems such as epoxies may help improve the economics of biorefineries.

Toward the oxidative deconstruction of lignin: oxidation of ß-1 and ß-5 linkages.

There have been numerous reports on methods for the oxidative cleavage of ß-O-4 linkages in lignin model compounds, but relatively few reports of how those methods affect other linkages that are present in lignin. We have investigated the effect of several of these oxidation methods on the ß-1 and the ß-5 lignin linkages, using four ß-1 and ß-5 model compounds. We observed that direct oxidative cleavage of C-C bonds occurs in metal-catalyzed TEMPO oxidation systems and with iron porphyrin oxidations, neither of which had we observed in similar oxidations on ß-O-4 models. The ß-5 linkage proved to be largely resistant to all of these oxidative systems, but the dihydrofuran ring in the ß-5 model 3 was opened when treated with KMnO4 at elevated temperature. Most promising was the oxidation of 2 with DDQ, which produced the benzylic ketone in high yield (84%), as it does in reactions with ß-O-4 models. This reaction exhibits selectivity for the benzylic position as well as compatibility with phenols, characteristics that are highly desirable for a two-step, benzylic oxidation/Baeyer-Villiger route for cleavage of lignin.

Selective cleavage of the C(α)-C(ß) linkage in lignin model compounds via Baeyer-Villiger oxidation.

Lignin is an amorphous aromatic polymer derived from plants and is a potential source of fuels and bulk chemicals. Herein, we present a survey of reagents for selective stepwise oxidation of lignin model compounds. Specifically, we have targeted the oxidative cleavage of Cα-Cß bonds as a means to depolymerize lignin and obtain useful aromatic compounds. In this work, we prepared several lignin model compounds that possess structures, characteristic reactivity, and linkages closely related to the parent lignin polymer. We observed that selective oxidation of benzylic hydroxyl groups, followed by Baeyer-Villiger oxidation of the resulting ketones, successfully cleaves the Cα-Cß linkage in these model compounds.

Linear and spiral forms of longitudinal cuts in graphitized N-doped multiwalled carbon nanotubes (g-N-MWCNTs).

Nitrogen-containing multiwalled nanotubes (N-MWCNTs), formed by CVD from a nitrogen-containing feedstock have a 'bamboo' structure in which the axes of the graphene planes are not parallel to the axis of the nanotube and the core is periodically bridged. We find that thermal and chemical treatment of these materials can produce nanotubes that have been cut longitudinally in either a linear or in a spiral manner. In addition, these longitudinally cut nanotubes can be partially or fully unrolled by sonication in an aqueous surfactant, producing graphite platelets as well as narrow structures that could be thin graphite ribbons or very narrow, intact N-MWCNTs. These different morphologies of graphite, available from one source, suggest that there are multiple structures of N-MWCNTs present, few as simple as stacked cups or nested scrolls.

Coupling biomolecules to fullerenes through a molecular adapter.

A novel biotinylated fullerene has been synthesized to facilitate the attachment of biotin-conjugated proteins to C(60) through the use of streptavidin as a molecular adapter. The strong biotin-streptavidin interaction enables the attachment of fullerenes to streptavidin and, because of the availability of four biotin-binding sites on streptavidin, to biotinylated biomolecules. The feasibility of this approach is demonstrated by using biotinylated alkaline phosphatase. Due to the insolubility of fullerenes in aqueous media, the immobilized enzyme can be eventually recovered by simple centrifugation with no significant loss in activity.

Methodology for the preparation of C1-monoalkylated 1,2-dihydro[C70)] derivatives: formation of the "other" regioisomer.

Deprotonation of 1,2-C(70)H(2) with TBAOH, followed by alkylation with methyl bromoacetate, results in formation of a C1-monoalkylated 1,2-dihydro-C(70) derivative. The position of the alkyl group (C1) was established by NMR spectroscopy and comparison with literature spectra of C2-monoalkylated analogs. Presumably, C1-alkylation is the major process due to selective deprotonation of 1,2-C(70)H(2) at C1. Substitution of benzyl bromide for methyl bromoacetate results in rapid dialkylation, unless the amount of base is carefully controlled, in which case C1-monobenzylation is the major process. This methodology for alkylation at C1 is complimentary to methods for the C2-monoalkylation of C(70) with Zn and methyl bromoacetate.

Hybrid nanoparticles based on organized protein immobilization on fullerenes.

Nanoscale carbon materials (i.e., fullerenes and nanotubes) are an attractive platform for applications in biotransformations and biosensors. The interesting properties displayed by nanoparticles demand new strategies for the manipulation of these materials on the nanoscale. Controlled modification of their surface with biomolecules is required to fully realize their potential in bionanotechnology. In this work, immobilization of a fullerene derivative with a mutant subtilisin is demonstrated, and the effect of the fullerene on the protein activity is determined. The fullerene-conjugated enzyme had improved catalytic properties in comparison to subtilisin immobilized on nonporous silica. Further, the pH profile of free and fullerene-conjugated subtilisin were almost identical.

Trends in chemical shift dispersion in fullerene derivatives. Local strain affects the magnetic environment of distant fullerene carbons.

13C NMR chemical shift assignments for 1,2-C60H2 (1) and a series of 13C-labeled fullerene derivatives with three-, four-, and five-membered annulated rings (2-4) were assigned using 2D INADEQUATE spectroscopy and examined for trends that correspond to the changes in strain in the fullerene cage. Chemical shifts of equivalent carbons from 1-4 show that eight carbons trend downfield (carbons 5, 7, 8, 9, 11, 15, 16, 17) and the remaining six carbons (4, 6, 10, 12, 13, 14) trend upfield with increasing ring size. While the average chemical shift is nearly constant, the dispersion is greatest when the local strain is the least, in 1,2-C60H2 (1). 13C chemical shifts are not well correlated with trends in ring size, with strain as measured by the pyramidalization angle of nearby carbons, or with the geometry of the fullerene cage. We interpret the results as evidence that subtle geometrical changes lead to modulation of the strength of ring currents near the site of addition and, in turn, the magnetic field generated by these ring currents affects the chemical shift of carbons on the far side of the fullerene core. These results highlight ring currents as being critically important to the determination of 13C chemical shifts in fullerene derivatives.

Monoalkylation of C60 and C70 with Zn and active alkyl bromides.

We report a convenient and simple solution-phase electron-transfer reaction of C(60) with zinc and alpha-bromoacetonitrile, alpha-bromo acetate esters, allyl bromide, benzyl bromide and alpha-bromo ketones in DMF, with which different types of monoalkylated C(60) derivatives can be prepared. When this method is employed with C(70), 2-carbomethoxymethyl-1,2-dihydro[70]fullerene (isomer 5a) is produced as one of the two 1,2-monoalkylated C(70) isomers, together with the first 5,6-monoalkylated C(70) derivative.

Alkylation of dihydrofullerenes.

The fulleride dianions C(60)(2-) and C(70)(2-) were generated by deprotonation of the corresponding hydrogenated fullerenes, 1,2-C(60)H(2) and 1,2-C(70)H(2). These anions were prepared in the presence of a variety of alkylating agents, and mono- or dialkylated products were obtained. Alkylation was not successful with sulfonate ester alkylating agents. Deprotonation of monoalkylated compounds, followed by second alkylation with a different alkylating agent, produced heterodialkylated compounds. The monoalkyated material was invariably the 1,2-isomers, while the dialkylated materials were generally 1,4-isomers, although some 1,2-isomer was observed in the C(70) context. The major product from alkylation of C(70)(2-) was the 7,23-isomer 13a, a structure where the alkylation took place near the equator of the fullerene cage, rather than at the more strained carbons near the poles.

A (13)C INADEQUATE and HF-GIAO study of C(60)H(2) and C(60)H(6) identification of ring currents in a 1,2-dihydrofullerene.

The hydrofullerenes C(60)H(2) (1) and C(60)H(6) (2) have been prepared in (13)C-enriched form and 2D INADEQUATE NMR spectra were measured. These spectra have provided unambiguous (13)C assignments for 2, and nearly unambiguous assignments for 1. In both cases, the most downfield resonances are immediately adjacent to the sp(3) carbons, despite the fact that these carbons are the least pyramidalized carbons in the molecule. Typically, (13)C chemical shifts move downfield with increasing pyramidalization (THETA(p)), but in these systems there is no strong correlation between THETA(p) and delta. HF-GIAO calculations are able to predict the chemical shifts, but provide little chemical insight into the origin of these chemical shifts. London theory reveals a significant paramagnetic ring current in 1, a feature that helps explain the (1)H shifts in these compounds and may contribute to the (13)C chemical shifts as well.

A novel synthesis of branched high-molecular-weight (C40+) long-chain alkanes.

Many biological and geochemical questions remain concerning the structures, functions, and properties of naturally occurring high-molecular-weight (C40+) alkanes with various mid-chain alkylation patterns. Above C40, these alkanes are exceedingly difficult to separate and purify, and syntheses can be blocked by the low solubility of intermediates. To overcome these problems, a facile three-step synthesis employing the alkylation of 1,3-dithiane with a suitable alpha,omega-dibromoalkane was developed. Bisalkylation of the bis(dithianyl)alkane intermediate with the appropriate 1-bromoalkane and subsequent desulfurization with Raney nickel furnished the desired long-chain alkane. Long-chain alkanes modified at mid-chain and/or symmetrically near the chain termini (or unmodified, i.e., long-chain n-paraffins) are accessible by the selection of appropriate bromoalkanes. Nine mid-chain methylated (C38H78 to C53H108), one symmetrical terminal-chain dimethylated (C40H82), and four linear (C44H90 to C58H118) long-chain alkanes were synthesized by using this approach. High-temperature gas chromatography (HTGC) was found to have important advantages for evaluating the purity of the synthetic high-molecular-weight alkanes.

Intramolecular Excited State Electronic Coupling Along an alpha-Helical Peptide.

Several families of peptides composed of alternating L-alanine (Ala) and alpha-aminoisobutyric acid (Aib) residues with an appended N,N-dimethylanilino and/or 2-naphthalenyl group exist in MeOH and CDCl(3) as alpha-helices. Steady state and time-resolved fluorescence measurements show that the distance and dihedral angle between the appended donor and acceptor and the alignment of the vectors for intramolecular charge transfer interaction (from donor to acceptor) with or against that of the helical dipole moment significantly influence the efficiency of photoinduced electronic coupling and, hence, of intramolecular fluorescence quenching.