Ionic Liquids as Both Solvent and Reagent in Electrophilic Addition Reactions

Ionic Liquids (ILs) are an environmentally friendly alternative to organic and aqueous reaction solvents. ILs do not emit hazardous gasses, are readily recycled and reused, and often do not require an excess volume of harmful reagents and purification solvents. The IL anion’s capacity to participate in a reaction as the nucleophile source is a much less understood area of this research, and it may advance the use of ILs in organic synthesis, particularly for addition and substitution reactions. Markovnikov hydrohalogenation of alkenes is one addition reaction that is generally taught as an introduction to organic chemical reactions; however, this reaction is difficult to successfully complete in the laboratory. ILs overcome the challenges posed in those traditional methods and can be used as both a reaction solvent and halogen source to successfully complete the Markovnikov addition of H-X across a double bond. The hydrohalogenation reaction was completed over 100 times using a variety of ionic liquids including imidazolium, pyridinium, pyrrolidinium, and piperidinium cations. Products were isolated using organic extraction and analyzed with NMR and GC-MS. Bromide anion ILs were consistently successful with all substrates, iodide was most efficient under nitrogen, and chloride was successful with additional heat. Hydrohalogenation of styrene was successful in all ILs used and most successful in the imidazolium-based bromide ILs. Reactions with cyclic aliphatic substrates were less successful with lower yields. Finally, hydrohalogenation of styrene derivatives was recently investigated with moderate success after modification of reaction conditions

Specific Isotopic Labeling and Photooxidation-linked Structural Changes in the Manganese-stabilizing Subunit of Photosystem II

Photosystem II (PSII) oxidizes water to molecular oxygen; the catalytic site is a cluster of four manganese ions. The catalytic site undergoes four sequential light-driven oxidation steps to form oxygen; these sequentially oxidized states are referred to as the Sn states, where n refers to the number of oxidizing equivalents stored. The extrinsic manganese stabilizing protein (MSP) of PSII influences the efficiency and stability of the manganese cluster, as well as the rates of the S state transitions. To understand how MSP influences photosynthetic water oxidation, we have employed isotope editing and difference Fourier transform infrared spectroscopy. MSP was expressed in Escherichia coli under conditions in which MSP aspartic and glutamic acid residues label at yields of 65 and 41%, respectively. Asparagine and glutamine were also labeled by this approach. GC/MS analysis was consistent with minimal scrambling of label into other amino acid residues and with no significant scrambling into the peptide bond. Selectively labeled MSP was then reconstituted to PSII, which had been stripped of native MSP. Difference Fourier transform infrared spectroscopy was used to probe the S 1QA to S2QA- transition at 200 K, as well as the S1QB to S2Q B- transition at 277 K. These experiments show that aspargine, glutamine, and glutamate residues in MSP are perturbed by photooxidation of manganese during the S1 to S2 transition

Isotope-based discrimination between the infrared modes of plastosemiquinone anion radicals and neutral tyrosyl radicals in photosystem II

Photosystem II (PSII) conducts the light-driven oxidation of water and reduction of plastoquinone. Difference Fourier transform infrared (FT-IR) spectroscopy can be used to obtain information about structural changes which occur in protein and cofactors when charge separation occurs. The focus of this work was the assignment of vibrational lines to two different species in PSII: the tyrosyl radical, Z, and the plastosemiquinone anion radical, QA-. Difference FT-IR experiments were conducted with cyanobacterial PSII samples, in which the tyrosine ring was uniformly 13C-labeled, in which tyrosine was 13C-labeled at carbon 4, and in which plastoquinone was methyl-deuterated. At 80 K, difference FT-IR spectra reflect the oxidation of chlorophyll/ carotenoid and the one-electron reduction of QA; no significant D or Z contribution to the spectrum is observed under these conditions. At 264 K, difference FT-IR spectra reflect the oxidation of redox-active tyrosines Z and D; no significant QA- contribution is observed under these conditions. At 80 K, isotope-induced shifts were observed in spectral features at 1482 and 1469 cm-1 upon deuteration of plastoquinone. At 264 K, isotope-induced shifts were observed in a 1478 cm-1 line upon 13C- labeling of tyrosine, but little change was observed upon plastoquinone deuteration. These data support the assignment of a positive 1478 cm-1 line to a tyrosyl radical vibrational mode and positive 1482 and 1469 cm-1 lines to plastosemiquinone anion vibrational modes. Hybrid Hartree-Fock/density functional calculations of p-cresyl radical\u27s vibrational frequencies and isotopic frequency shifts support this assignment. © 2000 American Chemical Society