Oxygen reduction and evolution reactions (ORRs and OERs) have been studied in ionic liquids containing singly charged cations having a range of ionic radii, or charge densities. Specifically, ORR and OER mechanisms were studied using cyclic and rotating disk electrode voltammetry in the neat ionic liquids (ILs), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) and 1-methyl-1-butyl-pyrrolidinium bis-(triflouromethanesulfonyl)imide (PYR 14 TFSI), and in their solutions containing LiTFSI, NaPF 6 , KPF 6 , and tetrabutylammonium hexafluorophosphate (TBAPF 6 ). A strong correlation was found between the ORR products and the ionic charge density, including those of the ionic liquids. The observed trend is explained in terms of the Lewis acidity of the cation present in the electrolyte using an acidity scale created from 13 C NMR chemical shifts and spin−lattice relaxation (T 1 ) times of 13 CO in solutions of these charged ions in propylene carbonate (PC). The ionic liquids lie in a continuum of a cascading Lewis acidity scale with respect to the charge density of alkali metal, IL, and TBA cations with the result that the ORR products in ionic liquids and in organic electrolytes containing any conducting cations can be predicted on the basis of a general theory based on the hard soft acid base (HSAB) concept. ■ INTRODUCTIONLithium-ion (Li-ion) batteries have an undeniable influence over our daily lives. Vested in this battery technology are mobile electronics, load-leveling infrastructures, and electric propulsion vehicles. Despite their ubiquity, limits on energy density and the high cost of commercialized Li-ion batteries have accelerated efforts for alternative rechargeable battery systems, such as the nonaqueous Li−O 2 battery, first realized 15 years ago. 1 The 5280 Wh/kg theoretical energy density of Li−O 2 is 7−8 times that of today's best Li-ion battery, and it offers a long-term solution to energy independence.A primary concern facing this power source is the inefficient rechargeability of insoluble Li x O y discharge products that accumulate on the O 2 electrode. 2 This leads to poor cell performance, stemming from large cathode impedances and the associated voltage gaps between oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs). 3,4 Our recent work on the mechanism of ORRs in nonaqueous electrolytes has revealed that the properties of the organic solvent play a significant role in the nature of the final reduction product formed, and the stability of the intermediates through which the conversion of O 2 to Li 2 O occurs in the discharge of a Li− O 2 cell. We have gained this insight from a detailed study of the ORR intermediates and products in a series of organic electrolytes 5,6 judicially selected on the basis of their Lewis acid−base properties as defined by Guttmann donor and acceptor numbers. The Guttmann donor number (DN) measures the electron-donating capacity of the solvent to form complexes with Lewis acids such as Li + .Ionic liquids as a class of elect...

Nuclear magnetic resonance (NMR) studies have shown that two distinct folded conformations of staphylococcal nuclease coexist in solution and that these two states can interconvert directly without passing through an unfolded state. These experiments have also revealed that the two forms have very different folding kinetics, although the possibility that one component is an obligatory intermediate for the folding of the other form could be discounted. Here we report NMR data which show that alternative unfolded states are also distinguishable. These observations led us to hypothesize that cis/trans isomerism at a single peptide bond between a proline and its preceding residue might be the origin of the conformational multiplicity. Proline 117 was identified as a likely candidate for the site concerned and a mutant protein, in which Pro 117 was replaced by Gly, was constructed in order to test this. Alternative conformations are not observed in the spectrum of this mutant, lending powerful support to this hypothesis.

The equilibrium between alternative folded states of a globular protein, staphylococcal nuclease, has been investigated by using 1H NMR. Magnetization-transfer experiments have revealed the existence of a related structural heterogeneity of the unfolded state, and quantitative analysis of a series of these experiments has permitted the kinetics of folding and interconversion of the different states to be explored. A model based on cis/trans isomerism at the peptide bond preceding Pro-117 has been developed to account for the results. This model, recently supported by a protein-engineering experiment [Evans et al. (1987) Nature (London) 329, 266], has been used to interpret the kinetic data, providing insight into the nature of the folding processes. The predominance of the cis-proline form in the folded state is shown to derive from a large favorable enthalpy term resulting from more effective overall folding interactions. The kinetics of folding and isomerization are shown to occur on similar time scales, such that more than one pathway between two states may be significant. It has been possible, however, to compare the direct folding and unfolding rates within the cis- and trans-proline-containing populations, with results suggesting that the specific stabilization of the cis peptide bond is effective only at a late stage in the folding process.

Inteins are protein splicing elements that mediate their excision from precursor proteins and the joining of the flanking protein sequences (exteins). In this study, protein splicing was controlled by splitting precursor proteins within the Psp Pol-1 intein and expressing the resultant fragments in separate hosts. Reconstitution of an active intein was achieved by in vitro assembly of precursor fragments. Both splicing and intein endonuclease activity were restored. Complementary fragments from two of the three fragmentation positions tested were able to splice in vitro. Fragments resulting in redundant overlaps of intein sequences or containing affinity tags at the fragmentation sites were able to splice. Fragment pairs resulting in a gap in the intein sequence failed to splice or cleave. However, similar deletions in unfragmented precursors also failed to splice or cleave. Single splice junction cleavage was not observed with single fragments. In vitro splicing of intein fragments under native conditions was achieved using mini exteins. Trans-splicing allows differential modification of defined regions of a protein prior to extein ligation, generating partially labeled proteins for NMR analysis or enabling the study of the effects of any type of protein modification on a limited region of a protein.