Science Among all-aliphatic olefins, re sults are spotty to date. Vinylcyclohexane gives enantiomeric excesses of 46%, and fnms-3-hexene gives 20%. Electron-withdrawing groups seem to help. Ethyl β-cyclohexylacrylate gives 74% enantiomeric excess. And Sharpless says that enantio meric excesses are improved in acetonitrile-water solvent. Working at —10 to —20 °C raises enantiomeric excesses by about 20%. Also, medic inal chemists can upgrade optical purities of solid products by crys tallization techniques. Sharpless' further research in cludes seeking conditions and cata lysts that raise enantiomeric ex cesses. He also sees promise for acetals in drug intermediate production. Cinnamaldehyde dimethyl acetal gives 80% enantiomeric excesses, and the MIT group is developing a com mercial route to chiral glyceraldehyde acetonides from acrolein acetals. Besides the value of asymmetric dihydroxylation for its own sake, Sharpless cites it as an example of ligand-accelerated catalysis. The al kaloid derivatives not only induce asymmetry but also accelerate the reaction. This behavior contrasts with that of added quinuclidine (the parent nucleus of both alkaloid de rivatives) and other amines, which retard reaction rates in comparison with osmium alone. Stephen Stinson, New York
Gel electrophoresis in capillaries advances Chemists at Northeastern Universi ty have adapted polyacrylamide gel electrophoresis (PAGE) to a capil lary format. Gel electrophoresis al lows analytical and preparative sep arations of peptides and nucleotides by molecular weight. And electro phoresis in capillaries narrower than about 80 μτη gives rapid, reproduc ible, quantitatively reliable separa tions with potentially millions of theoretical plates (C&EN, March 7, page 28). Analytical chemistry professor Barry L. Karger of Northeastern de scribed possible applications of the 20
March 21, 1988 C&EN
technique at the Pittsburgh Confer ence on Analytical Chemistry & Ap plied Spectroscopy, held last month in New Orleans. Working with staff scientist Aha ron S. Cohen, visiting scholar Carlos Diez-Masa, and postdoctoral fellows Andras Guttman and Aran Paulus and supported by the National Sci ence Foundation and Dow Chemi cal Co., Karger filled capillary tubes with aqueous solutions of acrylamide monomer, Ν,Ν'-methylenebis(acrylamide) crosslinking agent, am monium persulfate initiator, and tetramethylethylenediamine acceler ator. Capillaries were first treated with 3-methacryloyloxypropyltrimethoxysilane to bind curing gels covalently to inner walls. The Northeastern team added so dium dodecyl sulfate (SDS) and zinc salts to solutions of samples and buffers for electrophoresis. Addition of SDS l e n g t h e n e d analyses of polydeoxythymidines to 35 minutes from 15 minutes, for example, but made it possible to separate oligomers two to 18 bases long completely. The Boston chemists demonstrated that protein separations were indeed size-dependent (and therefore mo lecular-weight-dependent) by meas uring mobilities in gels made from solutions with varying concentra tions of acrylamide. Increasing monomer concentrations yielded gels of increasing density. Plotting l o g a r i t h m s of mobilities versus monomer concentrations and extrap olating to 0% monomer, they found that a-lactalbumin, β-lactoglobulin, trypsinogen, and pepsin all had the same mobility at 0% monomer. Moreover, plots of logarithms of mobilities versus logarithms of mo lecular weights were linear. This means that the method can deter mine molecular weights reliably. Karger's group also used capil lary PAGE to detect so-called twochain methionine-human growth hormone (Met-hGH) at levels of 0.1% within 10 minutes in prepara tions of Met-hGH. In some such preparations, a protease can cleave the peptide chain in two. The two chains remain connected by disul fide bridges. Conventional PAGE on slabs with Coomassie Blue staining cannot detect the two-chain impu rity at levels less than 5%.
Inclusion of cyclodextrin in gelforming solutions allowed separa tions of mixtures of racemic, dansylated (5-dimethylamino-l-naphthalenesulfonated) leucine, serine, and glutamic acid. Dansyl groups fluo resce for enhanced detection. Other separations achieved in cyclodextrin gels include those of penicillins, barbiturates, salicylic acid derivatives, peptides, prosta glandins, and parabens. In a preparative separation, the Northeastern workers injected from 100 to 800 ng of a crude oligonu cleotide 20 bases long into capillar ies. The oligonucleotide was part of the gene for staphylococcus nucle ase, which another group at Massa chusetts General Hospital had al tered by one amino acid residue. These researchers needed purified oligonucleotide for hybridization with complementary oligonucleo tide to verify the change. Karger's group collected fractions in microcentrifuge tubes contain ing a few microliters of water and a platinum electrode to complete the electrophoretic circuit. Each lower ing of a tube to change collectors broke the circuit and electrophore sis stopped. The gel prevented diffusion of sample molecules during these waits. Thus peak shapes were not degrad ed. The Northeastern team verified this by reversing the electrophoret ic field, causing migration of sam ple back through the ultraviolet ab sorption detector, where the re searchers could monitor peak shapes. After a preliminary analysis to visualize the entire electropherogram, they collected the major frac tion preparatively within 20 min utes. Reinjection of this fraction gave a single peak. The Boston scientists developed an injection method to measure oligonucleotide concentra tion so that the hospital workers would know how much to use in hybridization experiments. They fitted a narrow, polytetrafluoroethylene sleeve over the in jection end of the electrophoresis capillary so that a known length of it extended beyond the end. They could thus inject a known volume of liquid between the sleeve and the capillary end. Stephen Stinson, New York
