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Thin films in action changed the extent to which the polarizaother hand, were much less sensitive to To study the local properties of mesostruc- tion of the transmitted light was altered. such effects. The signal varied with time and modulatured thin films, Erwen Mei and Daniel A. Using the phase data, the researchers tion frequency and was detected with Higgins at Kansas State University develwere able to determine that the reorientation cross-polarized, transmitted-light nearoped a new form of near-field scanning opdynamics were faster near the polymer-LC tical microscopy (NSOM). Previously used field opttcal methods. interface than at the center of the dropto look at the static orgalets—a finding that connization of materials, tradicts studies conducted NSOM was adapted to with other methods. characterize the dynamBased on the new inforics of molecular reorienmation, the researchers tation processes, as well. developed a model in Mei and Higgins apwhich molecules near the plied a sinusoidally modpolymer-LC interface ulated voltage between cannot align strictly paralthe NSOM probe and a lel to their neighbors and conductive optically rapidly relax once the Topographic image (A) of a toroidal droplet. Amplitude (B) and phase (C) images transparent substrate electricfieldhas been acquired by recording the dynamic optical signal using NSOM. that supported the samremoved. To rule out other possible explanations, the scientists ple a polymer-dispersed liquid crystal Dynamic amplitude and phase images are investigating die source of the image (PDLC) film A concentrated electric field were taken. The light and dark regions of contrast. They plan additional work to dethe amplitude images showed how the loformed beneath the probe and the local termine the time scale for reorientation cal directors were aligned with respect to molecules reoriented themselves In most the incident and detected polarization axes. using local time- or frequency-dependent the molecules switched from an responses In the meantime they expect However, the data were qualitative, not alignment apDroximately parallel to the that the current method will work on some film surface to an alicnment normal lo the quantitative, because the amplitude images sample volumes as small as 10"15 cm3 surface Because the I C droplets are hire depended on local LC orientation and on (J Phvs Chem A 1998 102 7558-63) sample thickness. The phase data, on the frinp-ent chantring the path of the lip-ht H/D e x c h a n q e
controls. Their goal is to speed up gasphase H/D reactions so that they can be rapidly quantitated for a wide range of DNA and RNA mono- and polynucleotides. The ordering of the relative exchange rates for the nucleotides differs for D20 and D2S. The reason for the change is unclear but may indicate that D2S exchanges with the nucleotide anions via a mechanism that differs from D20. All exchange rates increase dramatically on changing from D20 to D2S because of the smaller gas-phase acidity difference between FT-ICRMS for GMP after a the exchange reatrent and reaction period of 360 s with the nucleotide (top) D20 and (bottom) D2S.
Hydrogen/deuterium (H/D) exchange is a powerful tool for examining the solution phase structure of proteins and peptides. Although most H/D exchange studies are done in the solution phase, gas-phase analysis eliminates solvation effects. Alan G. Marshall and colleagues at Florida State University use FTion cyclotron resonance to demonstrate the effect of gas-phase acidity on
the relative exchange rate of D 0 and D2 S with eight monophosphate nucleotides and two sugar phosphate
These results provide a fundamental starting point for elucidating the mechanism of H/D exchange in larger systems. The demonstrated increase in H/D exchange rate with D2S relative to D20 shortens, from hours to minutes, the time required to measure such rates. The results are more accurate, because fewer ions are lost from the ion trap during the measurement, and extend to less acidic analyte hydrogens, because slower exchange rates become accessible. The extension to even more acidic DC1, DBr, and DI as negativeion exchange reagents is being investigated. Analysis of di- and triphosphate nucleotides should reveal the effect of multiple negative charges on gas-phase nucleotide conformation. In addition H/D exchange with single-stranded and duplex RNAs and DNAs should now be possible. (J. Am. Chem. Soc. 1998 120,10187-93)
Analytical Chemistry News & Features, December 1, 1998 767 A
