News
Seasonal variation of solar radiation, sulfates, MSA, and nitrates determined by LAMMS.
STM describes outer Helmholtz layer at Pt electrode In situ scanning tunneling microscopy (STM) has been used extensively to study the structure of electrode surfaces in electrolyte solution. Although STM has been used to elucidate the structure of the inner Helmholtz layer at Pt electrodes, chemical species such
Sampling took place from February to December 1991 at two sites at the Syowa station, and particles found on the third stage (1.6- 5.4 um) of a multistage high-volume air sampler were analyzed. The behaviors of sulfates, MSA, and nitrates in the Antarctic atmosphere correlated with the seasonal change in solar radiation (except in March, April, and August), which indicates that photochemical reactions are responsible for generating the aerosol particles The researchers postulate that the presence of metallic elements including V Mn Zr Cr and Pb may be caused by the transport of soil particles into the reeion (Fnviron Sci Terhnol 1996 on Qg5_gn
as solvated cations and anions might be expected to move too rapidly to be observed by in situ STM. Kingo Itaya and colleagues of Tohuku University and the Research Institute of Electric and Magnetic Materials (Japan) have used STM for what they believe is the first in situ description of the outer Helmholtz layer. They examined a well-defined Pt(lll) electrode in acidic and alkaline solutions containing NaCN or KCN and found a CN adlayer of 6-membered rings in the double-layer region. STM images revealed a well-ordered array of cations adsorbed to the CN adlayer when K+ or Na* was introduced to the system. The cations appeared as bright spots in the STM images, and the corrugation height found to be potential dependent. On the basis of these results the researchers proposed a new model for the CN adlayer in which a hollow hexagon structure allowed the coordination of cations in the center of the hexaeon (7 Am Chem %oc 1996 111 found on
High-resolution STM image of the structure Pt(111) in a solution containing 0.1 mM KCN at 0.7 V.
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Analytical Chemistry News & Features, April 1, 1996
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Stretching DNA How double-stranded DNA (dsDNA) packs into phage heads, bends on interaction with proteins, and loops to connect enhancer and promoter regions is a function of how it bends and twists. An easy way to explore DNA elasticity is to stretch a single macromoleculefromboth ends and measure the force as a function of its end-to-end distance extension. Carlos Bustamante and colleagues at the University of Oregon used force-measuring laser tweezers to observe the behavior of single molecules of dsDNA They used a dual-beam laser trap instrument with low numerical aperture lenses, to form an optical trap. Each end of a single A,,phage dsDNA molecule was attached to a separate microscopic latex bead. One bead was held by suction with a glass micropipette and the other was held by the optical trap, which also functioned as a force transducer. The DNA molecule was extended by moving the pipette relative to the laser trap, and the force acting on the DNA was inferred from the displacement of the laser beams on position-sensitive photodetectors. At a force of ~ 65 pN, ,he molecule assumed an extended form ~ 1.7x its B-form contour length. When the force fell below 65 pN, the DNA contracted to its normal contour length. The force rose rapidly again at extensions > 28 um when the molecule was entirely converted to the overstretched form. The narrow range of forces over which the overstretching transition occurs indicates high cooperativity, which implies that it is energetically easier to spread an existing region than to nucleate a new one. (Sciencce196,271, 795-98)
Possible conformations of stretched DNA. (Adapted with permission of the American Association for the Advancement of Science.)
