News A hydrogel never forgets Hydrogels that respond to pH, temperature, electricfields,or saccharides already exist. Now, Takashi Miyata and colleagues at Kansai University Qapan) have created one that responds to a specific antigen and "remembers" its own initial structure, returning to it after the antigen disappears. The hydrogel also allows permeation of a model drug, suggesting that it could be used for drug delivery. The researchers began by creating a semi-interpenetrating network (semi-IPN) hydrogel with a modified version of the antigen rabbit immunoglobin (IgG) and a modified version of the goat anti-rabbit IgG antibody. The hydrogel is initially compact because the antigen and antibody bind, providing cross-linking. But in the presence of free antigen—which is unmodified and, thus, binds more strongly to the antibody—the cross-linking is undone, and the gel swells. The effect is specific: The researchers found no change in the hydrogel if free goat IgG was added instead of free rabbit IgG. But swelling in response to the stimulus is just the beginning. Miyata and co-workers found that a swollen hydrogel will shrink gradually if the free antigen is absent. Furthermore, the hydrogel recreates its
To catch a plasmid When it comes to purifying plasmid DNA, smaller is better, at teast tor chromatography. That's the conclusion of Anders Ljunglof and colleagues at Amersham Pharmacia Biotech (Uppsala, Sweden), who used confocal microscopy to visualize DNA bound to the adsorbent particles used in chromatography. The researchers incubated adsorbent particles bearing (CTT)7 oligonucleotide probes with double-stranded plasmid DNA (6.3 kilobase pairs), allowing triple helixes to form. Unbound
initial semi-IPN structure, indicating a "memorylike" ability. Finally, when a model drug was added along with rabbit IgG, it permeated through the membrane. Without rabbit IgG, there was no permeation, suggesting that such a hydrogel might be used to deliver a drug in response to a specific antigen. {Nature 1999 399 766-68)
Surface secrets revealed
The suggested mechanism for the ewelling of an antigen-antibody semi-interpenetrating hydrogel in response to a free antigen. (Adapted with permission. Copyright 1999 Macmillan Magazines.)
ther —25° or -46°; ;he diiference arising ffom a 180° rotation of the -COOH group. Because the acid molecules adsorb to the graphite surIf such "far out" concepts as molecular face with the carboxyl groups nearby for hydevices and electronics are going to bedrogen bonding, the different-OH conformcome practical, then researchers will ers affect how adjacent molecules arrange have to be able to determine the actual themselves. Thus, the 25° conformer leads to conformation of molecules on surfaces. a nearest neighbor on the graphite surface George W. Flynn and colleagues at Columbia University demonstrate this capa- shifted up by a half of a molecular width (see figure). The two 46° conformers adopt a simibility by imaging long chain carboxylic lar arrangement, but the second molecule is acid molecules on a graphite surface usshifted up by an enttre molecular wiith. ing scanning tunneling microscopy (STM). Of particular interest in this study STM images showed the 12-bromois that the -COOH group adopts two con- dodecanoic acid adsorbed on the graphite formations, which leads to different pack- lattice surface as the 25° conformer, ing of the molecules on the surface. whereas the 2-bromohexadecanoic acid had an angle of 53°. The former conformer 12-Bromododecanoic acid and 2-bromohexadecanoic acid, the molecules leads to an arrangement of molecules in a being studied, were adsorbed onto a Schematic representation nf two n-carboxylic graphite surface from phenyloctane soluacids adsorbed on a graphite surface, (a(a is tion. Theoretical considerations indicate calculated as 24.75°° and the edsorbed that the angle between the -OH in the molecules are shifted by one-hall molecular carboxyl end group and the molecular axis width; (b) p is 45.75', and the molecules sre through the long carbon chain can be eishifted by a full width. 586 A
Analytical Chemistry News & Features, September 1, 1999
plasmids were washed away, and then the dye YOYO-1—which forms a fluorescent complex when bound to doublestranded DNA but not single-stranded DNA—was added to distinguish particles with plasmids from those without. The particles were viewed with confocal microscopy, which focuses laser light on one spot at a time, thus limiting background fluorescence. Optically thin sections of the sample were acquired and translated into fluorescence intensity profiles. Based on these intensity profiles,
close-packed structure with no voids, like pieces in a puzzle, whereas the latter conformer creates a pattern with energetically unfavorable voids. (J. Phys. Chem. B 1999,103, 5711-15)
