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major component of the virus's protein coat. Higher resolution was necessary to characterize other mutations. MALDI FTMS was used to identify mutations that produced structural peptides differing by 1 or 2 Da. Foo example, a -1 Da change in a mutant TMV is shown to be due to a Glu->Gln mutation. MS2 Comparison of MALDI-TOF mass spectra for tryptic digests of experiments yielded patwild-type TMV and mutant TMV-Asp50Arg. (Adapted with terns consistent with the permission. Copyright 1998 National Academy of Sciences.) containing the Glu-»Gln mutation Mass spectral database gram amounts of high-purity material and searching was also used and the highits inability to identify mutations that don t resolution MALDI FTMS spectra were more change the mass of the peptide fragment. successful at yielding matches than the Nevertheless, the authors say that this MALDI-TOF spectra method complements DNA sequencing for The mass mapping approach is hampered by the need for nanogram to micro-
identifying viral mutants. (Proc. Ndtl. Acad. Sci. USA 1998,95,8596-601)
Nanotube probes
self-assembled monolayer (SAM) substrates did not depend on pH. In contrast, ,nteractions weee pH-dependent for nanotubes capped witii ethylenediamine. Finally, when die tips were used to make tapping-mode images of patterned SAM substrates, benzylamine-modified nanotubes produced a phase lag that corresponded to a difference in adhesion force. The tips were also used to ssudy receptorligand interactions. Lieber and colleagues demonstrated this application by attaching biotin to a tip's end and streptavidin to a plate. Most of the time, they recorded force displacement measurements of 200 pN, indicating binding of a single biotin-streptavidin pair. Sometimes, however, a force twice as high would be measured, suggesting simultaneous binding of two pairs. In addition, die researchers suggest that die functionalized nanotubes will be good for imaging selfassembled polymeric and biological materials. Using single-walled nanotubes rather dian multiwalled ones should increase the resolution, the scientists say. Finally, they speculate that use of such probes could lead to the creation of nanometer-scale interconnections for electronic devices and to the assembly Diagram illustrating the modification of a nanotube of new classes of nanotube materitip with an amine. (Adapted with permission. als. (Nature 1 9 9 8 394 52-55) Copyright 1998 Macmillan Magazines.)
Nanotubes were first used as tips for scanning probe microscopy a few years ago. Now, Charles M. Lieber and colleagues at Harvard University take the carbon creations one step further. The researchers modified nanotube ends to make atomic force microscopy probes with the ability to detect specific biological and chemical molecules. Using selective functionalization—that is, attaching amines to die carboxyl groups at the ends of multiwalled nanotubes—die researchers created specialized microscopy tips. Attaching benzylamine yielded a tip whose interactions with hydroxyl-terminated
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Analytical Chemistry News & Features, September 1, 1998
MALDI for DNA analysis Ion fragmentation has been a problem with MALDI MS analysis of large nucleic acids, but Franz Hillenkamp and colleagues at Universitat Miinster (Germany) and Odense University (Denmark) have found a solution. Using an IR laser and a liquid glycerol matrix in a single-stage reflectron, time-offlight instrument, the researchers were able to accurately determine the mass of a 2180-nucelotide (nt) DNA fragment (-700 kDa) and a 1206-nt RNA transcript. In the past, the largest DNA sample that could be analyzed was —90 kDa (—300 ntt and the largest RNA sample -150 kDa After testing both IR and UV lasers in combination with various matrixes, Hillenkamp and his colleagues found that an EnYAG laser (wavelength 2.94 um) with a liquid glycerol matrix was the "gentlest" approach, leaving nucleic acids of more than 2000 nt intact. Another advantage of the glycerol matrix was its reproducibility and mass precision (200-400 ppm). Analysis of 515-bp polymerase chain reaction products showed that the combination of IR laser and glycerol matrix provided high sensitivity even at the low S/N of —20 The researchers saw an improvement of 3 orders of magnitude over previous IR-MALDI experiments The accuracy of the technique ranged from 0.3% for a 21-nt piece of DNA to 1% or better for high-mass samples. For RNA 0.5% accuracy was obtained for a 1206-nt transcript. In contrast, agarose-gel electrophoresis, the most common way to determine the size of nucleic acids, has an accuracy of only 5-10% for DNA and is not reliable for large pieces of RNA. Now, wiih the high accuracy of the new IR-laser/glycerol matrix technique, MALDI-MS should be useful for applications such as sizing RNA transcripts and determining the number of repeats in micro satellite DNA. (Science 1998 281 260-62)
