News
Parents do some cleaning up
Parent ion scans of immonium, oxonium, and phospho group ions (all of which are in the low m/z range) can be The usefulness of parent ion scans for used to selectively identify molecules analyzing small molecules and peptides from a complex mixture. For example, has already been demonstrated. Now, Matthias Mann and Gitte Neubauer at the through a parent ion scan of the oxonium ion of HexNac (m/z 204) of a mixture of European Molecular Biology Laboratory RNAse B (a glycoprotein) and carbonic in Heidelberg, Germany, have extended anhydrase (a nonglycosylated protein), the applicability of these scans to large the RNAse B was selectively identified. molecules such as proteins and oligonucleotides. These parent ion scans allow Similarly, a parent ion scan of the phospho group (m/z 79) allowed the detection the selective detection of molecules that might otherwise be lost in chemical noise. of a phosphorylated protein (B-casein) in the presence of 3. nonphosphorylated protein (carbonic anhydrase) Mann and Neubauer also determined the applicability of parent ion scans for phospho groups in the analysis of oligonucleotides. When a parent ion scan was done with a synthetic oligonucleotide that still had contaminants from the synthesis process, the chemical noise from the salt adducts was greatly reduced and Mass spectrum of a 1:1 carbonic anhydrase and RNAse B the mass spectrum mixture with the major glycoform of RNAse B marked (A). The much simplified (/. parent ion scan of the oxonium ion of HexNac (m/z 204) Mass Spectrom 1999 selectively detects only RNAse B (B). (Adapted with 32 94-98) permission from John Wiley & Sons.)
Making DNA more like a protein
molecule that is chemically more like a peptide and that, in turn, improves the detectability of small modified oligonucleotides by MALDI by a factor of > 110 over unmodified DNA The authors also evaluated different matrices for laser desorption of the chemically modified DNA (Rapid Commun. Mass spectrom. 1997,11,43-50)
MALDI has emerged as a major technique for peptide analysis, but the same technique works poorly for studying DNA Key obstacles include the instability of DNA under the acidic conditions of sample preparation and desorption, as well as the large number of negative charges on the sugar phosphate backbone. Ivo G. Gut and colleagues at the Imperial Cancer Research Fund (United Kingdom) tackled this problem by chemically modifying DNA They introduced a new procedure that quantitatively attaches quaternary ammonium fixed charge tags to the 5' or 3' NH2 ends of DNA In addition, they used a previously reported technique to alkylate the backbone of phosphoroTwo-step modification of DNA. (Adapted with thioate DNA The result is a permission from John Wiley & Sons.) 226 A
Analytical Chemistry News & Features, April 1, 1997
Detecting tuberculosis Infection by Mycobacterium tuberculosis (MTB), the agent responsible for the respiratory disease, remains a major health care problem. Clinical tests for MTB are typically culture dependent and, because of the slow growth of the organism, time consuming. A faster, precise test would aid in patient treatment and in protection of health care workers. Joseph Wang and colleagues at New Mexico State University introduced a new sequence-specific electrochemical biosensor for short fragments of MTB DNA The sensor is a carbon-paste transducer modified with 27- and 36-mer oligonucleotide probes that hybridize to complementary strands from the MTB DNA direct repeat region. These sequences are known to be highly specific to MTB complex organisms. Chronopotentiometry, in connection with a Co(phen)|+ indicator, is used to transduce the hybridization event. Hybridization takes place in 5 to 15 min, and the MTB DNA is quantified at ng/mL levels. (Anal. Chim. Acta 1997, 337, 7,-48)
Chronopotentiograms for increasing concentrations of27-mer (top) and 36-mer (bottom) MTB DNA. (Adapted with permission from Elsevier Science.)
Novel laser sampling Laser ablation is an attractive method for generating samples for ICP techniques. Unfortunately, the method suffers from a host of problems such as preferential vaporization, laser-induced plasma shielding of the laser beam, and variations in the amount and stoichiometry of material ablated from the surface. W. T. Chan and Kenneth K K. Lam from the University of Hong Kong introduced a new approach, back-surface ablation, that circumvents some of these problems.
