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Sequencing complex sugars

Because of the heterogeneity and complexity of polysaccha-rides, the sequencing of sugars has lagged behind the sequenc-ing of polynucleotides and proteins. To address this problem,Ram Sasisekharan and colleagues at the Massachusetts Ins-titute of Technology developed a notational system forrepresenting sequences and combined it with analysis bymatrix-assisted laser desorption/ionization (MALDI) MS.The re-searchers applied this method to heparinlike gly-cosaminoglycans (HLGAGs)—sugars that had been con-sidered useless extracellular material but now are believedto have a role in processing signals from outside the cell.The notational system provides a compact way to represent

the more than 1 million possible sequences that can be formedfrom the 32 HLGAG building blocks. Because there are 4 siteswhere chemical modifications can occur (1 = present, 0 =absent), the system is hexadecimal (42 = 16). A positive or neg-ative sign is added to indicate the isomer of the uronic acid (Ior G, respectively). The code can be expanded to accommodateother modifications by moving to a higher numerical base.Using this notation, the researchers generated a list of

the theoretical molecular masses of all possible saccharidesequences. Then they fragmented the polysaccharides, usingMALDI MS to determine the masses of the oligos with anaccuracy of <1 Da. Computer analysis that combined thetwo types of information generated a set of possible identi-ties for each sample. Successive iterations of this process, inwhich different techniques were used to generate the frag-ments, narrowed the possibilities. The researchers noted thatthis method uses only 1 pmol of material and is fast enoughto allow complete sequencing of a sugar in only one day.(Science, 1999, 286, 537–542)

Hexadecimal notationalsystem for HLGAGs.I-containing units areshown. G-containing unitsare the same, except theyare preceded by a minussign. (Adapted withpermission. Copyright1999 American Associationfor the Advancement ofScience.)

Singling out airborne particlesThe size and composition of atmospheric particles change over time as a result of interactions with air pollutants. Because this interaction

occurs at the single-particle level, knowledge of single-particle composition is needed to better understand such processes. The majority of air-

borne particles, however, have been characterized from bulk chemical composition data taken from filters containing a mixture of particles. With

the advent of aerosol time-of-flight mass spectrometry (ATOFMS), it is now possible to meas-

ure the size and mass spectrum of single particles.

Among the first to delve into the world of single, airborne particles are Glen R. Cass and co-

workers at the California Institute of Technology and the University of California–Riverside. At

three air monitoring stations in southern California, they have collected particle size and com-

position data at the single-particle level continuously over a two-week period using ATOFMS.

More conventional reference method instruments, including electrical aerosol analyzers and

laser optical particle counters, were also used to measure particle-size distributions.

The data create profiles of the sodium-, ammonium-, nitrate-, and carbon-containing parti-

cles, showing differences in particle composition as a function of location. The results are compared with data obtained during experiments per-

formed in 1987 and are consistent with reductions in diesel soot and elemental carbon emissions. Ultimately, the authors say, the data will be used

to evaluate air quality models that predict the size distribution and chemical composition of individual particles in the atmosphere. (Environ. Sci.

Tech. 1999, 33, 3506–3515)