Airborne minerals and related aerosol particles: Effects on climate and the environment

Aerosol particles are ubiquitous in the troposphere and exert an important influence on global climate and the environment. They affect climate through scattering, transmission, and absorption of radiation as well as by acting as nuclei for cloud formation. A significant fraction of the aerosol particle burden consists of minerals, and most of the remainder— whether natural or anthropogenic—consists of materials that can be studied by the same methods as are used for fine-grained minerals. Our emphasis is on the study and character of the individual particles. Sulfate particles are the main cooling agents among aerosols; we found that in the remote oceanic atmosphere a significant fraction is aggregated with soot, a material that can diminish the cooling effect of sulfate. Our results suggest oxidization of SO2 may have occurred on soot surfaces, implying that even in the remote marine troposphere soot provided nuclei for heterogeneous sulfate formation. Sea salt is the dominant aerosol species (by mass) above the oceans. In addition to being important light scatterers and contributors to cloud condensation nuclei, sea-salt particles also provide large surface areas for heterogeneous atmospheric reactions. Minerals comprise the dominant mass fraction of the atmospheric aerosol burden. As all geologists know, they are a highly heterogeneous mixture. However, among atmospheric scientists they are commonly treated as a fairly uniform group, and one whose interaction with radiation is widely assumed to be unpredictable. Given their abundances, large total surface areas, and reactivities, their role in influencing climate will require increased attention as climate models are refined.

Three quaternary structures for a single protein.

The structure of a multisubunit protein (immunoglobulin light chain) was solved in three crystal forms, differing only in the solvent of crystallization. The three structures were obtained at high ionic strength and low pH, high ionic strength and high pH, and low ionic strength and neutral pH. The three resulting "snapshots" of possible structures show that their variable-domain interactions differ, reflecting their stabilities under specific solvent conditions. In the three crystal forms, the variable domains had different rotational and translational relationships, whereas no alteration of the constant domains was found. The critical residues involved in the observed effect of the solvent are tryptophans and histidines located between the two variable domains in the dimeric structure. Tryptophan residues are commonly found in interfaces between proteins and their subunits, and histidines have been implicated in pH-dependent conformation changes. The quaternary structure observed for a multisubunit protein or protein complex in a crystal may be influenced by the interactions of the constituents within the molecule or complex and/or by crystal packing interactions. The comparison of buried surface areas and hydrogen bonds between the domains forming the molecule and between the molecules forming the crystals suggest that, for this system, the interactions within the molecule are most likely the determining factors.

Sequence-specific DNA-binding dominated by dehydration.

Fluorescence spectroscopy and isothermal titration calorimetry were used to study the thermodynamics of binding of the glucocorticoid receptor DNA-binding domain to four different, but similar, DNA-binding sites. The binding sites are two naturally occurring sites that differ in the composition of one base pair, i.e., an A-T to G-C mutation, and two sites containing chemical intermediates of these base pairs. The calorimetrically determined heat capacity change (Delta C(p)o(obs)) for glucocorticoid receptor DNA-binding domain binding agrees with that calculated for dehydration of solvent-accessible surface areas. A dominating effect of dehydration or solvent reorganization on the thermodynamics is also consistent with an observed linear relationship between observed enthalpy change (Delta Ho(obs)) and observed entropy change (Delta So(obs)) with a slope close to the experimental temperature. Comparisons with structural data allow us to rationalize individual differences between Delta Ho(obs) (and Delta So(obs)) for the four complexes. For instance, we find that the removal of a methyl group at the DNA-protein interface is enthalpically favorable but entropically unfavorable, which is consistent with a replacement by an ordered water molecule.

Active site topology and reaction mechanism of GTP cyclohydrolase I.

GTP cyclohydrolase I of Escherichia coli is a torus-shaped homodecamer with D5 symmetry and catalyzes a complex ring expansion reaction conducive to the formation of dihydroneopterin triphosphate from GTP. The x-ray structure of a complex of the enzyme with the substrate analog, dGTP, bound at the active site was determined at a resolution of 3 A. In the decamer, 10 equivalent active sites are present, each of which contains a 10-A deep pocket formed by surface areas of 3 adjacent subunits. The substrate forms a complex hydrogen bond network with the protein. Active site residues were modified by site-directed mutagenesis, and enzyme activities of the mutant proteins were measured. On this basis, a mechanism of the enzyme-catalyzed reaction is proposed. Cleavage of the imidazole ring is initiated by protonation of N7 by His-179 followed by the attack of water at C8 of the purine system. Cystine Cys-110 Cys-181 may be involved in this reaction step. Opening of the imidazole ring may be in concert with cleavage of the furanose ring to generate a Schiff's base from the glycoside. The gamma-phosphate of GTP may be involved in the subsequent Amadori rearrangement of the carbohydrate side chain by activating the hydroxyl group of Ser-135.

Mapping striate and extrastriate visual areas in human cerebral cortex.

Functional magnetic resonance imaging (fMRI) was used to identify and map the representation of the visual field in seven areas of human cerebral cortex and to identify at least two additional visually responsive regions. The cortical locations of neurons responding to stimulation along the vertical or horizontal visual field meridia were charted on three-dimensional models of the cortex and on unfolded maps of the cortical surface. These maps were used to identify the borders among areas that would be topographically homologous to areas V1, V2, V3, VP, and parts of V3A and V4 of the macaque monkey. Visually responsive areas homologous to the middle temporal/medial superior temporal area complex and unidentified parietal visual areas were also observed. The topography of the visual areas identified thus far is consistent with the organization in macaque monkeys. However, these and other findings suggest that human and simian cortical organization may begin to differ in extrastriate cortex at, or beyond, V3A and V4.