26 research outputs found
Mediterranean climate variability during the Holocene
We present a study on four high sedimentation-rate marine cores with suppressed bioturbation effects, recovered along the northern margin of the eastern Mediterranean. We demonstrate that this region, central to the development of modern civilisation, was substantially affected throughout the Holocene by a distinct cycle of cooling events on the order of 2o C. In the best-preserved cases the onset of these events appears particularly abrupt, within less than a century. The cooling events typically lasted several centuries, and there are compelling indications that they were associated with increased aridity in the Levantine/NE African sector (Rossignol-Strick, 1995; 1998; Alley et al., 1997; Hassan, 1986; 1996; 1997a,b; McKim Malville et al., 1998). Several of these episodes appear coincident with cultural reorganisations, with indigenous developments (eg. cattle domestication, new technologies) and population migrations and fusion of peoples and ideas (Hassan, 1986; 1996; 1997a,b; McKim Malville, 1998). We infer that climatic events of a likely high-latitude origin (O’Brien et al., 1995; Bond et al., 1997; Mayewski et al., 1997; Alley et al., 1997) caused cooling and aridity in and around the eastern Mediterranean via a direct atmospheric link, and therefore played an important role in the development of modern civilisation
Structure of the Fundamental Lipopeptide Surfactin at the Air/Water Interface Investigated by Sum Frequency Generation Spectroscopy
The
lipopeptide surfactin produced by certain strains of <i>Bacillus
subtilis</i> is a powerful biosurfactant possessing
potentially useful antimicrobial properties. In order to better understand
its surface behavior, we have used surface sensitive sum frequency
generation (SFG) vibrational spectroscopy in the Cî—¸H and Cî—»O
stretching regions to determine its structure at the air/water interface.
Using surfactin with the leucine groups of the peptide ring perdeuterated,
we have shown that a majority of the SFG signals arise from the 4
leucine residues. We find that surfactin forms a robust film, and
that its structure is not affected by the number density at the interface
or by pH variation of the subphase. The spectra show that the ring
of the molecule lies in the plane of the surface rather than perpendicular
to it, with the tail lying above this, also in the plane of the interface
Crystallization and Crystallinity of Fluticasone Propionate
Solubilization of fluticasone propionate (FP) was effected using aqueous solutions of (i) different grades of poly(ethylene glycol) (PEG), (ii) methanol, and (iii) acetone to enable antisolvent crystallization by the addition of water. The solubility of FP in acetone was significantly higher than in PEG 400 or PEG 6000, and FP solubility was observed to be nonideal in either cosolvent. Crystallization of FP was instantaneous upon addition of water as antisolvent, with nucleation occurring during the mixing phase. The smallest crystals were produced in all cases from PEG solvents, which was attributed to a greater degree of nucleation and microcrystals of a size-range were produced. Crystals produced from PEG solvents also displayed a resistance to agglomeration and Ostwald ripening, which was observed to affect the morphology of FP crystallized from either methanol or acetone by the addition of water. In spite of the very rapid kinetics of solid formation, FP crystallized as the stable Form I polymorph from PEG 400 and PEG 6000. Conversely, mechanical milling of highly crystalline particles resulted in the generation of disorder in the crystals, which was apparent from surface dynamic vapor sorption analysis. The generation of FP microcrystals with a small size range from environmentally benign PEG solvents as the stable crystalline form represents an improvement over current crystallization and micronization techniques for the production of inhalable FP.Peer reviewe