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A J Weinheimer

Publications and source records attributed to A J Weinheimer.

14 recordsLinked to original sources
PubMed2004

Quantifying stratospheric ozone in the upper troposphere with in situ measurements of HCl.

We have developed a chemical ionization mass spectrometry technique for precise in situ measurements of hydrochloric acid (HCl) from a high-altitude aircraft. In measurements at subtropical latitudes, minimum HCl values found in the upper troposphere (UT) were often near or below the detection limit of the measurements (0.005 parts per billion by volume), indicating that background HCl values are much lower than a global mean estimate. However, significant abundances of HCl were observed in many UT air parcels, as a result of stratosphere-to-troposphere transport events. We developed a method for diagnosing the amount of stratospheric ozone in these UT parcels using the compact linear correlation of HCl with ozone found throughout the lower stratosphere (LS). Expanded use of this method will lead to improved quantification of cross-tropopause transport events and validation of global chemical transport models.

Journal Article↗
PubMed2004

Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds.

In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (Delta-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Delta-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Delta-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.

Journal Article↗
PubMed1988

Crystal and molecular structure of didemnin B, an antiviral and cytotoxic depsipeptide.

Didemnin B, a highly active depsipeptide isolated from a Caribbean tunicate, crystallizes from chloroform/benzene in the orthorhombic space group C2221, with cell parameters a = 14.990 +/- 0.003 A, b = 22.574 +/- 0.004 A, c = 41.112 +/- 0.009 A, V = 13911.7 A3 at 138 K and a calculated density of 1.143 g/cm3 based on C57H89N7O15, 1.5C6H6.H2O and eight formula units per cell. The overall agreement factor R = 0.052 for 7699 reflections, 20 theta max = 150 degrees, Cu K-alpha radiation. The structure determination revealed that didemnin B contains an isostatine residue instead of a statine residue. The conformation of the 23-membered depsipeptide ring is stabilized by one transannular hydrogen bond. The ring does not show the antiparallel beta-pleated-sheet structure but, instead, has a fold in the shape of a bent figure-eight. The linear peptide moiety, containing N-methylleucine and lactylproline, forms a beta (II)-bend and is folded back toward the cyclic backbone, giving the overall molecule a globular character. Comparison with the structure of cyclosporin A shows distinct stereochemical differences between the two molecules. It is suggested that didemnin B and cyclosporin A are unlikely to have a common receptor binding site.

Antiviral Agents↗
PubMed1983

Assignment of the 13C-nmr spectrum of Hippurin, a novel furospirostane: observation of both ends of the axis of anisotropic reorientation through the measurement of spin-lattice relaxation times.

Anisotropic reorientation of 3 alpha-substituted steroids has been well documented. Assignment of the 13C-nmr spectrum of Hippurin, a novel furospirostane and the measurement of the spin-lattice (T1) relaxation times has established both points in the molecule through which the axis of anisotropic reorientation passes, specifically the 3- and 24-positions.

Kinetics↗
PubMedDate not supplied

Isolation of the cembranolide diterpenes dihydrosinularin and 11-epi-sinulariolide from the marine mollusk Planaxis sulcatus.

Marine mollusks have been identified as a rich source of novel compounds. The diversity of the chemical nature of these compounds can be partially attributed to the dietary sources inasmuch as mollusks are known to concentrate selectively compounds present in their diet. Although cembranolide diterpenses have been isolated in ever-growing numbers from a variety of marine as well as terrestrial sources, there is only one report about their presence in a marine mollusk. In this communication, we report the isolation of dihydrosinularin and 11-epi-sinulariolide from the mollusk Planaxis sulcatus Born. Dihydrosinularin and 11-epi-sinulariolide acetate have been isolated previously from soft corals which, together with gorgonians, are the richest sources of marine cembranoids. It should be pointed out that this is the first report on the occurrence of the parent 11-epi-sinulariolide in a marine invertebrate. It is expected these diterpenes are of dietary origin although the possibility of biotransformation of a precusor by the mollusk cannot be ruled out at this time. Both dihydrosinularin and 11-epi-sinulariolide have been reported to have marginal antineoplastic activity against P-388 lymphocytic leukemia.

Antineoplastic Agents↗
PubMedDate not supplied

Crystal and molecular structure of didemnin A, an antiviral depsipeptide.

The molecular structure of didemnin A, the parent compound of a series of antiviral cytotoxic depsipeptides extracted from a marine tunicate Trididemnum solidum of the family of Didemnidae, has been determined by single-crystal X-ray diffraction. In the crystal, didemnin A molecules form pseudo-symmetric dimeric pair. The two molecules in the dimer are held together by strong N--H center dot center dot center dot O and N--H center dot center dot center dot N hydrogen bonds. A chloride ion, placed almost symmetrically between the dimeric pair, forms N--H center dot center dot center dot Cl hydrogen bonds (3.19 and 3.23 Angstrom) with both the molecules. The two independent molecules in the structure have closely similar geometry. For each molecule, the 23-membered depsipeptide ring assumes a folded conformation in the shape of a 'bent figure-of-eight' similar to that observed in the didemnin B crystal structure. The major conformational differences in the macrocycle of didemnin A and didemnin B are around the Hip residue. The root mean-square (RMS) difference of 20 of the 23 endocyclic torsion angles for the two structures is less than 10 degrees, while the three bond torsions in the Hip residue vary by about 50 degrees. The macrocycle conformation is stabilized by a transannular N--H center dot center dot center dot O hydrogen bond linking the isostatine amide group with the leucine carbonyl group. The truncated linear chain is folded back toward the macrocyclic ring and is held by a N--H center dot center dot center dot O hydrogen bond between the leucine amide group and Me-Leu carbonyl group. The transannular hydrogen bond in the didemnin A structure (N4--H center dot center dot center dot O3 = 2.83 Angstrom in both molecule a and molecule b) is noticeably stronger than that observed in the didemnin B structure (3.02 Angstrom). The X-ray structure of didemnin A is generally consistent with that obtained by NMR studies. Within the crystal, the molecules are packed in zig-zag chains formed by intermolecular O--H center dot center dot center dot O hydrogen bonds. The crystal structure and packing of didemnin A are quite different from that of the didemnin B structure.

Antiviral Agents↗