Hz In Water Research Articles

NMR and molecular modeling study of the conformations of taxol and of its side chain methylester in aqueous and non-aqueous solution.

The conformations of the antimitotic agent taxol and its side chain methyl ester have been studied by NMR-spectroscopy and molecular modeling in hydrophobic (CDCl 3) and hydrophilic (water, d 6-DMSO) solvents. For the side chain methyl ester ( 4), the coupling constant J H2′-H3′ changes from ≈2 Hz in chloroform to ≈5 Hz in d 6-DMSO or water : d 6-DMSO, 1 : 1 (v/v). The conformational equilibrium for 4 thus shifts from one favoring conformers with a gauche <H-C2′-C3′-H> torsion angle (chloroform), to one predominantly of conformers having this torsion angle anti. In the case of taxol, J H2′-H3′ changes from 2.7 Hz in CDCl 3 to ≈8 Hz in water, water - sodium dodecyl sulfate (SDS) and/or d 6-DMSO. Again, gauche conformations are implicated in chloroform, but molecular modeling suggests the anti conformer 27 to be dominant in aqueous media and in d 6-DMSO. No nuclear Overhauser effects (nOe's) between the side chain and the taxane ring-system are observed in chloroform solution. In water and/or d 6-DMSO, however, nOe's between the side chain (Ph3′ and H2′) and the OAc4 methyl group are detected.

The design and performance analysis of barrel stave projectors

There is growing interest in low-frequency, light-weight sound sources for underwater acoustics research and for potential future sonar applications. A design being studied at DREA is the barrel stave projector, which consists of a piezoceramic cylinder driving a set of annular staves in flexure. An electroelastic finite element model has been used to analyze the linear vibrations and acoustic radiations of projectors having outer diameters of about 8 cm and fundamental resonance frequencies below 1600 Hz in water. Prototype projectors that operate at electroacoustic efficiencies greater than 50% at resonance have been assembled and their measured acoustic properties have been compared to those predicted by the model. In addition, a brief discussion of the model parameters and limitations is given.

Cross-peptide bond 13C 15N coupling constants by 13C and J cross-polarization 15N NMR

Comparative 13C 15N coupling constants are reported for the linear dipeptide tBoc- l-[U- 13C]Ala-[ 15N]GlyOMe and the corresponding cyclic diketopiperazine, both in dimethylsulfoxide (DMSO) and, upon removal of the tBoc group, in water solutions. Spectra were obtained by 13C NMR and by the first application of J cross-polarization (JCP) 15 NMR, which greatly reduces the time required to accumulate 15N NMR spectra. In DMSO there was evidence for the formation of coplexed species which were not present in water. The values obtained for the cross-peptide bond coupling constant 2J 13Cα− 15N were consistently less (by 2.2 Hz in DMSO, 4.3 Hz in water) for the cyclic than for the linear peptide, which may be related to the cross-peptide bond conformation. The 15N resonance for the cyclic peptide was shifted only 2 ppm downfield only 2 ppm downfield from the linear peptide chemical shift value in both solvents.

Low-frequency spherical-shell projectors

The spherical-shell projector comprises a piezoelectric ceramic ring driving two concave spherical shell segments in flexure. One model resonant at 600 Hz in water weighs 15 kg and delivers a souce level of 201.5 dB re 1 μPa at 1 m. Its mechanical Q is 5 and efficiency 80% at resonance. An axially-symmetric finite element model is being used to study the performance of spherical-shell projectors and generally good agreement is achieved between the calculated and experimental results.