Die Molekül‐ und Kristallstrukturen der Titelverbindungen (I) (RG P21/c, Z=4), (II) (RG C2/c, Z=8), (III) (X: N0; RG P21/n, Z=4) und (III) (X: C102; RG P21/c, Z=4) werden mittels Röntgenanalysen bestimmt und diskutiert.

Ultrasonic treatment (0.5-2 min) stimulated multiple shoot regeneration to high levels in vitro from recalcitrant cotyledon explants of commercial squash (Cucurbita pepo L.) cultivars Ma'yan and Bareqet, on Murashige and Skoog [Physiol Plant 15:473-497, 1962] (regeneration) medium augmented with 4.4 microM benzyladenine. At this stage, unsonicated control explants regenerated only a few very small shoots or bud-like structures. Ultrasound also stimulated massive explant growth. Ultrasound treatment resulted in further multiple shoot production (five times greater than control) after explant transfer to elongation medium (Murashige and Skoog [Physiol Plant 15:473-497, 1962] medium with 0.44 microM benzyladenine and 2.9 microM gibberellic acid). Longer ultrasonic treatments (5 or 10 min) promoted multiple shoot regeneration and explant growth accompanied by hyperhydration. Scanning electron microscope observations showed that 2 min ultrasound changed the joint area between epidermal cells and removed some of the surface from the cotyledon epidermal cells, without gross surface injury to the explants. Longer periods of ultrasound (5-10 min) caused further surface erosion. Rubbing the explant contact surface with chloroform or sandpaper emulated the effect of sonication on shoot regeneration and explant growth, demonstrating that ultrasound exerts its morphogenic influence by surface removal. Sonication of explants from other batches of squash seeds (of cultivars Ma'yan and True French), that regenerated without such treatment, reduced regeneration and caused hyperhydration. This is the first report of stimulation of in vitro regeneration by ultrasound treatment.

The bidentate diphosphine ligdnd, 3,3'-oxybis[(dipenylphosphino)methylbenzene] (1) forms monomeric, truns-square-planar complexes MX2(1) (M = Ni, Pd, Pt; X = CI-., Br-, I-, and, in

where and 0=1, 2 J =(m ai 3)n 2 m 2 l+ ; m s V2Notice that v^{oo = 0) =^0ln(^m ax A min ) reduces to the Spitzer value whereas at co =Sl p Eqs. (9) and (11), one can take fe^ min/

Study of the Zn'+-containing D-alanyl-D-alanine-cleaving carboxypeptidase of Streptomyces albus G by smallangle X-ray scattering in solution yielded the following molecular parameters: radius of gyration R = 1.82 f 0.05 nm; largest diameter D = 5.9 f 0.2 nm; relative molecular mass M, = 17000 & 2000; volume V zz 35 f 2 nm3; degree of hydration: 0.25 f 0.02 g water/g protein. By reference to theoretical scattering curves of rigid triaxial homogeneous bodies, a model which fits all experimental data is an elliptical cylinder. Such a model is compatible with that observed in the crystal structure. At those high concentrations necessary to form inactive enzyme-ligand associations the non-competitive p-lactam inhibitors, cephalothin and cephalosporin C, drastically altered the scattering behaviour of the protein.The G, R61 and R39 D-alanyl-D-alanine-cleaving peptidases (in short DD-peptidases), isolated from Streptomyces albus G, Streptomyces R61 and Actinomadura R39 respectively, have been used extensively as model enzymes for the study of the reactions involved in the last stages of bacterial wall peptidoglycan synthesis and the mode of action of the p-lactam antibiotics [l]. The R61 and R39 enzymes perform catalysis via an active serine residue and are respectively very and exceedingly sensitive to penicillin. In contrast, the G enzyme operates via a Zn2+ cofactor and is highly resistant to penicillin. Knowledge of the exact atomic level structures and functioning of these enzymes is also relevant to the proposed peptidoglycan network models [2,3]. The G and R61 DD-peptidases have been crystallized. The solution of the R61 enzyme structure has proceeded to a resolution of 0.28 nm, showing, via difference Fourier maps, the binding site of cephalosporin C and 6,6-dichloro-4-deaza-2,2-didemethylpenicillanic acid [4, 51. The amino acid sequence (212 residues) [6] and the threedimensional structure at 0.45nm and 0.25 nm resolution of the G Zn2+ DD-peptidase have been established [7, 81 and a plausible picture of how this enzyme performs catalysis has been proposed [9]. The overall shape of the enzyme in the crystal structure is that of an elliptical cylinder with a height of 4.8nm and an axial ratio of 1.4: 1 :0.8 (height:long axis:short axis). The enzyme conformation in aqueous solution has now been studied by small-angle X-ray scattering. The results obtained are presented here. MATERIALS AND METHODS EnzymeThe enzyme was purified to protein homogeneity [lo]. The enzyme solutions were prepared by extensive dialysis at 4 "C against 50mM Tris/HCl buffer pH8.3 (buffer I) or 10mM Hepes/NaOH buffer pH 7.75 containing 5 mM MgC1, (buffer 11). Enzyme concentrations ranging between about 0.2 mM and 0.8 mM were determined spectrophotometrically at 280 nm, using A ] & = 10. Small-angle X-ray scatteringWeighed amounts of cephalosporin C and cephalothin (from Ely Lilly and Sigma respectively) were added to the enzyme solutions followed by rapid agitation. The molar ratios enzyme: antibiotic were 1 :20 for cephalosporin C in buffer...