Comparison of steric and electronic requirements for C-C and C-H bond activation. Chelating vs nonchelating case.

C-H bond activation was observed in a novel PCO ligand 1 (C(6)H(CH(3))(3)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)()]BF(4) (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degrees C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH(3))Rh(C(6)H(CH(3))(2)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))BF(4)), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF(4)(-) anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.

DNA bending by an adenine--thymine tract and its role in gene regulation.

To gain insight into the structural basis of DNA bending by adenine-thymine tracts (A-tracts) and their role in DNA recognition by gene-regulatory proteins, we have determined the crystal structure of the high-affinity DNA target of the cancer-associated human papillomavirus E2 protein. The three independent B-DNA molecules of the crystal structure determined at 2.2-A resolution are examples of A-tract-containing helices where the global direction and magnitude of curvature are in accord with solution data, thereby providing insights, at the base pair level, into the mechanism of DNA bending by such sequence motifs. A comparative analysis of E2-DNA conformations with respect to other structural and biochemical studies demonstrates that (i) the A-tract structure of the core region, which is not contacted by the protein, is critical for the formation of the high-affinity sequence-specific protein-DNA complex, and (ii) differential binding affinity is regulated by the intrinsic structure and deformability encoded in the base sequence of the DNA target.

[Laparoscopic sacral colpopexy: an attractive approach for prolapse repair]. / La promonto-fixation sous coelioscopie: une voie d'abord séduisante pour la cure des prolapsus.

OBJECTIVE: Evaluate laparoscopic sacral colpopexy. MATERIAL AND METHODS: 76 patients operated by laparoscopy between 1993 and 1999. The mean age of the patients was 59 years. 35 patients suffered from stress incontinence. A history of abdominal surgery was never a contraindication to laparoscopy. Hysterectomy was not systematically performed and the type of correction of urinary incontinence varied between suburethral sling, Burch procedure and TVT. The operating time ranged from 100 to 345 minutes with a mean of 199 minutes. Patients were discharged from hospital between the 2nd and the 10th postoperative day (mean: 4.9 days). Nine out of 10 patients operated (89.40%) were discharged before the 6th postoperative day. The results were satisfactory with correction of prolapse in 74 patients (96%). Only one intraoperative complication occurred in our series: a minimal injury of the colon complicated dissection of the posterior floor during resection of the pouch of Douglas. This wound was sutured during the same operating time. One conversion was required following sudden onset of oxygen desaturation during carbon monoxide insufflation. Laparoscopic prolapse surgery is used increasingly frequently and this study confirms that laparoscopic sacral colpopexy is effective, reliable and reproducible.

[Transperitoneal laparoscopic nephrectomy]. / La néphrectomie sous coelioscopie par voie transpéritonéale.

OBJECTIVES: To evaluate the feasibility and complications of transperitoneal laparoscopic nephrectomy. MATERIAL AND METHODS: 30 transperitoneal laparoscopic nephrectomies were performed between November 1992 and October 1998: 17 for malignant lesions (10 renal cell carcinomas, 7 urothelial tumours) and 13 for benign lesions. RESULTS: The operation was performed entirely by laparoscopy in 25 cases, with 4 conversions: 3 for technical problems (difficulties of dissection: 1, haemorrhage: 2). One patient underwent laparotomy on the same day and three patients were transfused. The mean operating time was 116 minutes. No laparoscopy-specific complications were observed. CONCLUSION: A standard technique must be performed regardless of the disease: systematic open laparoscopy, wide colonic dissection, exposure of the vessels and resection in the plane of the radical nephrectomy. The risk of cancer dissemination with laparoscopy appears to be non-existent for renal cell carcinomas and controversial for urothelial tumours. Complications were always benign, although there is always a risk of bleeding, regardless of the operator's experience. The current technical conditions of laparoscopic surgery and its extension to all forms of visceral surgery should encourage the use of this technique by urologists, particularly for renal surgery.

Structural code for DNA recognition revealed in crystal structures of papillomavirus E2-DNA targets.

Transcriptional regulation in papillomaviruses depends on sequence-specific binding of the regulatory protein E2 to several sites in the viral genome. Crystal structures of bovine papillomavirus E2 DNA targets reveal a conformational variant of B-DNA characterized by a roll-induced writhe and helical repeat of 10.5 bp per turn. A comparison between the free and the protein-bound DNA demonstrates that the intrinsic structure of the DNA regions contacted directly by the protein and the deformability of the DNA region that is not contacted by the protein are critical for sequence-specific protein/DNA recognition and hence for gene-regulatory signals in the viral system. We show that the selection of dinucleotide or longer segments with appropriate conformational characteristics, when positioned at correct intervals along the DNA helix, can constitute a structural code for DNA recognition by regulatory proteins. This structural code facilitates the formation of a complementary protein-DNA interface that can be further specified by hydrogen bonds and nonpolar interactions between the protein amino acids and the DNA bases.

Molecular replacement: the revival of the molecular Fourier transform method.

The molecular Fourier transform method, perhaps the first application of the molecular-replacement approach, used in the 1950s for the two-dimensional structure determination of small molecules, has been modernised for the efficient solution of complex structures. In the modern application of the molecular Fourier transform (MFT), the three-dimensional transform of the molecular model is calculated and fitting is achieved by rotating the weighted reciprocal lattice with respect to the calculated transform. The fit between the transform and the weighted reciprocal lattice is gauged by three different criteria corresponding to R factor, correlation coefficient and product function. Since the procedure involves the rotation of indices and is, therefore, independent of the number of atoms, it is much faster than other methods which employ the rotation of the molecular model. This feature enabled the renovation of the rotation-translation search method ULTIMA, which utilizes low-order data and packing considerations for the efficient solution of large structures.