Phase measurement for accurate mapping of chemical bonds in acentric space groups.

Although the electron density is fundamental to the study of chemical bonding and density-functional theory, it cannot be accurately mapped experimentally for the important class of crystals lacking inversion symmetry, since structure factor phase information is normally inaccessible. We report the combination of x-ray and electron diffraction experiments for the determination of the electron density in acentric AlN, using multiple-scattering effects in convergent-beam electron diffraction to obtain sensitivity to structure factor phases, and describe a new error metric and weighting scheme for multipole refinement using combined measurements of structure factor magnitudes and phases.

Application of a modified Oszlányi and Süto ab initio charge-flipping algorithm to experimental data.

The structures of two crystals have been solved using a new iterative phasing method. The iterative phasing algorithm is developed from the 'charge-flipping' method proposed by Oszlányi & Süto [Acta Cryst. (2004), A60, 134-141]. Positivity and point-atom constraints are incorporated within this extremely simple and effective algorithm by flipping (sign reversal) of less-positive density values during the iterations. Convergence is reliably achieved and the two structures were solved. This structure solution method does not require information on atomic scattering factors or symmetry. Heavy atoms can be distinguished from light ones by their charge-density values.

Recognition and cleavage at the DNA major groove.

DNA recognition agents based on the indole-based aziridinyl eneimine and the cyclopent[b]indole methide species were designed and evaluated. The recognition process involved either selective alkylation or intercalating interactions in the major groove. DNA cleavage resulted from phosphate backbone alkylation (hydrolytic cleavage) and N(7) -alkylation (piperidine cleavage). The formation and fate of the eneimine was studied using enriched 13C NMR spectra and X-ray crystallography. The aziridinyl eneimine specifically alkylates the N(7) position of DNA resulting in direction of the aziridinyl alkylating center to either the 3'- or 5'-phosphate of the alkylated base. The eneimine species forms dimers and trimers that appear to recognize DNA at up to three base pairs. The cyclopent[b]indole quinone methide recognizes the 3'-GT-5' sequence and alkylates the guanine N(7) and the thymine 6-carbonyl oxygen causing the hydrolytic removal of these bases. In summary, new classes of DNA recognition agents are described and the utility of 13C-enrichment and 13C NMR to study DNA alkylation reactions is illustrated.

Synthesis and structures of heterocyclic azidogallanes [(CH3)ClGaN3]4 and [(CH3)BrGaN3]3 en route to [(CH3)HGaN3]x: an inorganic precursor to GaN.

The synthesis of [(CH3)ClGaN3]4 (1) with a heterocyclic cyclooctane-like structure and [(CH3)BrGaN3]3 (2) with a trimeric structure has been demonstrated. X-ray structural determinations reveal that 1 and 2 consist of Ga4N4 eight-membered rings and Ga3N3 six-membered rings, respectively, in which the Ga atoms are bridged by the alpha nitrogens of the azide groups. [(CH3)ClGaN3]4 crystallizes in the tetragonal space group P42(1)c with a = 11.017(4) A, c = 8.699(7) A, and Z = 8. [(CH3)BrGaN3]3 crystallizes in the triclinic space group P1 with a = 8.1080(10) A, b = 9.9390(13) A, c = 10.4439(13) A, alpha = 86.069(3) degrees, beta = 86.771(3) degrees, gamma = 80.829(2) degrees, and Z = 6. The reaction of 1 and 2 with LiGaH4 yields [(CH3)HGaN3]x, which is a new low-temperature source of GaN.