TDSIR phasing: direct use of phase-invariant distributions in macromolecular crystallography

Improving the convergence rate of a hybrid input-output phasing algorithm by varying the reflection data weight

In an iterative projection algorithm proposed for ab initio phasing, the error metrics typically exhibit little improvement until a sharp decrease takes place as the iteration converges to the correct high-resolution structure. Related to that is the small convergence probability for certain structures. As a remedy, a variable weighting scheme on the diffraction data is proposed. It focuses on phasing low- and medium-resolution data first. The weighting shifts to incorporate more high-resolution reflections when the iteration proceeds. It is found that the precipitous drop in error metrics is replaced by a less dramatic drop at an earlier stage of the iteration. It seems that once a good configuration is formed at medium resolution, convergence towards the correct high-resolution structure is almost guaranteed. The original problem of phasing all diffraction data at once is reduced to a much more manageable one due to the dramatically smaller number of reflections involved. As a result, the success rate is significantly enhanced and the speed of convergence is raised. This is illustrated by applying the new algorithm to several structures, some of which are very difficult to solve without data weighting.

Synthesis, biological activity, and SAR of antimycobacterial 9-aryl-, 9-arylsulfonyl-, and 9-benzyl-6-(2-furyl)purines

9-Aryl-, 9-arylsulfonyl- and 9-benzyl-6-(2-furyl)purines were synthesized by N-alkylation or N-arylation of the purine followed by Stille coupling to introduce the furyl substituent in the 6-position and the compounds screened for activity against Mycobacterium tuberculosis. The 9-aryl- and 9-sulfonylarylpurines exhibited weak activity toward the bacteria, but 9-benzylpurines were good inhibitors especially those carrying electron-donating substituents on the phenyl ring. A chlorine atom in the purine 2-position further enhanced activity. The high antimycobacterial activity (MIC 0.39 microg/mL against M. tuberculosis), low toxicity against mammalian cells and activity inside macrophages found for 2-chloro-6-(2-furyl)-9-(4-methoxyphenylmethyl)-9H-purine makes this compound a highly interesting potential antituberculosis drug.

Phasing methods for protein crystallography

The phase problem has been formulated as one of constrained global minimization and it leads to the minimal principle, which is the theoretical basis of 'Shake-and-Bake'--an algorithm for the automatic solution of the phase problem, ab initio. A related approach (termed 'Half-Baked') that, as in Shake-and-Bake, alternates phase refinement in reciprocal space with density modification in real space, has recently been formulated. The traditional techniques of direct methods have been integrated with isomorphous replacement and anomalous scattering and the first applications have been made. The accurate measurement of a limited number of structure invariants by means of multiple beam X-ray diffraction has become a reality. A feasibility study shows that, in combination with the direct method, the ability to measure the values of the structure invariants experimentally will strengthen existing techniques.