Direct space strategy

A number of oligopeptide structures have been determined from powder XRD data using the direct-space strategy for structure solution. The first example concerns Phe-Gly-Gly-Phe [100], for which the stmcture solution calculation (employing... 

Thus, reaching high resolution is always a matter of compromise depending on the kind of informations needed and strategies to cancel or reintroduce anisotropies of selected interactions were developed in direct space (powdered or single crystal, mechanical spinning (MAS), etc.), in the space of coherences (averaged Hamiltonians, multidimensional NMR sequences, etc.) or combined methods (MQMAS, etc.). 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

Selufien The basic strategy behind the direct SCF method is recomputing certain intermediate quantities within the calculation—specifically the two-electron integrals—as needed, rather than storing them on disk. This has the advantage of making it possible to study systems which would require more disk space than is available on the system. 

These initial systematic studies regarding the directed evolution of PAL allowed several conclusions to be made. Protein sequence space can be explored successfully by applying the following strategies [8c,33j ... 

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