Structural data

Four-co-ordinated aluminium is also present in the three methylaluminate 

Different tautomeric forms can be envisaged for both 1 and 2. In the trans configuration, the protons are located on the opposite nitrogen atoms, whereas in the cis structure they are positioned on the adjacent ones (Fig. 8.3). Of the two cis forms of 1, the vicinal arrangement of two protons on the same bipyrrole unit (cis-1) is energetically much more favorable than that of the other possible species 

Strong hydrogen bonds in 1 should result in the NH vibrations being shifted to low frequencies. Actually, the NH stretching vibration has not yet been identified in the IR spectrum of 1, even though the calculations predict that it should correspond to the most intense band [41, 42]. The IR and Raman experiments could 

The differences in HB strength in 1 and 2 are nicely reflected in the values of the NMR chemical shifts of the inner protons (Table 8.2). The values for porphycene are shifted downfield from those of porphyrin, and correlate clearly with the N-N separation. The chemical shifts of the peripheral protons are very similar in both isomers. 

Numerous NMR experiments have been reported for variously substituted porphyrins, both in solution [43-66] and in the solid phase [58, 61, 63, 67, 68]. At elevated temperature, the trans-trans interconversion is fast on the NMR time scale. For example, for i N-enriched porphyrin in toluene-dg the rate constant at 298 K is more than 2 x s i at 254 K, = 1300 S [63]. Further lowering of the temperature results in a complete localization of protons. Below about 230 K, the two trans tautomers do not interconvert in the dark. They can still be transformed into each other, however, upon photoirradiation, even at cryogenic temperatures [38, 39]. 

On the basis of recent electron-nuclear double resonance (ENDOR) studies, Hoffman and Seefeldt suggested that the interstitial X-atom is not exchanged during catalysis [20, 21]. This would seem to suggest that the X-atom, even if it is a nitride, is not derived from N2 during turnover. Most recently, Hoffman and Seefeldt have gone further in their analysis and have proposed that the central light atom is not an N-atom [21]. They noted a loss of electron 

---

Figure Bl.24.17. An example of scanning transmission ion microscopy (STIM) measurements of a human oral cancer cell. The different images indicate different windows in the energy of transmitted helium ions as indicated in the figure. White indicate areas of high counts. The teclmique offers a thickness scan through the sample, and, in this case, the cell walls of one specific cell can be seen in the areas dominated by thicker structures (data from C A Pineda, National Accelerator Centre, Fame, South Africa).

During the process of conversion, a program may drop some information produced by other software because the format conventions cannot handle this additional information. For example, when the J ACMP format is converted to a Molfile, its content is reduced to structural data only, without spectra data. In other cases, a... 

SDfile. sdf Structure-Data file extension of the MDL Molfile containing one or more compounds umm.mdli.com 50... 

New ways to represent structure data became available through molecular modeling by computer-based methods. The birth of interactive computer representation of molecular graphics was in the 196Ds. The first dynamic molecular pictures of small molecules were generated in 1964 by Lcvinthal in the Mathematics and Computation (MAC) project at the Electronic Systems Laboratoiy of the Massachusetts... 

A strict separation of these three types of databases is difficult hence most databases contain a mixture of data types. Therefore the classification given here is based on the predominating data type. For example, the major emphasis of a patent database is on hterature, whereas it also comprises numeric and structural data. Another type is the integrated database, which provides a supplement of additional information, especially bibhographic data. Thus, different database types are merged, a textual database and one or more factual databases. 

ICSD FIZ Karlsruhe. Germany NIST, USA crystal structure data of inorganic compounds nu- meric. factual 65 000 journals STN online, CD-ROM biannu- ally wuTw.fiz-infoT- mationsdien- ste.de... 

PDB Research Col-laboratory for Structural Bioinformat-ics (RCSB) macromole-cular structure data on proteins, nucleic acids, protein-nucleic acid complexes, and viruses nu- meric. biblio. -20000 records experi- ments Research Col-laboratory for Structural Bioinformatics online, CD-ROM periodi- cally WU7W.TCsh.0Tg/ pdh/... 

Table 8. Properties and Crystal Structure Data for Important Actinide Binary Compounds...

X-Ray Crystallography. Structural data exists on more than one thousand steroids. Comparisons of the conformations obtained by x-ray crystallography have been made to the conformations obtained by other methods (161). Several studies use x-ray crystal stmctures in the study of progestins. 

S. E. DeVoe, M. Storms, and K. Haraki, A.ntibiotics Properties and Structures Data Base, maintained at the Medical Research Division, American... 

Nuclear Data Sheets, Academic Press, San Diego, Calif., TvaluatedNuclear Structure Data File (ENSDF), a computer database of nuclear stmcture data evaluated by an international network of evaluators, is maintained at the National Nuclear Data Center, Brookhaven National Laboratory. NUDNTis a computer database of decay data extracted from the ENSDF. 

The pyrimidine ring is virtually flat. Its corrected bond lengths, as determined by a least-squares analysis of the crystal structure data for a unit cell of four molecules, are shown in formula (2) (60AX80), and the bond angles derived from these data show good agreement with those (3) derived by other means (63JCS5893) for comparison, each bond... 

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

Structural data of a diaziridine come from gas phase electron diffraction measurements (74CC397). The N—N bond of 3-methyldiaziridine (24) is longer than in hydrazine (1.449 A) the C—N bond distances in (24) and in diazirine are nearly equal (1.479 versus 1.482 A),... 

Figure 1 The principal sources of structural data are the NOEs, which give information on the spatial proximity d of protons coupling constants, which give information on dihedral angles < i and residual dipolar couplings, which give information on the relative orientation 0 of a bond vector with respect to the molecule (to the magnetic anisotropy tensor or an alignment tensor). Protons are shown as spheres. The dashed line indicates a coordinate system rigidly attached to the molecule.

Restraints due to artifacts may, by chance, be completely consistent with the correct structure of the molecule. However, the majority of incorrect restraints will be inconsistent with the correct structural data (i.e., the correct restraints and information from the force field). Inconsistencies in the data produce distortions in the structure and violations in some restraints. Structural consistency is often taken as the final criterion to identify problematic restraints. It is, for example, the central idea in the bound-smoothing part of distance geometry algorithms, and it is intimately related to the way distance data are usually specified The error bounds are set wide enough that all data are geometrically consistent. 

To gain the most predictive utility as well as conceptual understanding from the sequence and structure data available, careful statistical analysis will be required. The statistical methods needed must be robust to the variation in amounts and quality of data in different protein families and for structural features. They must be updatable as new data become available. And they should help us generate as much understanding of the determinants of protein sequence, structure, dynamics, and functional relationships as possible. 

Genetics and model building provide a framework for the interpretation of structural data, but they cannot reliably be used to predict the details of the stereochemistry of specific interactions within or between molecules. 

Reaction center hydropathy plots agree with crystal structural data... 

Figure 14.17 A sequence of events combining the swinging cross-bridge model of actin and myosin filament sliding with structural data of myosin with and without bound nucleotides.

---