Images, chemical atom labels

Figure 3.3 Example objects with (a) Q, (b) Q and (c) C, symmetry. The differently shaded points should be thought of as atoms of different chemical elements. The label 1 has been added to suggest points that are the initial positions used to generate symmetry images of the same shading.

Radiochemical tracers, compounds labeled with radioisotopes (qv), have become one of the most powerful tools for detection and analysis in research, and to a limited extent in clinical diagnosis (see Medical IMAGING TECHNOLOGY). A molecule or chemical is labeled using a radioisotope either by substituting a radioactive atom for a corresponding stable atom in the compound, such as substituting for H, for or for P, and for for... 

Figure 7.15 The P280 thrombus imaging agent peptide for labeling with technetium-99m. Possible Tc ligand atoms shown in hold type. (Adapted with permission from Figure 23 of Liu, S. Edwards, D. S. Chem. R(.v., 1999, 99(9), 2235-2268. Copyright 1999, American Chemical Society.)...

Tool-tips provide additional information on atoms, bonds and chemical features and allow the user to measure angles and distances (Fig. 6.7). Tool-tips are semi-transparent, two-dimensional labels that are overlaid over the molecular image and point to the object for which they provide information. 

Two atoms or groups are said to the diastereotopic if the two atoms or groups appear to be similar but are not actually related by a symmetry operation. Selectively labeling one diastereotopic atom or group wUl not produce the mirror image of the molecule in which the other atom or group is labeled. For example, the methylene protons in phenylalanine are diastereotopic. The resonances from diastereotopic atoms and groups will normally be anisochronous, unless they happen to fortuitously have the same chemical shift. 

---