Closed groups

There is more to this story, however. Enzymes not only bring substrates and catalytic groups close together, they orient them in a manner suitable for catalysis as well. Comparison of the rates of reaction of the molecules shown... 

The structures of (4) the cyanuric tricyanamide ion, C8H9 , (B) the cyamelurate ion, CeNiOs" and (C) the cyameluric tricyanamide (hydromelonate) ion, C9NJ3 . Predicted values of interatomic distances are shown. The molecules are coplanar, with all bond angles (except in the linear cyanamide groups) close to 120°. 

There are also some examples of retention of configuration in nucleophilic displacement reactions where the common feature is an atom or group—close to the carbon undergoing attack—which has an electron... 

Structural characterization of many quinolizidine derivatives has been established by X-ray diffraction. For example, this technique, in combination with spectroscopic methods, showed that (+)-2-thionosparteine 21 and (+)-2,17-dithionospartine 22 are conformationally rigid and have their lactam and thiolactam groups close to planarity, with the exception of the lactam group in 21, and that rings A and C adopt distorted sofa conformations <2005JST75>. 

The theory of discrete infinite groups closely parallels that of finite groups, but for infinite continuous groups there are several important differences. 

We will begin by redrawing the molecules to place the functional groups close together. 

Fig. 7.7. Topographical model of the active site of pig liver esterase as proposed by Jones and co-workers [70] [71]. The model postulates two hydrophobic sites, one large (HL) and one small (Hs), and two polar binding sites, one in the front (PF) and one in the back (PB). The serine sphere shows the approximate zone of action of the catalytic OH group, a) A view from the top with the dimensions in A the sites HL, Hs, and PF are at ground level and have an elevation of 3.1 A, 2.3 A, and 1.6 A, respectively, while PB is located 1.5 A above ground level and has an elevation of 0.8 A. b) A computer-generated perspective view with dimethyl phenylmalonate positioned to have its pro-S ester group close to the catalytic site [72a].

The above mechanism of intramolecularly catalyzed cleavage is possible only for compounds with a N02 group close to the departing NO group. In other words, it can be postulated to follow nitro reduction in erythritol tetra-nitrate (9.8), mannitol hexanitrate (9.9), pentaerythritol tetranitrate (9.10), and perhaps trolnitrate (9.14), but not in isosorbide dinitrate (9.11). 

The chemical shifts caused by different substituent groups closely resemble those observed for benzene derivatives [76T2339, 76ZN(B)I641]. 

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