Substituents unrestricted

TABLE 14. Effect of the first atom from the Rf substituent (unrestricted Rj = R ) on selected structural descriptors of enamine grouping expressed by statistical parameters0... 

Phospholipids are ideal compounds for making membranes because of their amphipathic nature (see chapter 17). The polar head-groups of phospholipids prefer an aqueous environment, whereas the nonpolar acyl substituents do not. As a result, phospholipids spontaneously form bilayer structures (see fig. 17.6), which are a dominant feature of most membranes. The phospholipid bilayer is the barrier of the cell membrane that prevents the unrestricted transport of most molecules other than water into the cell. Entry of other molecules is allowed if a specific transport protein is present in the cell membrane. Similarly, the phospholipid bilayer prevents leakage of metabolites from the cell. The amphipathic nature of phospholipids has a great influence on the mode of their biosynthesis. Thus, most of the reactions involved in lipid synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic. 

TABLE 1. Frequency of elements appearing as the first atom of the unrestricted substituents R R5... 

TABLE 2. Selected structural and statistical data for enamine fragments (sample size 474) with unrestricted substituents R1, R2, R3, R4 and R5... 

TABLE 4. Selected statistical data for the absolute values of angular out-of-plane parameter, xNi (deg), and twist parameters, tni C2 and tc2 C3 (deg), together with pyramidality coefficients P (%) for the N1, C2 and C3 atoms of the enamine grouping both for a sample containing enamines with unrestricted substituents (size 474) and with carbon as the first atom in each of the RJ-R5 substituents (sample size 178)... 

The value of the average twist , tNi c2, also varies over a wide range from 0 ° to 94 ° with an average of 23.35 ° slightly higher for the set of enamine fragments with unrestricted substituents. The most frequent, however, are conformations with small twist as shown by almost 60% of fragments with t < 17 °. 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

Sperling, D., Fabian, J. Substituent effects on the vinylcyclopropane-cyclopentene rearrangement. A theoretical study by restricted and unrestricted density functional theory. Eur. J. Org. Chem. 1999, 215-220. 

The values of the out-of-plane deformation parameter, for the enamine fragments with unrestricted substituents vary in a wide range (0 °-77.5 °) and have a high dispersion around the average value of 19,10 The frequency distribution of Xn> is not normal. Its maximum corresponds to 0 < 18 ° (64% of all considered fragments) and, for a... 

The average pyramidality value for N (15% in the case of unrestricted substituents) is much higher than for and C, which is in agreement with the significant contribution of sp -hybridized N atoms in the studied sample of the enamine population. The pyramidality varies over a very wide range, 0 < 133.7%, in which the values... 

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