Activated complex, “loose tight

In other words, whether or not an Sn2 reaction has a tight or loose activated complex will not only depend upon the nature of the reactants Y and R-X, in solution it will also be affected by the nature of the solvent. Better solvation of the activated complex of a type II Sn2 reaction by solvents with improved EPD/EPA properties will lead to a loosening of the activated complex. Transferring this activated complex from solution to the gas phase, with subsequent loss of the charge-separation stabilizing solvation, will therefore increase its tightness cf. also [499]. 

When the molecule can be considered freely rotating in the transition state, this is called a loose activated complex. When rotation is restricted, it is considered a tight activated complex (Figure 4.15). Table 4.7 gives some values of rotational... 

In the nuclei of eukaryotes (see p. 196), DNA is closely associated with proteins and RNA. These nucleoprotein complexes, with a DNA proportion of approximately one-third, are known as chromatin. It is only during cell division (see p. 394) that chromatin condenses into chromosomes that are visible under light microscopy. During interphase, most of the chromatin is loose, and in these conditions a morphological distinction can be made between tightly packed heterochromatin and the less dense euchromatin. Euchro-matin is the site of active transcription. 

The sequence of events in gene activation contains several steps. First, in the hormone-free state, the unoccupied receptor is bound to the nuclear membrane and loosely to chromatin. (Chromatin is the DNA-protein complex of chromosomes.) After the hormone binds the receptor, it changes its location. The receptor-hormone complex binds DNA tightly and thereby activates or inactivates the synthesis of mRNA from these genes. The specificity of these receptors lies in two properties their ability to bind different hormones and their ability to bind different DNA sequences. 

This would assume a very loosely bound complex. For more tightly bound complexes the values would be lower, corresponding to a negative entropy of activation. 

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

The enzyme is Mg " dependent, activation of the hydrolysis of O-glycosides in both loose and tight complexes being seen. The Mg " (presumably... 

Iron is transported in forms in which it is tightly complexed to small chelators called siderophores (microorganisms) or to proteins called transferrins (animals) or to citrate or mugeneic acid (plants). The problem of how the iron is released in a controlled fashion is largely unresolved. The process of mineral formation, called biomineralization, is a subject of active investigation. Vanadium and molybdenum are transported as stable anions. Zinc and copper appear to be transported loosely associated with peptides or proteins (plants) and possibly mugeneic acid in plants. Much remains to be learned about the biological transport of nonferrous metal ions. 

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