Rotation steric inhibition

A bulky substituent close to the reaction centre may increase the non-bonded compression energy as the transition state is formed this will cause an increase in A//. It will also hinder the close approach of solvent molecules to the reaction centre, thus reducing the maximum amount of stabilization possible (steric inhibition of solvation). This will result in a further increase in AH, but since decreased solvation means less ordering of solvent molecules about the transition state, there is a compensating increase in AS. Another effect of the bulky substituent may be to block certain vibrational and rotational degrees of freedom more in the (more crowded) transition state than in the initial state, and so to reduce AS. These are the most important of the simple effects of a bulky substituent and can be used to explain most of the relationships of Table 25. 

The degree of steric inhibition of resonance depends on the bulk of the Q-alkyl substituents. Rotation of the aryl ring to increase the overlap between... 

The n.m.r. spectrum of the orchid alkaloid dendrocrepine (12) shows that the phenyl groups give rise to six resonances, and the non-equivalence of the two ortho and meta positions is considered to be caused by steric inhibition of rotation of the phenyl groups. Similar steric inhibition of the free rotation of the phenyl group in 2,3-dimethyl-l-phenylcyclohexanol was demonstrated, and in the spectrum of dendrocrepine the six phenyl resonances were reduced to four at -1-57 C. 

The energy threshold for rotation is quite low, for example 11 kJ mol for the ethane molecule and 16.7 kJ mol for the polyethylene molecule. This segment rotation is more or less hindered by substituents attached to the main chain (atoms or atom groups) or branches (steric inhibition), for example, less so in polyethylene, more so in polystyrene due to the residual phenyl. 

Clark, Studitsky, and Felsenfeld [88-92] have extensively evaluated transcription on single nucleosomes utilizing SP6 RNA polymerase. Based on their data they have developed a model which is shown in Fig. lA. This model, referred to as the spooling model, proposes that when transcription occurs into the first 25 bp of the nucleosome, the polymerase pauses (step 2). DNA behind the polymerase binds the octamer surface, which further inhibits the action of the polymerase due to steric hinderance of the polymerase s rotation (step 3). For transcription to continue the DNA behind the polymerase must dissociate to allow the polymerase to continue to transcribe an additional 35 bp (step 4). At this point sufficient surface of the octamer is now available to permit substantial levels of... 

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