Single conformation

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

Larger polymers are known as proteins. Aside from the amide linkages, the polymer chain is very flexible, giving rise to the possibility of an enormous number of different conformers. It is nothing short of remarkable, therefore, that proteins rapidly fold into a single conformation. Very strong forces must be at work. 

It is unrealistic to expect that any single conformer of a polymer will adequately represent the overall size and shape of the polymer. The low-energy conformer for each polymer strand shown here is merely meant to allow identification of the polymer in terms of its components. 

NMR investigations of A (l)-methyl 117 and A (5)-methyl 118 derivatives of 112 (R = H) revealed that they adopted a similar conformation as the parent 112 (R = H) (99T6759). Both A -methyl derivatives contain the methyl group in an axial position. All of them have a trans-ring junction. Whereas, cA-2/7,4//,9n//-dimethyl derivative 112 (R = Me) exists in a single conformation, its cis-2H,AH-trans-9aH diastereomer 119 is conformation-ally complex, consisting of al, 0 2, and fi anomers in equilbrium in D2O (OOJOC889). 

One possible conclusion to be drawn from this comparison is that there is not one single conformation that is reactive with the enzyme, but a range of conformations. The flexibility of positioning of some portions of the molecule. 

Similarly, the 2,8,10-triene 3a gives a mixture of four isomers, but introduction of a TMS group as in 3b gives a single stereoisomer in 89% yield. The reason for the improved stereoselectivity is that the steric effect introduced by the TMS substituent favors a single conformer. 

Figure 10 Ribbon diagrams of a single conformer of inhibited sfSTR from residues 83 to 250. (A) The complex is viewed from above the (1-sheet. The positions of the two zincs are indicated as large balls. The strands of the [1-sheet (l-V) and helices (A-C) are indicated. The heavy atoms of the inhibitor and residues of the protein that ligate zinc are shown. The inhibitor runs antiparallel to strand IV. The structural zinc lies above the (1-sheet and is... 

Of course, with these alkane spacers rotation is possible about any number ofo-bonds and the convergent conformation is only one of many available to the molecule. This flexibility permits variable distances between opposing carboxyl functions. The price paid for this adjustability is the relatively low probability of any single conformation. The advantage of fine-tuning then, is somewhat offset by the entropic... 

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