Methylene tail modes, conformational

Considerable experimental evidence (1-6,11) suggests that the methylene chains inside of a spherical micelle are almost as disordered as in the bulk liquid state (i.e. they contain a significant proportion of gauche conformers). The FTTR spectra of micellar SDS support this assertion, exhibiting CH2 stretching and scissoring band frequencies which are comparable to those found in the spectra of liquid hydrocarbons (1-6,11). A recent quantitative analysis of the CH2 defect modes of SDS has shown that the disorder of me methylene tails is similar to that found in liquid tridecane (11). 

This unexpected result was interpreted to mean that the reverse prenyl transferase presents the olefinic n-system of DMAPP in a manner in which both faces of the n-system are susceptible to attack by the 2-position of the indole moiety. The simplest explanation is to invoke binding of the DMAPP in an upside down orientation relative to normal prenyl transferases which permits a facially non-selective S attack on the rr-system as shown in Scheme 20. It was speculated that in this situation the pyrophosphate group is likely anchored in the enzyme active site with the hydrophobic isopropenyl moiety being presented in a conformationally flexible (A B) disposition with respect to the tryptophan-derived substrate (Scheme 20). This is in contrast to the normal mode of prenyl transfer where the nucleophilic displacement at the pyrophosphate-bearing methylene carbon occurs with inversion of stereochemistry at carbon with the hydrophobic tail of DMAPP buried in the enzyme active site. 

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