Methylene tail modes

Methylene Tail Modes. Figure 4 presents representative spectra in the C-H stretching region of 70 mM SDS as a function of added NaCl (T=25°C). Also plotted are several difference spectra, obtained by subtraction of die spectrum of SDS in water from the spectra of the SDS samples containing added electrolyte, and a spectrum of a SDS coagel phase (T=6°C). 

Methylene Tail Modes. Figure 12a shows the frequency of the composite symmetric CH2 stretching band, and the zero shear viscosities (m) of mixed micelles formed between SDS and C14AO at 10% total surfactant (T = 20 °C). These compositions are all drawn from the L. phase. However, as the plot of the zero shear viscosities (t)0) shows, these micellar solutions are quite diverse, with a... 

Methylene Tail Modes. Figure 9 shows a plot of the aggregation numbers and the frequency of the symmetric CHj stretching band for 0.3 M DTAC/SDS mixtures (25 °C). The CH2 stretching bands (symmetric and asymmetric) of DTAC and SDS are highly overlapped and, therefore yield a "composite" CH2 band with... 

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). 

Figure 2 Conventional representation of micelles formed by an ionic surfactant, such as sodium dodecyl sulfate. The inner core region consists of the methylene tails of the surfactants. The Stem layer consists of surfactant headgroups and bound counterion species. The diffuse double layer consists of unbound counterions and coions which preserve the electrical neutrality of the overall solution. Also pictured are the transition moment vectors for the S-O stretching modes of sodium dodecyl sulfate.

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

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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