Molecule 1, methyl-terminated branches

Fig. 4.10 (a) Molecule 1, methyl-terminated branches and (b) molecule 2, hydroxyl-terminated branches. Chemical structures are accompanied with molecular models. (From ref. [73])... 

Calculation of the methyl branch is based on the assumption that one double bond exists per molecule. The methyl absorbance at 1378 cm"1, in excess of that required for methyls terminating the backbone, was interpreted as methyl branches. The value may be incorrect if the assumption is not valid. Also, a part or all of the methyl may correspond to long branches rather than methyl branch. The number average molecular weight by interpretation 1 is based on the assumption that there is one double bond per molecule. This assumes the presence of branches. 

The linear molecule (1) is called butane, or normal butane ( -butane), whereas the branched molecule (2) is methylpropane (rather than 2-methyl-propane, as the methyl group has to be in a 2-position). If the methyl group of (2) were attached to a terminal carbon, the resultant molecule would be the same as (1). Methylpropane (2) is also called /robutane. 

Nakanishi s method is based on the splitting of CD waves when two chromopho-res on a chiral molecule are close in space (the dibenzoate rule). Such is the case with dibromobenzoates of sugars which have a 1,2 or 1,3 relationship. In the first version of the method, saponins were permethylated, methanolyzed with acid and the liberated OH positions were p-bromobenzoylated (44). As terminal sugars are fully methylated and UV transparent, they need not be considered. Branched sugars (two substitutions at least) 3deld di-or tri-benzoates with exciton-split CD curves. The difference in Ae values of the two extrema of split CD curves is directly related to the respective positions of the benzoates (1, 2 eq-eq = 1, 2 eq-ax = 62 1, 2 ax-ax = 6 1, 3 eq-eq = 0 1, 3 eq-ax = 16). The sensitivity of circular dichroism makes the method suitable for microassays. Typical analyses are performed on a 100 pg scale (nanomolar levels). 

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