Solvent Effect on Chiral Aggregate Formation of Acylamino Acids

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We studied formation of chiral aggregates of optical active N-lauroyl-L-valine (L-LVA), N-lauroyl-L-glutamic acid (L-LGA) and methylester of L-LVA (L-LVMe) in various solvents such as acetonitrile, 1,4-dioxane, tetrahydrofuran and 1,2-dichloroethane through the measurements of circular dichroism (CD), NMR chemical shift of N-H proton and vapor-pressure osmometry (VPO). The above acylamino acids and L-LVMe showed CD bands at 212-215 nm, whose intensities depended on temperature. The CD bands have been believed to come from the formation of chiral aggregates. However, by means of NMR and VPO analyses, the acylamino acids and L-LVMe were found to be present mostly as monomers in relatively polar solvents such as acetonitrile, 1,4-dioxane and tetrahydrofuran. On the other hand, L-LVA in 1,2-dichloroethane forms chiral aggregates; the mean aggregation number of L-LVA increases with increasing in its concentration.
2002-11-01