Substituent Effect on Intramolecular Hydrogen Bonding in beta-Amino Acid-Containing Polyamides.
An alkyl substitution in the beta-amino acid-containing polyamides 1, 2, and 3 leads to an increase in the population of intramolecularly hydrogen bonded conformations. However, the most stable conformation is not always the same one when the substituent is changed from a methyl group to an isopropyl group. For the beta-amino acid derivatives of succinic acid, a methyl group enhances the formation of a head-to-tail type of folding pattern through an 11-membered ring (2b), but an isopropyl group appears to decrease the enhancement. For beta-amino acid derivatives of glutaric acid, an isopropyl group promotes the formation of a bifurcated conformation (3c). The thermodynamic parameters for the equilibrium between non-hydrogen-bonded states and the head-to-tail type of folding of 2a and 2b are obtained by a van't Hoff analysis of the variable-temperature NMR data, which gives a DeltaH of -1.0 kcal/mol and a DeltaS of -4.7 eu for triamide 2a and a DeltaH of -1.0 kcal/mol and a DeltaS of -3.7 e.u. for triamide 2b, each with a correlation coefficient of better than 0.99. The increased proportion of the intramolecular amide-amide hydrogen bond observed for triamide 2b is entirely due to entropic effects according to this analysis.
Gung
Benjamin W.
BW
Department of Chemistry, Miami University, Oxford, Ohio 45056.
MacKay
James A.
JA
Zou
Dong
D