Etude conformationnelle des longues chaines en solution: Generalite d'un phenomene repliement au seuil de 9 atomes de carbone de chaine

Abstract

ΔpK values obtained by potentiometric titration, 13C Spin Latice relaxation time (T 1) measurements, and proton chemical shift-concentration relationhips are utilised to investigate the conformation of bifunctional aliphatic chains in solution. It was found that variations in the physical-constants in the physical-constants determined occured between n = 2 and n = 9. From 9 to n = 18 ΔpK values were constant and T 1 values were constant close together. No micellization occured at the concentration at which measurements were made. The validity of the use of the electrostatic theory for unequivocal explanation of ΔpK values is demonstrated; in addition, the effect of the value of the microscopic dielectric constant is also invoked. A conformational analysis is used to explain the global effect of all parameters examined. A correlation between the average conformation in solution and the effect of these diammonium salts on DNA was discussed. The results are interpreted in terms of electrostatic interactions and torsional and hydrophobic energies for the ionic bifunctional compounds: from n = 2 to n = 5 the length chain is maximal. From n = 5 to n = 9 length is not maximal but it is increasing regularly; from n = 9 to n = 18 the distance between the two ionised groups is approximately constant, and the conformation of the chain is coiled. The results also seem to be valid for diacids, and the conformation of similar diionic compounds may reflect an intramolecular organization preceding the pre-miscellization stage.