Temperature Dependence of Chiral Discrimination in Langmuir Monolayers of <i>N</i>-Acyl Amino Acids As Inferred from Π/<i>A</i> Measurements and Infrared Reflection−Absorption Spectroscopy

Abstract

N-Hexadecanoylalanine monolayers were investigated on pure aqueous subphases and in the presence of 1 mM solutions of CaCl2 and ZnCl2, respectively, in the temperature range between 293 and 308 K using Langmuir trough measurements and IR reflection−absorption spectroscopy (IRRAS). The overwhelming importance of the temperature for chiral recognition processes was particularly clearly shown in the presence of Zn2+ in the subphase, where a change from homo- to heterochiral preference was observed by IRRAS measurements within a temperature range of 5 deg only. This change was not reflected by corresponding changes in Langmuir curves. This result implies that chiral interactions on a molecular scale inferred from IRRAS and on a macroscopic scale (Langmuir curves) may be at variance because of a different importance of hydrogen bond and complex formation at these scales. Calcium ions exert strong expanding effects, thus weakening the homochiral effect on the macroscopic level and even inducing a heterochiral effect on the molecular level at 293 K. A comparison between the results obtained in the presence of N-hexadecanoylalanine and its methyl ester, respectively, supports the hypothesis that hydrogen bond formation via the carboxyl group also plays an important role for chiral recognition.