The Determination of the Molecular Order of Chiral Monolayers at the Air−water Interface by Infrared Reflection−absorption Spectroscopy −A Bridge between Physico- and Biochemistry

The importance of inter- and intramolecular hydrogen-bond formation as well as of the complex formation with bivalent counterions in the aqueous subphase (Pb2+, Zn2+) for chiral discrimination in monolayers was studied in the presence of N-acylamino acid derivatives and 2-hydroxyhexadecanoic acid (HHDA) using Langmuir-trough measurements (“macroscopic information”) and infrared reflection-absorption spectroscopy (IRRAS) (“molecular order of the film molecules”). For L-and DL-N-octadecanoyltyrosine and its methyl ester, respectively, both methods suggest preferential heterochiral interactions at areas > 0.35 nm2 molecule−1, while upon compression of the monolayer, homochiral interactions are prevailing. However, in the case of HHDA monolayers, the macroscopic information inferred from Π-A isotherms and the IRRAS results are in contradiction. Although both in the presence of Pb2+ and Zn2+ cations the Π-A isotherms of the L-enantiomer exhibit the more condensed characteristics, the IRRAS measurements revealed that Pb2+ cations induce heterochiral interactions in compressed HHDA films, while the presence of Zn2+ leads to homochiral interactions.