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 nm(2) molecule(-1) while upon compression of the monolayer, homochiral interactions are prevailing. However, in the case of HHDA monolayers, the macroscopic information inferred from Pi-A isotherms and the IRRAS results are in contradiction. Although both in the presence of Pb2+ and Zn2+ cations the Pi-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.