Conformational behavior of oxygenated mycobacterial mycolic acids from Mycobacterium bovis BCG.

Phase diagrams of Langmuir monolayers of oxygenated mycolic acids, i.e. methoxy mycolic acid (MeO-MA), ketomycolic acid (Keto-MA), and artificially obtained deoxo-mycolic acid (deoxo-MA) from Mycobacterium bovis BCG were obtained by thermodynamic analysis of the surface pressure (pi) vs. average molecular area (A) isotherms. At lower temperatures and lower surface pressures, both Keto- and MeO-MAs formed rigid condensed monolayers where each MA molecule was considered to be in a 4-chain form, in which the three carbon chain segments due to bending of the 3-hydroxy aliphatic carboxylate chain and the 2-side chain were in compact parallel arrangement. At higher temperatures and surface pressures, MeO-MA and deoxo-MA tended to take stretched-out conformations in which the 3-hydroxy aliphatic carboxylate chain was more or less in an extended form, but Keto-MA retained the original 4-chain structure. The thickness measurement of the monolayers in situ by ellipsometry at different pi values and temperatures supported the above conclusions derived from the phase diagrams. The enthalpy changes associated with the phase transitions of MeO-MA and deoxo-MA implied that the MeO-MA needed larger energy to change from a compact conformation to an extended one, possibly and partly due to the dehydration of the methoxy group from water surface involved. Molecular dynamics studies of MA models derived from Monte Carlo calculations were also performed, which confirmed the conformational behavior of MAs suggested by the thermodynamic studies on the Langmuir monolayers.

Characteristics of monolayer behavior and multilayer film structures of comb polymers having different kinds of fluorocarbon side-chains.

With comb polymers with different kinds of fluorocarbon side-chains, significant effects of spreading solvents on the monolayers at the air/water interface and the molecular arrangement in the deposited films could be found by the use of trifluoroacetic acid and its mixed solvents with n-hexane, depending on the atoms, fluorine or hydrogen, substituted at the end of the fluorocarbon side-chains. These facts are probably due to intralayer hydrogen bonding cooperating with the carbonyl groups of the long-chain esters. The deposited films of solid substrates were characterized by polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as well as in-plane X-ray diffraction and scanning electron and atomic force microscopy.