**2. Air/water interface as a biological model membrane**

All the biological cells are covered with membranes, through which selective materials are allowed to diffuse in and/or out. It is needless to say that almost all biological activities are intermediated by membranes.

In a real biological membrane system, there always exists specific intermolecular interaction among the membrane-forming components, which is essential for biological activity however sometimes hinders simple understanding of the mechanism. Air/water interface across which dielectric constant changes drastically is a similar self-assembly field to biological interface without particular intermolecular interaction and therefore an ideal model for a biological membrane to study how a certain membrane component behaves there.

Interfacial tension or surface tension, when the interface is between air and water, *γ* is an essential thermodynamic property for studying interfacial phenomena. A monolayer formed at the air/water interface by a water-insoluble film-forming material is called "Langmuir monolayer." They have been widely studied by using a so-called trough, a shallow container of wide base area equipped with surface tensiometer and a movable barrier for changing the surface area. The performance of amphiphilic molecules such as MAs, lipids, or other biologically significant substance, at interfaces depends not only on the nature of the interface but also strongly on the environmental conditions such as temperature, pressure, pH and so on. Therefore, we studied the temperature effect on the surface tension and the molecular area in the interfacial monolayer of the representative MAs.

2 Will-be-set-by-IN-TECH

1991). Both in the monolayer on the water surface and in the proposed cell envelope lipid structure models, MA is considered to take the same structural arrangement, with the hydrophilic 3-hydroxy and 2-carboxyl groups touching the hydrophilic surface and with the aliphatic chains stretching out in parallel, and normal to the hydrophilic surface. Therefore, detailed studies on the artificial MA layers on water surface should help elucidation of the

Recently, limited Langmuir monolayer studies have been performed on a selection of MA (Hasegawa et al., 2000; 2002; Hasegawa & Leblanc, 2003; Hasegawa et al., 2003). Those studies reported that, in a compressed monolayer, *α*-mycolic acid from *Mycobacterium avium*, apparently took a conformation with three parallel chains, and on further compression, an extended structure, but that the corresponding *M. tb* mycolate appeared to take an extended conformation. As regards the MeO and Keto MAs from *M. tb*, they were reported to take triple chain folded conformations (Hasegawa & Leblanc, 2003; Hasegawa et al., 2003). Regrettably, their monolayer experiments were limited at a single temperature of 25 ◦C whereas temperature is one of the important factors that influence biological activities of the

In this chapter, the temperature effect on the Langmuir monolayer packing of all three *α*-, Keto-, and MeO-MAs from representative slow growing mycobacteria are analyzed to elucidate the conformational behavior of MAs in the monolayer and to understand their roles

All the biological cells are covered with membranes, through which selective materials are allowed to diffuse in and/or out. It is needless to say that almost all biological activities are

In a real biological membrane system, there always exists specific intermolecular interaction among the membrane-forming components, which is essential for biological activity however sometimes hinders simple understanding of the mechanism. Air/water interface across which dielectric constant changes drastically is a similar self-assembly field to biological interface without particular intermolecular interaction and therefore an ideal model for a biological

Interfacial tension or surface tension, when the interface is between air and water, *γ* is an essential thermodynamic property for studying interfacial phenomena. A monolayer formed at the air/water interface by a water-insoluble film-forming material is called "Langmuir monolayer." They have been widely studied by using a so-called trough, a shallow container of wide base area equipped with surface tensiometer and a movable barrier for changing the surface area. The performance of amphiphilic molecules such as MAs, lipids, or other biologically significant substance, at interfaces depends not only on the nature of the interface but also strongly on the environmental conditions such as temperature, pressure, pH and so on. Therefore, we studied the temperature effect on the surface tension and the molecular area

roles and the nature of actual mycolate layers in the mycobacterial cell envelope.

living cells.

in the mycobacterial cell envelope.

intermediated by membranes.

**2. Air/water interface as a biological model membrane**

membrane to study how a certain membrane component behaves there.

in the interfacial monolayer of the representative MAs.
