Model studies of the interfacial ordering of oleanolic acid in the cuticula.

Triterpenoids represent a promising and expanding platform for biologically active natural compounds whose capability to unique and potentially usable biological effects is currently only partly utilized. They are present in a variety of common plants and fruits and the triterpenoid isolation and identification from such natural sources is of special interest. From the biological standpoint, oleane triterpenoids belong to the most important triterpenoid structures. Besides the continuing extraction and isolation of natural material, synthetic derivatives with higher therapeutic potential have been developed. Oleanolic acid is an omnipresent triterpenoid in plant kingdom, medical herbs and is integral part of the human diet. It is one of the most important components of the cuticular membrane of plant leaves and fruits. Important functions for the performance of the cuticular membrane, such as protection from diseases, insects and drought, are related to the composition and molecular arrangement of the epicuticular waxes. Recent pharmacological activities on the basis of oleanolic acid comprised the isolation and purification of oleanolic acid from various plants and herbs, the chemical modifications to create more effective derivatives, the toxicity studies and the clinical use in various diseases including anticancer chemotherapies. The omnipresence of oleanolic acid in epicuticular waxes suggests its important role for the cuticle performance. Though nature and chemical composition of the epicuticular waxes are variable, most of the cuticle components are amphiphiles so that a layered structure of the cuticle morphology can be expected. However, the chemical structure of oleanolic acid with a multicyclic planar structure deviates from the typical structure of amphiphiles consisting of a polar head group and alkyl chain(s). Despite the untypical amphiphilic character of oleanolic acid, it is located exclusively in the cuticula. Therefore, studies of well-defined model monolayers at the air/water interface are a promising approach for getting first insight into the specific behaviour of oleanolic acid. A recent paper about mixed oleanolic acid/stearic acid monolayers could not provide convincing information about the structural ordering of pure oleanolic monolayers. Therefore the present work focuses on the interfacial ordering of pure oleanolic acid monolayers. Grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity (SR) measurements have proven to be optimal on the basis of lateral pressure results. First information about the features of the oleanolic acid spread on pure water at 25°C can be obtained from π-A isotherm shown in Fig. 1. The chemical structure in the inset of Fig. 1 indicates the untypical amphiphilic character of the bulky triterpenoid oleanolic acid. It is seen that a correct π-A isotherm for oleanolic acid monolayers can be measured only up to π ~ 8 mN·m. In the 0 ≤ π ≤ 8 mN·m range, repeated compression and expansion confirmed the measured shape of the isotherm. This result shows unambiguously that oleanolic acid forms insoluble monolayers. However, the extreme stiffness of the compressed monolayers prevents artifactfree recording of π-A isotherms up to high lateral pressures so that at π > 8 mN·m the recorded π values deviate increasingly from the expected real value.