THE USE OF HIGH-RESOLUTION SOLID-STATE NMR-SPECTROSCOPY AND DIFFERENTIAL SCANNING CALORIMETRY TO STUDY INTERACTIONS OF ANESTHETIC STEROIDS WITH MEMBRANE

We have used a combination of high-resolution solid-state C-13-NMR and DSC (differential scanning calorimetry) to study the distinctively di fferent thermotropic and dynamic properties of the anaesthetic steroid alphaxalone and its inactive congener Delta(16)-alphaxalone in dipalm itoylphosphatidylcholine (DPPC) model membranes, In the solid-state C- 13-NMR, the techniques included cross polarization (CP) and/or magic a ngle spinning (MAS). The observed data revealed the following importan t results. (a) DSC as a bulk method showed that the active steroid low ers the main phase transition temperature and broadens the pretransiti on more significantly than the inactive congener. The C-13-CP/MAS expe riments allowed us to detect the pretransition temperature in the alph axalone-containing preparation, which was not discernible in DSC. (b) The chemical shift values varied with temperature, indicating differen t degrees of trans-gauche isomerization in the lipid acyl chains when the bilayer is in the liquid crystalline phase. (c) Only specific addi tional peaks appeared in the C-13-CP/MAS spectra when each of the ster oids was present in the preparation. Delta(16)-alphaxalone gives rise to more additional peaks than alphaxalone, indicating a different mobi lity of the corresponding molecular moiety in the phospholipid bilayer environment. (d) The relative intensities of these peaks also confirm ed that alphaxalone is fully incorporated in the bilayer, whereas Delt a(16)-alphaxalone is only partially so. These results suggest that the differential effects of these two analogues in the membrane may, at l east in part, explain the reason for their different biological activi ties. (C) 1997 Elsevier Science B.V.