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A *H and 13C NMR study of the interaction between sodium deoxycholate (NaDC) micelles and a nitroxide probe (cholestane spin-label, CSL) is presented. It is shown that CSL primarily interacts with the ß face of NaDC. The NMR relaxation data enable the spatial geometry of the interaction to be defined, van der Waals energy calculations show that the CSL-NaDC interaction corresponds to a minimum if a helical model, previously proposed, is assumed for the NaDC micelle. At high concentrations of CSL, the CSL-NaDC interaction changes and the a face of NaDC may also become involved in the interaction.* To whom the correspondence should be addressed. f Eniricerche spa.

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Through the joint use of CD, Fourier transform ir (FTIR), and attenuated total reflectance FTIR we have found that synthetic polypeptide models of the Plasmodium falciparum circumsporozoite (CS) protein repeat domain bind calcium ions in helicogenic environments. Ca(2+)-(NANP)n complexes (n greater than or equal to 20) interact vectorially with model phospholipid membranes orienting their polypeptide axes preferentially along those of the lipid acyl chains. It is proposed that the P. falciparum CS protein central region, rather than acting as a molecular lure helping the parasite to evade host immune control, plays, as a specific Ca2+ macroligand, a critical functional role during attachment, invasion, and development of the malaria parasite in the hepatic cell.