Characteristics of pyrene phospholipid/gamma-cyclodextrin complex

Recently, it was demonstrated that gamma -cyclodextrins (gamma -CDs) greatl y accelerates transfer of hydrophobic pyrene-labeled and other fluorescent phospholipid derivatives from vesicles to cells in culture (Tanhuanpaa and Somerharju, 1999). To understand better the characteristics of this process , we studied the interaction of gamma -CD with pyrene-labeled phosphatidylc holines (PyrPCs) using a variety of physical methods. Either one or both of the acyl chains of PC was labeled with a pyrene moiety (monoPyrPCs and diP yrPCs, respectively), and the length of the labeled chain(s) varied from 4 to 14 carbons. Fluorescent binding assays showed that the association const ant decreases strongly with increasing acyl chain length. PyrPC/gamma -CD s toichiometry was 1:2 for the shorter chain species, but changed to 1:3 when the acyl chain length exceeded 8 (diPyrPCs) or 10 (monoPyrPCs) carbons. Th e activation energy for the formation of diPyr(10)PC/gamma -CD complex was high, i.e., +92 kJ/mol, indicating that the phospholipid molecule has to fu lly emerge from the bilayer before complex formation can take place. The fr ee energy, enthalpy, and entropy of transfer of monoPyrPC from bilayer to g amma -CD complex were close to zero. The absorption, Fourier transform infr ared, and fluorescence spectral measurements and lifetime analysis indicate d that the pyrene moiety lies inside the CID cavity and is conformationally restricted, particularly when the labeled chain is short. The acyl chains of a PyrPC molecule seem to share a CID cavity rather than occupy different ones. The present data provide strong evidence that the ability of gamma - CD to enhance intermembrane transfer of pyrene-labeled phospholipids is bas ed on the formation of stoichiometric complexes in the aqueous phase. This information should help in designing CID derivatives that are more efficien t lipid carriers then those available at present.