Micellar aggregates from short-chain phospholiponucleosides: A SANS study

The structure of micelles formed by a novel group of synthetic phospholipid amphiphiles functionalized with complementary nucleic bases, dioctanoylpho sphatidyl-adenosine (diC(8)P-Ade), dioctaneylphosphatidyluridine (diC(8)P-U ri), and their 1/1 mole ratio mixture, has been studied at 37 degrees C by small-angle neutron scattering, SANS, as a function of pH and amphiphile co ncentration. In the explored pH range (3.5-7.5), pure diC(8)P-nucleosides f orm quasispherical micelles with an axial ratio smaller than 2, composed of 30-45 monomers. These micellar aggregates do not grow as lipid concentrati on increases, while the homologous phosphatidylcholine forms rodlike micell es with an aggregation number of about 500, Micellar charge strongly depend s on the pH, indicating a progressive charging of the phosphatidylnucleosid e micelle polar headgroup. The 1:1 mixture of the two phospholiponucleoside s shows deviations from the ideal behavior, indicating specific interaction s at the micelle aggregate polar headgroups region. The analysis of SANS da ta shows that the recognition process between the two complementary bases o f the pho;phosphatidylnucleoside. demostrated by NMR, UV. and CD data, is a ssociated with a consistent decrease of the mean area per polar headgroup, while the aggregation number and the micellar shape remain almost unchanged . The recognition process occurs. as in biological systems, on a local scal e without perturbing the overall structure of the system.