The structure of micelles formed by a novel group of synthetic phospholipid amphiphiles functionalized with complementary nucleic bases, dioctanoylphosphatidyl-adenosine (diC(8)P-Ade), dioctaneylphosphatidyluridine (diC(8)P-Uri), 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 concentration. In the explored pH range (3.5-7.5), pure diC(8)P-nucleosides form quasispherical micelles with an axial ratio smaller than 2, composed of 30-45 monomers. These micellar aggregates do not grow as lipid concentration increases, while the homologous phosphatidylcholine forms rodlike micelles with an aggregation number of about 500, Micellar charge strongly depends on the pH, indicating a progressive charging of the phosphatidylnucleoside micelle polar headgroup. The 1:1 mixture of the two phospholiponucleosides shows deviations from the ideal behavior, indicating specific interactions at the micelle aggregate polar headgroups region. The analysis of SANS data shows that the recognition process between the two complementary bases of the pho;phosphatidylnucleoside. demostrated by NMR, UV. and CD data, is associated 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 scale without perturbing the overall structure of the system.