MOLECULAR RECOGNITION IN MONOLAYERS - COMPLEMENTARY BASE-PAIRING IN DIOLEOYLPHOSPHATIDYL DERIVATIVES OF ADENOSINE, URIDINE, AND CYTIDINE

Chemical recognition by base complementarity in DNA and RNA is strictl y related to their stereochemical order. The way in which this high st ereoregular order has been achieved in a prebiotic world is not fully understood yet. More primitive systems that display complementary base recognition as a prerequisite to information and, eventually, self-re plication might represent a possible route. This study investigates ph osphatidylnucleosides bearing complementary bases, adenine and uridine , that can mutually recognize each other, giving mixed structures with features characteristic of complementary base pairing. Dioleoylphosph atidyl derivatives of adenosine (DOP-adenosine), uridine (DOP-uridine) , and cytidine (DOP-Cytidine) have been studied at the water-air inter face as a function of pH and subphase composition. When monovalent cat ions (Li+, Na+, and K+) are dissolved in the subphase, the phosphatidy l derivative monolayers show expansion or compression depending on the cation nature. In particular DOP-adenosine shows a preferential inter action with Li+. The properties of mixtures of the DOP-adenosine/DOP-u ridine complementary bases mere investigated and compared to those of the non-complementary bases (DOP-adenosine/DOP-cytidine) The results i ndicate a preferential interaction in a hydrophilic environment only f or complementary nucleophospholipids at physiological pH, suggesting t hat the specific interfacial orientation of the phospholiponucleoside imposed by the interface promotes the molecular recognition between th e two complementary bases in a way that resembles the Watson-Crick pai ring In natural nucleic acids. Moreover, mixed monolayers of adenosine -uridine derivatives show a minimum of the free energy of mixing for D OP-uridine rich mixtures (around the DOP-adenosine/DOP-uridine = 0.2-0 .3 mole fraction) close to the stoichiometry of the trimeric adduct (u ridine)(2) adenosine that forms in highly concentrated solutions of ur idine and adenosine, where adenosine displays simultaneously the Watso n-Crick and the Hoogsten hydrogen bond patterns.