Interactions of cyclic AMP and its dibutyryl analogue with a lipid layer in the aqueous mixtures of monoolein preparation and dioleoyl phosphatidylcholine as probed by X-ray diffraction and Raman spectroscopy

Interactions of adenosine 3':5'-cyclic monophosphate (cAMP) and N-6,2'-O-dibutyryl-adenosine 3':5'-cyclic monophosphate (dbcAMP) with a lipid layer composed of monoolein-based preparation and dioleoyl phosphatidylcholine ( DOPC) were investigated by small-angle X-ray diffraction (SAXD) and Raman spectroscopy. The reversed hexagonal (H-II) MO/DOPC/H2O phase of 65:15:20 wt.% composition was selected as a reference system. SAXD revealed that entrapment (at the expense of water) of 3 wt.% cAMP into the reference system did not change the polymorphic form and structural parameters of the phase. The same content of dbcAMP induced the transition from the HII phase to the reversed bicontinuous cubic phase of space group Ia3d. This transition is explained by the increase of lipid head-group area due to the penetration of the acylated adenine group of dbcAMP into the polar/apolar region of lipid layer. The conclusion is supported by Raman spectroscopy, showing the disruption/weakening of hydrogen bonding in the MO/DOPC-based matrix at the N1- and N3-sites of the dbcAMP adenine ring. As distinct from dbcAMP, cAMP remains mostly in the water channels of the HII phase, although the phosphate residue of nucleotide interacts with the quaternary ammonium group of DOPC. Both nucleotides increase the population of gauche isomers in the DOPC choline group.