Ternary complexes of poly(ethylene imine), single-stranded oligodeoxynucleotides and glutamic acid moieties

Abstract

Ternary non-covalent complexes composed of poly(ethylene imine) (P), a pentadeoxynucleotide (N) and glutamic acid monomer or dipeptide (E or EE) were prepared by mixing aqueous P, N and E (or EE) solutions in various molar ratios. Five nucleotides were examined, viz., d(TTTTT), d(CCCCC), d(AAAAA), d(GGGGG) and d(GCGAT). The compositions, solution stabilities and intrinsic stabilities of the ternary complexes (“terplexes”) were probed by electrospray ionization mass spectrometry (ESI-MS), tandem mass spectrometry (MS2) and ion mobility mass spectrometry (IM-MS). ESI-MS experiments confirmed the formation of terplexes with four of the five N molecules tested, the favored stoichiometry being 1:1:1 P-to-N-to-E (or EE) in all cases; d(GGGGG) did not form any detectable ternary complexes. Other compositions, involving higher order terplexes with multiple units or P, N and/or E (or EE), as well as several binary combinations, could be identified by IM-MS. The solution stabilities of the ternary complexes, assessed from their relative intensities in ESI mass spectra, depend on the sequence of N for PNE terplexes, maximizing with thymine-rich oligonucleotides. This selectivity is lost in the more weakly bound PN(EE) terplexes, whose binding interactions are barely influenced by the oligonucleotide sequence. Gas-phase (intrinsic) stabilities, assessed by dissociation extents in MS2 experiments, follow the same order as the corresponding solution stabilities, suggesting similar terplex structures in both media.