title: Parallel-stranded DNA with mixed AT/GC composition: role of trans G.C base pairs in sequence dependent helical stability.
abstract: Parallel-stranded (ps) DNAs with mixed AT/GC content comprising G.C pairs in a varying sequence context have been investigated. Oligonucleotides were devised consisting of two 10-nt strands complementary either in a parallel or in an antiparallel orientation and joined via nonnucleotide linkers so as to form 10-bp ps or aps hairpins. A predominance of intramolecular hairpins over intermolecular duplexes was achieved by choice of experimental conditions and verified by fluorescence determinations yielding estimations of rotational relaxation times and fractional base pairing. A multistate mode of ps hairpin melting was revealed by temperature gradient gel electrophoresis (TGGE). The thermal stability of the ps hairpins with mixed AT/GC content depends strongly on the specific sequence in a manner peculiar to the ps double helix. The thermodynamic effects of incorporating trans G.C base pairs into an AT sequence are context-dependent: an isolated G. C base pair destabilizes the duplex whereas a block of > or =2 consecutive G.C base pairs exerts a stabilizing effect. A multistate heterogeneous zipper model for the thermal denaturation of the hairpins was derived and used in a global minimization procedure to compute the thermodynamic parameters of the ps hairpins from experimental melting data. In 0.1 M LiCl at 3 degrees C, the formation of a trans G.C pair in a GG/CC sequence context is approximately 3 kJ mol(-)(1) more favorable than the formation of a trans A.T pair in an AT/TA sequence context. However, GC/AT contacts contribute a substantial unfavorable free energy difference of approximately 2 kJ mol(-)(1). As a consequence, the base composition and fractional distribution of isolated and clustered G.C base pairs determine the overall stability of ps-DNA with mixed AT/GC sequences. Thus, the stability of ps-DNA comprising successive > or =2 G.C base pairs is greater than that of ps-DNA with an alternating AT sequence, whereas increasing the number of AT/GC contacts by isolating G.C base pairs exerts a destabilizing effect on the ps duplex. Molecular modeling of the various helices by force field techniques provides insight into the structural basis for these distinctions.
authors.affiliation: Engelhardt Institute of Molecular Biology, Russian Academy of Sciences, Moscow, Russia.
authors.forename: A K
authors.initials: AK
authors.lastname: Shchyolkina
authors.forename: O F
authors.initials: OF
authors.lastname: Borisova
authors.forename: M A
authors.initials: MA
authors.lastname: Livshits
authors.forename: G E
authors.initials: GE
authors.lastname: Pozmogova
authors.forename: B K
authors.initials: BK
authors.lastname: Chernov
authors.forename: R
authors.initials: R
authors.lastname: Klement
authors.forename: T M
authors.initials: TM
authors.lastname: Jovin
mesh_terms: D001482:Base Composition; D020029:Base Pairing; D002942:Circular Dichroism; D003198:Computer Simulation; D004247:DNA / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009838:Oligodeoxyribonucleotides / Q000737:chemistry; D013696:Temperature; D013816:Thermodynamics
chemical_list: D009838:Oligodeoxyribonucleotides; D004247:DNA