title: Incorporation of a cationic aminopropyl chain in DNA hairpins: thermodynamics and hydration.
abstract: We report on the physicochemical effects resulting from incorporating a 5-(3-aminopropyl) side chain onto a 2'-deoxyuridine (dU) residue in a short DNA hairpin. A combination of spectroscopy, calorimetry, density and ultrasound techniques were used to investigate both the helix-coil transition of a set of hairpins with the following sequence: d(GCGACTTTTTGNCGC) [N = dU, deoxythymidine (dT) or 5-(3-aminopropyl)-2'-deoxyuridine (dU*)], and the interaction of each hairpin with Mg(2+). All three molecules undergo two-state transitions with melting temperatures (T(M)) independent of strand concentration that indicates their intramolecular hairpin formation. The unfolding of each hairpin takes place with similar T(M) values of 64-66 degrees C and similar thermodynamic profiles. The unfavorable unfolding free energies of 6.4-6.9 kcal/mol result from the typical compensation of unfavorable enthalpies, 36-39 kcal/mol, and favorable entropies of approximately 110 cal/mol. Furthermore, the stability of each hairpin increases as the salt concentration increases, the T(M)-dependence on salt yielded slopes of 2.3-2.9 degrees C, which correspond to counterion releases of 0.53 (dU and dT) and 0.44 (dU*) moles of Na(+) per mole of hairpin. Absolute volumetric and compressibility measurements reveal that all three hairpins have similar hydration levels. The electrostatic interaction of Mg(2+) with each hairpin yielded binding affinities in the order: dU > dT > dU*, and a similar release of 2-4 electrostricted water molecules. The main result is that the incorporation of the cationic 3-aminopropyl side chain in the major groove of the hairpin stem neutralizes some local negative charges yielding a hairpin molecule with lower charge density.
authors.affiliation: Department of Pharmaceutical Sciences, University of Nebraska Medical Center, Omaha, NE 68198, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Soto
authors.forename: B I
authors.initials: BI
authors.lastname: Kankia
authors.forename: P
authors.initials: P
authors.lastname: Dande
authors.forename: B
authors.initials: B
authors.lastname: Gold
authors.forename: L A
authors.initials: LA
authors.lastname: Marky
mesh_terms: D001483:Base Sequence; D002151:Calorimetry; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D008274:Magnesium / Q000737:chemistry; D009690:Nucleic Acid Conformation; D009691:Nucleic Acid Denaturation; D009841:Oligonucleotides / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D013816:Thermodynamics; D014465:Ultrasonics; D014867:Water / Q000737:chemistry
chemical_list: D009841:Oligonucleotides; D014867:Water; D004247:DNA; D008274:Magnesium