Interfacial properties of uracil films adsorbed at the mercury/electrolyte interface

Remarkable Differences in Spin Couplings for Various Self-Paired Dimers of Ring-Expansion-Radicalized Uracil: A Basis for the Design of Magnetically Anisotropic Assemblies

The spin-coupling properties of a series of radicalized uracil (rU) dimer diradicals with different H-bonding modes is examined. Each rU has four double H-bonding sites [the amide units: two at the Watson-Crick face (upper site WC₁ and lower site WC₂ ), a Hoogsteen site (HO), and a minor-groove site (MI)], and ten homogeneous dimers (rU-rU) can self-pair with well-defined diradical characters and comparable stability to the native U dimers. More interestingly, all these dimers exhibit distinctly different spin-coupling characters (ferromagnetic (FM) versus antiferromagnetic (AFM) and large- versus small-magnitude spin couplings), indicative of remarkable magnetic-coupling anisotropy of rU. This observation originates from the fusion of a cyclopentadienyl radical to U, which leads to uneven spin-density distribution. In rU, the fused five-membered radical ring can spin-polarize to the edge in the minor groove, and thus dimerization of rU leads to different H-bonded structures with remarkably different magnetic couplings. The calculated larger magnetic coupling constants J are 1003.7 and 540.2 cm^-1 for the WC₂ -HO and MI-HO H-bonding modes between rU, which exhibit considerably large FM couplings, the MI-MI, WC₁ -WC₂ and WC₂ -WC₂ modes show moderate FM couplings (J=0.4-77 cm^-1 ), and the other modes exhibit moderate or weak AFM couplings. These observations indicate that the HO and MI sites are favorable spin-coupling sites. In addition, the H-bond lengths and electronic structures of the H-bonding sites, proton transfer, and extra H-bonding interaction with the surroundings can also affect the magnetic couplings of the base pairs. Clearly, the unique magnetic coupling anisotropy of rU provides a promising application basis for the design and assembly of bio-inspired anisotropically magnetic membranes and even magnetism-tunable building blocks for novel magnetic nanoscale devices.

Condensation of nucleobases at mercury/aqueous solution interface--a structural perspective using hydrogen bonding considerations

The two-dimensional condensation behavior exhibited by nucleobases at a mercury/aqueous solution interface is analyzed on the basis of their hydrogen-bonded quadruplex structures, and the experimentally observed critical temperatures are rationalized incorporating different types of hydrogen bonding, surface coordination effects, and stacking considerations. The proposed methodology provides a structural basis for the interpretation of critical temperatures and enables the calculation of the same pertaining to different modified nucleobases. The applicability of the procedure to order-disorder transitions of water dipoles at Hg electrodes is also indicated.