Yekaterina Rokhlenko

Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

The mechanistic aspects of hydration of guanine radical cations, G•+ in double- and single-stranded oligonucleotides were investigated by direct time-resolved spectroscopic monitoring methods. The G•+ radical one-electron oxidation products were generated by SO4•– radical anions derived from the photolysis of S2O82– anions by 308 nm laser pulses. In neutral aqueous solutions (pH 7.0), after the complete decay of SO4•– radicals (∼5 μs after the actinic laser flash) the transient absorbance of neutral guanine radicals, G(-H)• with maximum at 312 nm, is dominant. The kinetics of decay of G(-H)• radicals depend strongly on the DNA secondary structure. In double-stranded DNA, the G(-H)• decay is biphasic with one component decaying with a lifetime of ∼2.2 ms and the other with a lifetime of ∼0.18 s. By contrast, in single-stranded DNA the G(-H)• radicals decay monophasically with a ∼ 0.28 s lifetime. The ms decay component in double-stranded DNA is correlated with the enhancement of 8-oxo-7,8-dihydroguanine (8-oxoG) yields which are ∼7 greater than in single-stranded DNA. In double-stranded DNA, it is proposed that the G(-H)• radicals retain radical cation character by sharing the N1-proton with the N3-site of C in the [G•+:C] base pair. This [G(-H)•:H+C ⇆ G•+:C] equilibrium allows for the hydration of G•+ followed by formation of 8-oxoG. By contrast, in single-stranded DNA, deprotonation of G•+ and the irreversible escape of the proton into the aqueous phase competes more effectively with the hydration mechanism, thus diminishing the yield of 8-oxoG, as observed experimentally.

Experimental evidence for proposed transformation pathway from the inverse hexagonal to inverse diamond cubic phase from oriented lipid samples

A macroscopically oriented inverse hexagonal phase (HII) of the lipid phytantriol in water is converted to an oriented inverse double diamond bicontinuous cubic phase (QII(D)). The initial HII phase is uniaxially oriented about the long axis of a capillary with the cylinders parallel to the capillary axis. The HII phase is converted by cooling to a QII(D) phase which is also highly oriented, where the cylindrical axis of the former phase has been converted to a ⟨110⟩ axis in the latter, as demonstrated by small-angle X-ray scattering. This epitaxial relationship allows us to discriminate between two competing proposed geometric pathways to convert HII to QII(D). Our findings also suggest a new route to highly oriented cubic phase coatings, with applications as nanomaterial templates.

Controlling orientational order in block copolymers using low-intensity magnetic fields

Significance Magnetic field interactions with condensed matter can produce orientationally ordered states that are important for fundamental research and technological applications. Block copolymer (BCP) mesophases typically exhibit weak field coupling, requiring high-intensity fields generated by superconducting magnets to produce such states. This work advances a strategy for circumventing such field intensity limitations and creates highly aligned mesophases using fields an order of magnitude smaller than typically required and that can be produced by simple permanent magnets. We elucidate the roles of molecular mobility, grain size, and ordering kinetics on the mesophase field response. Low-intensity field-directed BCP ordering has potentially profound implications for processing functional materials and developing complex textures by field shaping. The interaction of fields with condensed matter during phase transitions produces a rich variety of physical phenomena. Self-assembly of liquid crystalline block copolymers (LC BCPs) in the presence of a magnetic field, for example, can result in highly oriented microstructures due to the LC BCP’s anisotropic magnetic susceptibility. We show that such oriented mesophases can be produced using low-intensity fields (<0.5 T) that are accessible using permanent magnets, in contrast to the high fields (>4 T) and superconducting magnets required to date. Low-intensity field alignment is enabled by the addition of labile mesogens that coassemble with the system’s nematic and smectic A mesophases. The alignment saturation field strength and alignment kinetics have pronounced dependences on the free mesogen concentration. Highly aligned states with orientation distribution coefficients close to unity were obtained at fields as small as 0.2 T. This remarkable field response originates in an enhancement of alignment kinetics due to a reduction in viscosity, and increased magnetostatic energy due to increases in grain size, in the presence of labile mesogens. These developments provide routes for controlling structural order in BCPs, including the possibility of producing nontrivial textures and patterns of alignment by locally screening fields using magnetic nanoparticles.

Directing block copolymer self-assembly with permanent magnets: photopatterning microdomain alignment and generating oriented nanopores

Magnetic fields are useful for directing block copolymer (BCP) self-assembly, but to date such a field alignment has required large fields (>5 T) necessitating the use of superconducting magnets. We report an approach that circumvents this limitation by introducing labile reactive mesogens into a liquid crystalline (LC) BCP based on a norbornene backbone with a poly(lactide) minority block that forms hexagonally packed cylinders. The free mesogens co-assemble with the smectic A mesophase of the BCP and enable alignment at fields as low as 0.5 T. The remarkable field response originates from the combined effects of enhanced mobility and decreased segregation strength, and the presence of large micron-scale grains in the system. We demonstrate a robust alignment of mesogen-blended samples using simple permanent magnets. The etching of poly(lactide) yields nanoporous films, while the spatially selective microdomain immobilization by UV-induced crosslinking through a photomask provides a versatile mechanism for creating alignment patterns. We anticipate that the nanoporous materials as generated here may find application in membrane fabrication or BCP lithography, while the ability to spatially pattern alignment is promising for the design of mechanical metamaterials exploiting the shape memory effect of LC elastomers.

Hexagonally Ordered Arrays of α-Helical Bundles Formed from Peptide-Dendron Hybrids

Combining monodisperse building blocks that have distinct folding properties serves as a modular strategy for controlling structural complexity in hierarchically organized materials. We combine an α-helical bundle-forming peptide with self-assembling dendrons to better control the arrangement of functional groups within cylindrical nanostructures. Site-specific grafting of dendrons to amino acid residues on the exterior of the α-helical bundle yields monodisperse macromolecules with programmable folding and self-assembly properties. The resulting hybrid biomaterials form thermotropic columnar hexagonal mesophases in which the peptides adopt an α-helical conformation. Bundling of the α-helical peptides accompanies self-assembly of the peptide-dendron hybrids into cylindrical nanostructures. The bundle stoichiometry in the mesophase agrees well with the size found in solution for α-helical bundles of peptides with a similar amino acid sequence.