The exposure of guanine in the oligonucleotide 5’-d(TCGCT) to one-electron oxidants leads initially to the formation of the guanine radical cation G•+, its deptotonation product G(−H)• and, ultimately, to various two- and four-electron oxidation products via pathways that depend on the oxidants and reaction conditions. We utilized single or successive multiple laser pulses (308 nm, 1 Hz rate) to generate the oxidants CO3•− and SO4•− (via the photolysis of S2O82− in aqueous solutions in the presence and absence of bicarbonate, respectively) at concentrations/pulse that were ~20-fold lower than the concentration of 5’-d(TCGCT). Time-resolved absorption spectroscopy measurements following single-pulse excitation show that the G•+ radical (pKa = 3.9) can be observed only at low pH and is hydrated within ≥ 3 ms at pH 2.5, thus forming the two-electron oxidation product 8-oxo-7,8-dihydroguanosine (8-oxoG). At neutral pH, and single pulse excitation, the principal reactive intermediate is G(−H)• that at best, reacts only slowly with H2O, and lives for ≥ 70 ms in the absence of oxidants/other radicals to form base sequence-dependent intrastrand cross-links via the nucleophilic addition of N3-thymidine to C8-guanine (5'-G*CT* and 5'-T*CG*). Alternatively, G(−H)• can be oxidized further by reaction with CO3•− generating the two electron products 8-oxoG (C8 addition), and 5-carboxamido-5-formamido-2-iminohydantoin (2Ih, by C5 addition). The four-electron oxidation products, guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), appear only after a second (or more) laser pulses. The levels of all products, except 8-oxoG, which remains at a low constant value, increase with the number of laser pulses.

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.

We report the successful synthesis of previously inaccessible poly­(3-hydroxystyrene)-block-poly­(dimethylsiloxane) (P3HS-b-PDMS) block copolymers (BCPs) with varying volume fractions, molecular weights, and narrow dispersities by sequential living anionic polymerization. The chemical structure and molecular weight were fully characterized by 1H NMR and gel permeation chromatography. The BCP phase behavior was investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy. Temperature-resolved SAXS measurements from symmetric disordered sample were used to determine the interaction parameter (χ) using mean-field theory. The results provide an estimate for interaction parameter, χHS/DMS(T) = 33.491/T + 0.3126, with an upper bound value of 0.39 at 150 °C. The calculated χ for P3HS-b-PDMS is approximately 4 times higher than that observed in a commonly studied high-χ system, PS-b-PDMS. The ultrahigh interaction parameter observed here affords the formation of well-ordered materials at remarkably low molecular weight. The presence of both PDMS and P3HS provides significant versatility in terms of etch selectivity, while the hydroxystyrene domain offers additional functionality as it can be exploited for immobilizing functional organic moieties.

We examine the role of intrinsic chain susceptibility anisotropy in magnetic field directed selfassembly of a block copolymer using in situ X-ray scattering. Alignment of a lamellar mesophase is observed on cooling across the disorder-order transition with the resulting orientational order inversely proportional to the cooling rate. We discuss the origin of the susceptibility anisotropy, ∆χ, that drives alignment, and calculate its magnitude using coarse-grained molecular dynamics to sample conformations of surface-tethered chains, finding ∆χ ≈ 2 × 10 −8 . From field-dependent scattering data we estimate grains of ≈ 1.2 µm are present during alignment. These results demonstrate that intrinsic anisotropy is sufficient to support strong field-induced mesophase alignment and suggest a versatile strategy for field control of orientational order in block copolymers.

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.