Our understanding of oxidative damage to double helical DNA and the design of DNA-based devices for molecular electronics is crucially dependent upon elucidation of the mechanism and dynamics of electron and hole transport in DNA. Electrons and holes can migrate from the locus of formation to trap sites, and such migration can occur through either a single-step "superexchange" mechanism or a multistep charge transport "hopping" mechanism. The rates of single-step charge separation and charge recombination processes are found to decrease rapidly with increasing transfer distances, whereas multistep hole transport processes are only weakly distance dependent. However, the dynamics of hole transport has not yet been directly determined. Here we report spectroscopic measurements of photoinduced electron transfer in synthetic DNA that yield rate constants of approximately 5 x 10(7) s(-1) and 5 x 10(6) s(-1), respectively, for the forward and return hole transport from a single guanine base to a double guanine base step across a single adenine. These rates are faster than processes leading to strand cleavage, such as the reaction of guanine cation radical with water, thus permitting holes to migrate over long distances in DNA. However, they are too slow to compete with charge recombination in contact ion pairs, a process which protects DNA from photochemical damage.

The driving force dependence of photoinduced electron-transfer dynamics in duplex DNA has been investigated for 16 synthetic DNA hairpins in which an acceptor chromophore serves as a linker connecting two complementary oligonucleotide arms containing a single donor nucleobase located either adjacent to the linker or separated from the linker by two unreactive base pairs. The rate constants for both charge separation and charge recombination processes have been determined by means of subpicosecond time-resolved transient absorption spectroscopy and the results analyzed using quantum mechanical Marcus theory. This analysis provides intimate details about electron-transfer processes in DNA including the distance dependence of the electronic coupling between the acceptor and nucleobase donor and the solvent and nuclear reorganization energies.

The dynamics of photoinduced charge separation and charge recombination in synthetic DNA hairpins have been investigated by means of femtosecond and nanosecond transient spectroscopy. The hairpins consist of a stilbene linker connecting two complementary 6-mer or 7-mer oligonucleotide strands. Base pairing between these strands results in formation of hairpins in which the stilbene is approximately parallel to the adjacent base pair. The singlet stilbene is selectively quenched by guanine, but not by the other nucleobases, via an electron-transfer mechanism in which the stilbene singlet state is the electron acceptor and guanine is the electron donor. In a hairpin containing only A:T base pairs, no quenching occurs and the restricted geometry results in a long stilbene lifetime and high fluorescence quantum yield. In families of hairpins which contain a single G:C base pair at varying locations in the hairpin stem, the stilbene fluorescence lifetime and quantum yield decrease as the stilbene-guanine distance decreases. Transient absorption spectroscopy is used to monitor the disappearance of the stilbene singlet and the formation and decay of the stilbene anion radical. Analysis of these data provides the rate constants for charge separation and charge recombination. Both processes show an exponential decrease in rate constant with increasing stilbene-guanine distance. Thus, electron transfer is concluded to occur via a single-step superexchange mechanism with a distance dependence β ) 0.7 Å -1 for charge separation and 0.9 Å -1 for charge recombination. The rate constants for charge separation and charge recombination via polyA vs polyT strands are remarkably similar, slightly larger values being observed for polyA strands. The dynamics of electron transfer in hairpins containing two adjacent G:C base pairs have also been investigated. When the guanines are in different strands, the second guanine has little effect on the efficiency or dynamics of electron transfer. However, when the guanines are in the same strand, somewhat faster charge separation and slower charge recombination are observed than in the case of hairpins with a single G:C base pair. Thus, the GG step functions as a shallow hole trap. The relationship of these results to other theoretical and experimental studies of electron transfer in DNA is discussed.

Evaluation of prophylactic treatment of haemophilia requires sensitive methods. To design and test a new magnetic resonance imaging (MRI) scale for haemophilic arthropathy, two scales of a combined MRI scoring scheme were merged into a single scale which includes soft tissue and osteochondral subscores. Sixty-one joint MRI's of 46 patients with haemophilia were evaluated by four radiologists using the new and older scales. Forty-six of the joints were evaluated using two X-ray scales. For all MRI scores, interreader agreement and correlations with X-ray scores and lifetime number of haemarthroses were analysed. The interreader agreement intraclass correlation coefficient was 0.82, 0.89 and 0.88 for the soft tissue and osteochondral subscores and the total score, as evaluated according to the new MRI scale, compared to 0.80 and 0.89 as for the older scales. The total score and osteochondral subscore according to the new scale, as well as scores according to the older scales were correlated (P < 0.01) with number of haemarthroses (Spearman correlation 0.35-0.68) and with the X-ray scores (Spearman correlation 0.40-0.76), but no correlation (P > 0.05) was found between the soft tissue subscore of the new MRI scale and the X-ray scores. The new MRI scale is simpler to apply than the older and has similar reader reliability and correlation with lifetime number of haemarthroses, and by separating soft tissue and osteochondral changes it gives additional information. The new scale is useful for analyses of early and moderate stages of arthropathy, and may help to evaluate prophylactic haemophilia treatment.

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We have prepared a green chromophore, 1,7-bis(pyrrolidin-1-yl)-3,4:9,10-perylene-bis(dicarboximide) (5PDI), that exhibits photophysical and redox properties similar to those of chlorophyll a (Chl a), yet unlike Chl a, it can be easily functionalized and incorporated into a wide variety of biomimetic electron donor−acceptor systems. The N,N ‘-dicyclohexyl derivative (5PDI) absorbs strongly (ε = 46 000 M-1 cm-1) at 686 nm in toluene and fluoresces at 721 nm with a 35% quantum yield. Additionally, 5PDI is both oxidized and reduced in CH2Cl2 at 0.57 V and −0.76 V vs SCE, respectively, making it a facile electron donor or acceptor. Rodlike covalent electron donor−acceptor molecules 5PDI−PI, 5PDI−NI, and 5PDI−PDI were prepared by linking the imide group of the 5PDI donor to pyromellitimide (PI), 1,8:4,5-naphthalenebis(dicarboximide) (NI), and 1,7-bis(3,5-di-tert-butylphenoxy)-3,4:9,10-perylene-bis(dicarboximide) (PDI) acceptors via an N−N bond. Following femtosecond laser excitation of 5PDI, 5PDI−PI, 5PDI−NI, and 5PDI−PDI in both toluene and 2-methyltetrahydrofuran, the formation and decay of their excited and radical ion pair states were monitored directly by transient absorption spectroscopy. We also report steady state emission and spectro-electrochemistry data for these molecules, which aid in elucidation of the transient spectra and the mechanisms of photoinduced charge separation. In toluene, charge separation occurs with high yield only in 5PDI−NI and 5PDI−PDI, whereas for 5PDI−PI charge separation is slow relative to excited-state decay of 1*5PDI−PI indicating that ΔG CS ≅ 0. This fact provides a means of estimating the ionic radii of the photogenerated ions, which for perylene chromophores 5PDI and PDI are 7.6 ± 0.5 Å, whereas those of the PI and NI electron acceptors are 5.6 ± 0.5 Å. These ionic radii are used in turn to determine the free energies of reaction of the remaining molecules with the series. Electroabsorption measurements are used to show that the change in dipole moment, Δμ that occurs upon formation of 1*5PDI is 3.5 D. The rates of charge separation in 5PDI−NI, 5PDI−PI, and 5PDI−PDI are compared to those of related donor−acceptor molecules having a 9-(pyrrolidin-1-yl)-perylene-3,4-dicarboximide (5PMI) donor. The 5PMI donor with Δμ = 15.4 D has a lowest excited singlet state with significantly higher charge-transfer character than does 5PDI, and has greater electron density near the imide group to which the acceptor is attached. The rate constants for charge separation from 1*5PMI are greater than those from 1*5PDI, which suggests that the rates of electron transfer from donors with CT excited states to an attached acceptor depend on the charge distribution in the CT excited state.

The competition between conformational dynamics and electron transfer within a series of phenothiazine-(phenyl) n -pyrene (n ) 0, 1) electron donor-acceptor dyads of potential use in organic light emitting diodes was examined using femtosecond transient absorption spectroscopy. The molecular structures of these dyads permit only torsional motions around the single bonds joining each aromatic subunit. The redox properties of these molecules are nearly independent of the phenyl bridging group, whereas spectroelectrochemistry shows that the UV/vis absorption spectra of the oxidized and reduced species vary with the bridge. Each molecule exhibits dual fluorescence emission which provides evidence for conformational heterogeneity. Emission from a locally excited state originates from a minority conformation, in which electron transfer is negligible, whereas emission because of ion pair recombination results from the majority conformation which undergoes electron transfer. The electron-transfer reactions proceed with time constants <25 ps except in the dyad with the longest donor-acceptor distance in nonpolar solution, where the free energy of the charge separation reaction is positive. If electron transfer is sufficiently fast, conformational relaxation within the ion pair state product occurs on a 100-400 ps time scale, whereas if electron transfer is slow, conformation relaxation with the locally excited state centered on phenothiazine occurs. In two of the dyads in nonpolar solvents, wherein the free energy for charge separation is estimated to be very small, strong mixing between the ion pair state and the locally excited state of phenothiazine is found. The results show that competitive conformational relaxations can have a strong influence on the charge separation dynamics of donor-bridge-acceptor molecules with single bond linkages. In turn, these conformational dynamics will undoubtedly have an important influence on the photophysics of these molecules in the solid-state environment characteristic of light-emitting diodes. † Part of the special issue "Edward Schlag Festschrift".

The international MRI expert subgroup of the International Prophylaxis Study Group (IPSG) has developed a consensus for magnetic resonance imaging (MRI) scales for assessment of haemophilic arthropathy. A MRI scoring scheme including a 10 step progressive scale and a 20 step additive scale with identical definitions of mutual steps is presented. Using the progressive scale, effusion/haemarthrosis can correspond to progressive scores of 1, 2, or 3, and synovial hypertrophy and/or haemosiderin deposition to 4, 5, or 6. The progressive score can be 7 or 8 if there are subchondral cysts and/or surface erosions, and it is 9 or 10 if there is loss of cartilage. Using the additive scale, synovial hypertrophy contributes 1-3 points to the additive score and haemosiderin deposition contributes 1 point. For osteochondral changes, 16 statements are evaluated as to whether they are true or false, and each true statement contributes 1 point to the additive score. The use of these two compatible scales for progressive and additive MRI assessments can facilitate international comparison of data and enhance the accumulation of experience on MRI scoring of haemophilic arthropathy.