Formation and decay of localized contact radical ion pairs in DNA hairpins.

The dynamics of charge separation and charge recombination in synthetic DNA hairpins possessing diphenylacetylene-4,4'-dicarboxamide linkers have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. The lowest excited singlet state of the linker is capable of oxidizing nearest neighbor adenine as well as guanine. A large wavelength shift in the transient absorption spectrum accompanies the conversion of the singlet linker to its anion radical, facilitating the investigation of electron-transfer dynamics.

Dynamics of electron injection in DNA hairpins.

The dynamics of electron injection has been investigated in DNA hairpins possessing a stilbenediether electron donor linker by means of femtosecond transient absorption spectroscopy. Ultrafast electron injection and charge recombination are observed with neighboring cytosine or thymine bases; however, guanine-guanine base pairs are not reduced, permitting the investigation of the distance dependence of charge injection.

Charge transport in photofunctional nanoparticles self-assembled from zinc 5,10,15,20-tetrakis(perylenediimide)porphyrin building blocks.

Molecules designed to carry out photochemical energy conversion typically employ several sequential electron transfers, as do photosynthetic proteins. Yet, these molecules typically do not achieve the extensive charge transport characteristic of semiconductor devices. We have prepared a large molecule in which four perylene-3,4:9,10-tetracarboxydiimide (PDI) molecules that both collect photons and accept electrons are attached to a central zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) electron donor.

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation.

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer.

A phase II Hoosier Oncology Group study of vinorelbine and estramustine phosphate in hormone-refractory prostate cancer.

Background: The purpose was to evaluate the combined anti-microtubular regimen of vinorelbine and estramustine phosphate (EMP) in hormone refractory prostate cancer.

Patients And Methods: Weekly vinorelbine 20 mg/m2 (or 15 mg/m2 if a history of prior pelvic radiotherapy) was combined with EMP at 280 mg orally tds for 3 days (the day before, the day of and the day after vinorelbine infusion). After 8 weeks of therapy the combination was given every other week.

Dynamics of inter- and intrastrand hole transport in DNA hairpins.

The dynamics of photoinduced electron transfer has been investigated in DNA hairpins possessing a stilbenedicarboxamide (Sa) electron acceptor, a guanine (G) primary donor, and two adjacent guanines (GG) as secondary donors. Hole transport from G to GG across a single A is more rapid than across AA or T by factors of 20 +/- 7 and 40 +/- 15, respectively. Intrastrand hole transport across a single A is more rapid than interstrand transport by a factor of 7 +/- 3.

Low-penetrance susceptibility to breast cancer due to CHEK2(*)1100delC in noncarriers of BRCA1 or BRCA2 mutations.

Mutations in BRCA1 and BRCA2 confer a high risk of breast and ovarian cancer, but account for only a small fraction of breast cancer susceptibility. To find additional genes conferring susceptibility to breast cancer, we analyzed CHEK2 (also known as CHK2), which encodes a cell-cycle checkpoint kinase that is implicated in DNA repair processes involving BRCA1 and p53 (refs 3,4,5). We show that CHEK2(*)1100delC, a truncating variant that abrogates the kinase activity, has a frequency of 1.

Functional asymmetry of photosystem II D1 and D2 peripheral chlorophyll mutants of Chlamydomonas reinhardtii.

The peripheral accessory chlorophylls (Chls) of the photosystem II (PSII) reaction center (RC) are coordinated by a pair of symmetry-related histidine residues (D1-H118 and D2-H117). These Chls participate in energy transfer from the proximal antennae complexes (CP43 and CP47) to the RC core chromophores. In addition, one or both of the peripheral Chls are redox-active and participate in a low-quantum-yield electron transfer cycle around PSII.

Photophysics of pi-conjugated metal-organic oligomers: aryleneethynylenes that contain the (bpy)Re(CO)(3)Cl chromophore.

A comprehensive study of a series of four monodisperse, metal-organic pi-conjugated oligomers of varying length is reported. The oligomers are based on the aryleneethynylene architecture, and they contain a 2,2'-bipyridine-5,5'-diyl (bpy) metal binding unit. The photophysical properties of the free oligomers and their complexes with the (L)Re(I)(CO)(3)X chromophore (where L = the bpy-oligomer and X = Cl or NCCH(3)) were explored by a variety of methods including electrochemistry, UV-visible absorption, variable temperature photoluminescence (PL), transient absorption (TA), and time-resolved electron paramagnetic spectroscopy (TREPR).

Conformational gating of long distance electron transfer through wire-like bridges in donor-bridge-acceptor molecules.

A series of five donor-bridge-acceptor (DBA) molecules in which the donor is tetracene, the acceptor is pyromellitimide, and the bridge molecules are oligo-p-phenylenevinylenes (OPV) of increasing length has been shown to undergo electron transfer (ET) by means of two mechanisms. When the bridge is short, strongly distance dependent superexchange dynamics dominates, whereas when the bridge is longer, bridge-assisted hopping dynamics prevails. The latter mechanism results in relatively soft distance dependence for ET in which the OPV oligomers act effectively as molecular wires.

Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins.

The dynamics of photoinduced charge separation and charge recombination processes in synthetic DNA hairpins have been investigated by means of femtosecond transient absorption spectroscopy. The driving force and distance dependence of charge-transfer processes involving singlet acceptors and nucleobase donors are consistent with a single-step superexchange mechanism in which the electronic coupling between the donor and acceptor is strongly distance dependent. The dynamics of reversible hole transport between a primary guanine donor and nearby GG or GGG sequences has also been determined and establishes that these sequences are very shallow hole traps.

Direct measurement of hole transport dynamics in DNA.

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.

Distance-dependent electron transfer in DNA hairpins.

The distance dependence of photoinduced electron transfer in duplex DNA was determined for a family of synthetic DNA hairpins in which a stilbene dicarboxamide forms a bridge connecting two oligonucleotide arms. Investigation of the fluorescence and transient absorption spectra of these hairpins established that no photoinduced electron transfer occurs for a hairpin that has six deoxyadenosine-deoxythymidine base pairs. However, the introduction of a single deoxyguanosine-deoxycytidine base pair resulted in distance-dependent fluorescence quenching and the formation of the stilbene anion radical.

Antenna excited state decay kinetics establish primary electron transfer in reaction centers as heterogeneous.

The decay of the excited primary electron donor P* in bacterial photosynthetic reaction centers (both membrane-bound and detergent-isolated) has been observed to be nonexponential on a time scale of some tens of picoseconds. Although the multipicosecond nonexponentiality of P* has been ascribed to heterogeneity in teh rate of primary electron transfer (PET), the decay kinetics can be interpreted equally well using homogeneous models. To address this ambiguity, we studied the decay of excited bacteriochlorophyll (Bchl) in the membrane-bound core antenna/reaction center complexes of wild-type and mutant reaction center strains of Rhodobacter capsulatus.

The lifetimes and energies of the first excited singlet states of diadinoxanthin and diatoxanthin: the role of these molecules in excess energy dissipation in algae.

The lifetimes of the first excited singlet states (2(1)A(g)) of diadinoxanthin and diatoxanthin, carotenoids involved in the xanthophyll cycle in some genera of algae, have been measured by femtosecond time-resolved optical spectroscopy to be 22.8 +/- 0.1 ps and 13.

Excitation energy transfer and charge separation in the isolated Photosystem II reaction center.

The nature of excitation energy transfer and charge separation in isolated Photosystem II reaction centers is an area of considerable interest and controversy. Excitation energy transfer from accessory chlorophyll a to the primary electron donor P680 takes place in tens of picoseconds, although there is some evidence that thermal equilibration of the excitation between P680 and a subset of the accessory chlorophyll a occurs on a 100-fs timescale. The intrinsic rate for charge separation at low temperature is accepted to be ca.

Near-transform-limited visible and near-IR femtosecond pulses from optical parametric amplification using Type II beta-barium borate.

Type II phase-matched beta-barium borate is used in the f irst stage of amplif ication of a white-light continuum in a two-stage optical parametric amplifier pumped by the second harmonic of a regeneratively amplified Ti:sapphire laser system operating at 824 nm. Near-transform-limited sub-190-fs pulses with microjoule energies are achieved in the signal branch, which is tunable from 475 nm to degeneracy. This system effectively bridges the wavelength gap between the fundamental and the second harmonic of amplified Ti:sapphire laser systems.

Optical parametric amplification of femtosecond pulses tunable from the blue to the infrared with microjoule energies.

A white-light continuum is used to seed a two-stage optical parametric amplifier pumped by the second harmonic of a regeneratively amplified Ti:sapphire laser system operating at 824 nm. Microjoule energies are achieved in the signal branch, which is tunable from 472 to 785 nm. Near-transform-limited sub-200-fs pulses are attainable over the vast majority of the tuning range.

Femtosecond photodichroism studies of isolated photosystem II reaction centers.

Photosynthetic conversion of light energy into chemical potential begins in reaction center protein complexes, where rapid charge separation occurs with nearly unit quantum efficiency. Primary charge separation was studied in isolated photosystem II reaction centers from spinach containing 6 chlorophyll a, 2 pheophytin a (Pheo), 1 cytochrome b559, and 2 beta-carotene molecules. Time-resolved pump-probe kinetic spectroscopy was carried out with 105-fs time resolution and with the pump laser polarized parallel, perpendicular, and at the magic angle (54.

Photophysics of the carotenoids associated with the xanthophyll cycle in photosynthesis.

Green plants use the xanthophyll cycle to regulate the flow of energy to chlorophylla within photosynthetic proteins. Under conditions of low light intensity violaxanthin, a carotenoid possessing nine conjugated double bonds, functions as an antenna pigment by transferring energy from its lowest excited singlet state to that of chlorophylla within light-harvesting proteins. When the light intensity increases, violaxanthin is biochemically transformed into zeaxanthin, a carotenoid that possesses eleven conjugated double bonds.

Picosecond optical switching based on biphotonic excitation of an electron donor-acceptor-donor molecule.

An electron donor-acceptor-donor molecule consisting of two porphyrin donors rigidly attached to the two-electron acceptor N,N'-diphenyl-3,4,9,10-perylenebis(dicarboximide) acts as a light intensity-dependent molecular switch on a picosecond time scale. Excitation of the porphyrins within this molecule with subpicosecond laser pulses results in single or double reduction of the acceptor depending on the light intensity. The singly and doubly reduced electron acceptors absorb light strongly at 713 and 546 nanometers, respectively.

Determination of the primary charge separation rate in Photosystem II reaction centers at 15 K.

We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a (-) (Pheo a (-)) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D1, D2, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450-840 nm as a function of time with 500fs resolution following 610 nm laser excitation.

Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis.

We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, HM, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PHL (-)HM state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, HL, is blocked and the yield of transient P(+)HM (-) can be estimated with respect to the lifetime of P(*).