PubMed Health⌕ Search

Biomedical subjects

Ryan T Hayes

Publications and source records attributed to Ryan T Hayes.

10 recordsLinked to original sources
PubMed2004

DNA-mediated exciton coupling and electron transfer between donor and acceptor stilbenes separated by a variable number of base pairs.

The synthesis, steady-state spectroscopy, and transient absorption spectroscopy of DNA conjugates possessing both stilbene electron donor and electron acceptor chromophores are described. These conjugates are proposed to form nicked DNA dumbbell structures in which a stilbenedicarboxamide acceptor and stilbenediether donor are separated by variable numbers of A-T or G-C base pairs. The nick is located either adjacent to one of the chromophores or between two of the bases. Thermal dissociation profiles indicate that stable structures are formed possessing as few as two A-T base pairs. Circular dichroism (CD) spectra in the base pair region are characteristic of B-DNA duplex structures, whereas CD spectra at longer wavelengths display two bands attributed to exciton coupling between the two stilbenes. The sign and intensity of these bands are dependent upon both the distance between the chromophores and the dihedral angle between their transition dipoles [Deltaepsilon approximately Rda(-2) sin(2theta)]. Pulsed laser excitation of the stilbenediamide results in creation of the acceptor-donor radical ion pair, which decays via charge recombination. The dynamics of charge separation and charge recombination display an exponential distance dependence, similar to that observed previously for systems in which guanine serves as the electron donor. Unlike exciton coupling between the stilbenes, there is no apparent dependence of the charge-transfer rates upon the dihedral angle between donor and acceptor stilbenes. The introduction of a single G-C base pair between the donor and acceptor results in a change in the mechanism for charge separation from single step superexchange to hole hopping.

Chromatography, High Pressure Liquid↗
PubMed2003

Dynamics and energetics of single-step hole transport in DNA hairpins.

The dynamics of single-step hole transport processes have been investigated in a number of DNA conjugates possessing a stilbenedicarboxamide electron acceptor, a guanine primary donor, and several secondary donors. Rate constants for both forward and return hole transport between the primary and secondary donor are obtained from kinetic modeling of the nanosecond transient absorption decay profiles of the stilbene anion radical. The kinetic model requires that the hole be localized on either the primary or the secondary donor and not delocalized over both the primary and the secondary donor. Rate constants for hole transport are found to be dependent upon the identity of the secondary donor, the intervening bases, and the location of the secondary donor in the same strand as the primary donor or in the complementary strand. Rate constants for hole transport are much slower than those for the superexchange process used to inject the hole on the primary donor. This difference is attributed to the larger solvent reorganization energy for charge transport versus charge separation. The hole transport rate constants obtained in these experiments are consistent with experimental data for single-step hole transport from other transient absorption studies. Their relevance to long-distance hole migration over tens of base pairs remains to be determined. The forward and return hole transport rate constants provide equilibrium constants and free energies for hole transport equilibria. Secondary GG and GGG donors are found to form very shallow hole traps, whereas the nucleobase deazaguanine forms a relatively deep hole trap. This conclusion is in accord with selected strand cleavage data and thus appears to be representative of the behavior of holes in duplex DNA. Our results are discussed in the context of current theoretical models of hole transport in DNA.

Base Pairing↗
PubMed2002

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. The rate constants for charge separation are dependent upon the oxidation potentials of the neighboring nucleobase donors but not upon the identity of nonnearest neighbors. Thus, the charge separation processes yield a contact radical ion pair in which the positive charge is localized on the neighboring nucleobase. Rate constants for charge recombination are dependent upon the identity of the first and second nearest-neighbor nucleobases but not more remote bases. This dependence is attributed to stabilization of the contact radical ion pair by interaction with its nearest neighbor. The absence of charge migration to form a base-pair separated radical ion pair is a consequence of Coulombic attraction in the contact radical ion pair and the low effective dielectric constant (epsilon < 7) experienced by the contact radical ion pair. Photoinduced charge injection to form a base-pair separated radical ion pair is necessary in order to observe charge migration.

Acetylene↗
PubMed2002

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.

Base Pairing↗
PubMed2002

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. This molecule self-assembles into ordered nanoparticles both in solution and in the solid-state, driven by van der Waals stacking of the PDI molecules. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to form ZnTPP(+)PDI(-) radical ion pairs, in which the radical anion rapidly migrates to PDI molecules that are, on average, 21 A away, as evidenced by magnetic field effects on the yield of the PDI triplet state that results from radical ion pair recombination. These nanoparticles exhibit charge transport properties that combine important features from both photosynthetic and semiconductor photoconversion systems.

Journal Article↗
PubMed2002

Dynamic docking and electron transfer between Zn-myoglobin and cytochrome b(5).

We present a broad study of the effect of neutralizing the two negative charges of the Mb propionates on the interaction and electron transfer (ET) between horse Mb and bovine cyt b(5), through use of Zn-substituted Mb (ZnMb, 1) to study the photoinitiated reaction, ((3)ZnP)Mb + Fe(3+)cyt b(5) --> (ZnP)(+)Mb + Fe(2+)cyt b(5). The charge neutralization has been carried out both by replacing the Mb heme with zinc-deuteroporphyrin dimethylester (ZnMb(dme), 2), which replaces the charges by small neutral hydrophobic patches, and also by replacement with the newly prepared zinc-deuteroporphyrin diamide (ZnMb(diamide), 3), which converts the charged groups to neutral, hydrophilic ones. The effect of propionate neutralization on the conformation of the zinc-porphyrin in the Mb heme pocket has been studied by multinuclear NMR with an (15)N labeled zinc porphyrin derivative (ZnMb((15)N-diamide), 4). The rates of photoinitiated ET between the Mb's (1-3) and cyt b(5) have been measured over a range of pH values and ionic strengths. Isothermal titration calorimetry (ITC) and NMR methods have been used to independently investigate the effect of charge neutralization on Mb/b(5) binding. The neutralization of the two heme propionates of ZnMb by formation of the heme diester or, for the first time, the diamide increases the second-order rate constant of the ET reaction between ZnMb and cyt b(5) by as much as several 100-fold, depending on pH and ionic strength, while causing negligible changes in binding affinity. Brownian dynamic (BD) simulations and ET pathway calculations provide insight into the protein docking and ET process. The results support a new "dynamic docking" paradigm for protein-protein reactions in which numerous weakly bound conformations of the docked complex contribute to the binding of cyt b(5) to Mb and Hb, but only a very small subset of these are ET active, and this subset does not include the conformations most favorable for binding; the Mb surface is a large "target" with a small "bullseye" for the cyt b(5) "arrow". This paradigm differs sharply from the more familiar, "simple" docking within a single, or narrow range of conformations, where binding strength and ET reactivity increase in parallel. Likewise, it is distinct from, although complementary to, the well-known picture of conformational control of ET through "gating", or a related picture of "conformational coupling". The new model describes situations in which tight binding does not correlate with efficient ET reactivity, and explains how it is possible to modulate reactivity without changing affinity. Such "decoupling" of reactivity from binding clearly is of physiological relevance for the reduction of met-Mb in muscle and of met-Hb in a red cell, where tight binding of cyt b(5) to the high concentration of ferrous-Mb/Hb would prevent the cytochrome from finding and reducing the oxidized proteins; it likely is of physiological relevance in other situations, as well.

Animals↗
PubMed2002

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.

Base Sequence↗