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. Kinetic analysis suggests that duplex DNA is somewhat more effective than proteins as a medium for electron transfer but that it does not function as a molecular wire.

Risperidone in the management of violent, treatment-resistant schizophrenics hospitalized in a maximum security forensic facility.

This study examines the effectiveness of risperidone compared with traditional neuroleptic medications in the areas of clinical functioning and aggressive behaviors in a sample of inpatients diagnosed with chronic schizophrenia. Similar to the methodology of Menditto et al. (Psychiatr Serv 47:46-51, 1996), two groups of 10 patients were selected from those being treated in a comprehensive psychosocial rehabilitation program. Group 1 subjects were placed on risperidone at various times during their treatment. Group 2 subjects, who were matched with Group 1 subjects on pre-study levels of clinical functioning as measured by the Time-Sample Behavioral Checklist (TSBC), remained on traditional neuroleptics throughout the study period. For each subject, scores on six TSBC subscales were examined at four time points; data were analyzed with repeated-measures multivariate analyses of variance and univariate analyses of variance. Frequency counts of aggressive behaviors (threats and assaults) were compiled into two six-month time periods and analyzed with nonparametric techniques. The risperidone group did not differ from the traditional neuroleptic group on measures of clinical functioning and aggressiveness measured over time. Both groups evidenced improvements in bizarre motor behaviors over the study period. The risperidone group evidenced some deterioration in measures of appropriate interpersonal interaction over time. No differences in aggressive behaviors were noted for either group. The study concludes that for forensic patients with chronic schizophrenia, risperidone failed to produce therapeutic effects in overall clinical functioning and aggressive behaviors that were significantly different from traditional neuroleptics. Descriptive comparisons are made between the receptor-binding profiles and clinical effectiveness of risperidone and clozapine in an attempt to explain these findings.

TRIDENT high-energy-density facility experimental capabilities and diagnostics.

The newly upgraded TRIDENT high-energy-density (HED) facility provides high-energy short-pulse laser-matter interactions with powers in excess of 200 TW and energies greater than 120 J. In addition, TRIDENT retains two long-pulse (nanoseconds to microseconds) beams that are available for simultaneous use in either the same experiment or a separate one. The facility's flexibility is enhanced by the presence of two separate target chambers with a third undergoing commissioning. This capability allows the experimental configuration to be optimized by choosing the chamber with the most advantageous geometry and features. The TRIDENT facility also provides a wide range of standard instruments including optical, x-ray, and particle diagnostics. In addition, one chamber has a 10 in. manipulator allowing OMEGA and National Ignition Facility (NIF) diagnostics to be prototyped and calibrated.

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. (2 ps)(-1), and is based on several measurements using different experimental techniques. This rate is in good agreement with estimates based on larger sized particles, and is similar to the rate observed with bacterial reaction centers. However, near room temperature there is considerable disagreement as to the observed rate for charge separation, with several experiments pointing to a ca. (3 ps)(-1) rate, and others to a ca. (20 ps)(-1) rate. These processes and the experiments used to measure them will be reviewed.

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.

Influence of iron-removal procedures on sequential electron transfer in photosynthetic bacterial reaction centers studied by transient EPR spectroscopy.

Electron spin polarized electron paramagentic resonance (ESP EPR) spectra were obtained with deuterated iron-removed photosynthetic bacterial reaction centers (RCs) to specifically investigate the effect of the rate of primary charge separation, metal-site occupancy, and H-subunit content on the observed P865+QA- charge-separated state. Fe-removed and Zn-substituted RCs from Rb. sphaeroides R-26 were prepared by refined procedures, and specific electron transfer rates (kQ) from the intermediate acceptor H- to the primary acceptor QA of (200 ps)-1 vs (3-6 ns)-1 were observed. Correlation of the transient EPR and optical results shows that the observed slow kQ rate in Fe-removed RCs is H-subunit-independent, and, in some cases, independent of Fe-site occupancy as Zn2+ substitution does not ensure retention of the native kQ. In addition, shifts in the optical spectrum of P865 and differences in the high-field region of the Q-band ESP spectrum for Fe-removed RCs with slow kQ indicate possible structural changes near P865. The experimental X-band and Q-band spin-polarized EPR spectra for deuterated Fe-removed RCs where kQ is at least 15-fold slower at room temperature than the (200 ps)-1 rate observed for native Fe-containing RCs have different relative amplitudes and small g-value shifts compared to the spectra of Zn-RCs which have a kQ unchanged from native RCs. These differences reflect the trends in polarization predicted from the sequential electron transfer polarization (SETP) model [Morris et al. (1995) J. Phys. Chem. 99, 3854-3866; Tang et al. (1996) Chem. Phys. Lett. 253, 293-298]. Thus, SETP modeling of these highly resolved ESP spectra obtained with well-characterized proteins will provide definitive information about any light-induced structural changes of P865, H, and QA that occur upon formation of the P865+QA- charge-separated state.

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. Reaction centers isolated from these same strains display a range of multiexponentiality in primary charge separation. The mutant strains carry substitutions of amino acids residing near the monomeric Bchl on the active and/or inactive sides of the reaction center. Transient absorption measurements monitoring the Qy bleach of antenna Bchls require at least two exponential components to fit all decays. The wild type was fitted with equal-amplitude components whose lifetimes are 24 and 65 ps. The shortest-lived component is relatively insensitive to mutation, in contrast to the longer-lived component(s) whose amplitude and magnitude were dramatically perturbed by amino acid substitutions. Unlike the situation with isolated reaction centers, here the only kinetic models consistent with the data are those in which the primary electron-transfer rate constant is heterogeneous, suggesting at least two structural populations of RCs. PET in the population with the shortest-lived antenna decay causes the kinetics to be transfer-to-trap-limited, whereas the kinetics in the other population(s)--having longer-lived antenna decays--are limited by the rate of PET. Observation of both types of kinetic limitation within a single light-harvesting system is unexpected and complicates any discussion of the rate-limiting step of light energy utilization in photosynthesis.