I. Y. Lee

Molecular photovoltaics and the photoactivation of mammalian cells

Photosynthetic reaction centers are integral plant membrane protein complexes and molecular photovoltaic structures. We report here that addition of Photosystem I (PSI)-proteoliposomes to retinoblastoma cells imparts photosensitivity to these mammalian cells, as demonstrated by light-induced movement of calcium ions. Control experiments with liposomes lacking PSI demonstrated no photosensitivity. The data demonstrate that PSI, a nanoscale molecular photovoltaic structure extracted from plants, can impart a photoresponse to mammalian cells in vitro.

New Spontaneous Fission Mode for 252Cf: Indication of Hyperdeformed 144,145,146Ba at Scission.

From {gamma}-ray coincidence studies following spontaneous fission of {sup 252}Cf, direct measurements of yields and neutron multiplicities were made for Sr-Nd, Zr-Ce, Mo-Ba, Ru-Xe, and Pd-Te correlated pairs. A strong enhancement of the 7-10 neutron channels for Mo-Ba pairs is observed. A new fission mode associated with the enhanced neutron yields is identified. These data can be interpreted in terms of one or more of {sup 144,145,146}Ba being hyperdeformed at scission. Mean field calculations predict a hyperdeformed third minimum in {sup 252}Cf and an extremely deformed {sup 146}Ba fragment at scission. {copyright} {ital 1996 The American Physical Society.}

Platinization : a novel technique to anchor photosystem I reaction centres onto a metal surface at biological temperature and pH

Abstract Photosystem I (PSI) reaction centre core antenna complexes containing about 40 chlorophylls per P700 (PSI-40) are 6 nm biological photovoltaic device. PSI particles carry positively-charged domain(s) and can therefore interact electrostatically with a negatively-charged surface. PSI particles were platinized by reduction of [PtCl 6 ] −2 at neutral pH and 20°C. The presence of metallic platinum on PSI particles was observed directly by scanning tunnelling microscopy (STM). The platinized PSI particles were larger than control PSIs (without platinization), due to the presence of metallic platinum. The electric properties of PSI and metallic platinum were characterized by STM tunnelling conductance (I–V) measurements. Diodic response was obtained in platinized PSI. Metallic platinum formed during the reduction of [PtCl 6 ] 2− caused bonding between PSI particles and a gold metal surface. Using the technique of chemical platinization, PSI particles can be “welded” onto gold plates under biologically compatible conditions, resulting in a 2-dimensional spatial array of PSI particles anchored on the metal surface. Metallic platinum is not only a good conductor but also an active catalyst. This platinization “welding” technique is potentially important for both bioelectronics and biometallocatalysis.

Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I.

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively. The amplitudes of these two components were exceptionally small in both of the samples. These results provide direct evidence that although platinization dramatically alters the photocatalytic properties of PSI, it does not alter the intrinsic excitation dynamics and initial electron transfer reactions in PSI.

High-spin states in 183Pt

Abstract High-spin states in 183 Pt have been studied for the first time using the reactions 154 Sm( 34 S, 5n) and 170 Yb( 16 O,3n). Rotational bands built on the Nilsson configurations 1 2 − [521], 7 2 − [514] and 9 2 + [624] were observed up to spin values of 39 2 − 49 2 h . Quasiparticle alignments and band crossing frequencies were investigated in these bands. A large signature splitting was observed in the νi 13 2 - band structure . The experimental results were compared with total routhian surface calculations, in which the shape of the nucleus could be followed as a function of rotational frequency for different quasiparticle configurations.

Transition quadrupole moments of high-spin states in 172W and their implications for the interpretation of band crossings in the light isotopes of W to Pt

Abstract Lifetimes of states in 172W have been measured by the Doppler-shift recoil-distance method using the reaction 124Sn(52Cr, 4n)172W at a bombarding energy of 225 MeV. The data were collected in the γγ-coincidence mode rather than the customary singles mode in order to reduce the complexities of the γ-ray spectra and to avoid some of the problems associated with side feeding to excited states. At low spin (I⩽8 h ) the experimental transition quadrupole moments, Qt, are all near a constant value of 7.0 e·b. For states in the yrast band with I⩾10 h , the experimental Qt values are all near a constant value of 6.8 e.b. This is the trend predicted by cranked Hartree-Fock-Bogoliubov (HFB) calculations and by the calculated systematics of triaxial shape-driving forces which originate from the aligned i i 3 2 neutron orbitals around N = 98. From the extensive total routhian surface (TRS) calculations in this mass region, it now seems clear that the low lying band crossings in the light W to Pt isotopes are due to the alignment of various neutron

Observation of a vh{sub 11/2} pair alignment in neutron-rich {sup 118}Pd

The yrast band was significantly extended to 14{sup +} and the {gamma} band to 5{sup +} in neutron-rich {sup 118}Pd by measuring the {gamma}-{gamma}-{gamma} coincidences emitted from the spontaneous fission of {sup 252}Cf with Gammasphere. The first band crossing was observed in the yrast band in {sup 118}Pd at a frequency of {Dirac_h}{omega}{approx}0.36 MeV at the starting point of the backbending, which is similar to that found in {sup 112-116}Pd. The first bandbending in the yrast cascade in {sup 118}Pd is interpreted to be built on a two h{sub 11/2} neutron configuration based on its similarity to the yrast bands in even-even {sup 112-116}Pd. Our result indicates {sup 118}Pd still maintains a prolate shape. The quasineutron Routhian calculations indicate a lower crossing frequency for the h{sub 11/2} level.

High-spin single-particle states in 193Hg

Abstract A decay sequence above the I= 47 2 , 5.4 MeV level in 193Hg has been established through discrete γ-ray spectroscopy. The new energy levels exhibit single-particle character implying the dominant role of individual particle angular momentum alignment at moderate spins in the neutron-deficient Hg isotopes.

Possible octupole correlation in {sup 147}Pr and {pi}h{sub 11/2} bands in {sup 149,151}Pr

Neutron-rich {sup 147,149,151}Pr nuclei, produced in the spontaneous fission of {sup 252}Cf, were studied using the Gammasphere array. Possible parity doublets in {sup 147}Pr with N=88 and {pi}h{sub 11/2} bands in {sup 149,151}Pr are proposed. These new data on the level structures of odd Pr isotopes suggest that octupole correlations may also be present in the neutron-rich {sub 59}{sup 147}Pr{sub 88} nucleus such as those observed in {sub 58}{sup 146}Ce{sub 88}, and also that the h{sub 11/2} bands in the {sup 149,151}Pr track in energy the yrast bands in {sup 148,150}Ce. The backbending related to the breaking of the neutron i{sub 13/2} pair is observed at {Dirac_h}{omega}{approx}0.27 (MeV) for the proton h{sub 11/2} band of {sup 149}Pr.

Transition quadrupole moments of high-spin states in 162,163Yb

Abstract Lifetimes of states in 162,163 Yb have been measured by the Doppler-shift recoil-distance method using the reaction 116 Cd ( 50 Ti , x n ) 166−x Yb at 215 MeV . The data were collected in coincidence with a total-energy filter to provide some discrimination on reaction channel. The experimental transition quadrupole moments, Q t , do not agree in detail with predictions from cranked Hartree-Fock-Bogoliubov (HFB) calculations for the yrast band of 162 Yb. In particular, the loss of collectivity occurs at much lower angular momentum than predicted by the calculations. These discrepancies are not understood in state of the art calculations of the nuclear shape. The interaction strength V between the g- and the s-band extracted from the data is very small (| V | ≲ 1 keV). The E1/E2 branching ratios and E1 transition probabilities for decay of states at low spins in the (−, 1) band are large ( B (E1) = 10 −3 B (E1) sp ). The origin of these effects suggests that the (−, 1) band at low frequencies in 162 Yb is predominately an octupole band. Although no stable octupole deformation has been predicted in this mass region, the experimental evidence points to some octupole instability for 162 Yb.