2,278 research outputs found
Low temperature superlattice in monoclinic PZT
TEM has shown that the strongly piezoelectric material Pb(Zr0.52Ti0.48)O3
separates into two phases at low temperatures. The majority phase is the
monoclinic phase previously found by x-ray diffraction. The minority phase,
with a nanoscale coherence length, is a slightly distorted variant of the first
resulting from the anti-phase rotation of the oxygen octahedra about [111].
This work clears up a recent controversy about the origin of superlattice peaks
in these materials, and supports recent theoretical results predicting the
coexistence of ferroelectric and rotational instabilities.Comment: REVTeX4, 4 eps figures embedded. JPG version of figs. 2&4 is also
include
FLUORESCENCE AND CIRCULAR DICHROISM STUDIES ON THE PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM
Two phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobac-terium Westiellopsis prolifica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (Ī±-84) to the cyanin (Ī²-84, 155) chromophores is efficient in the trimer (Ī±Ī² it is impeded after dissociation to the monomer (Ī±,Ī²). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol. 55,119ā124, 1992), exhibited the spectral properties similar to that of the Ī±-subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained
THERMAL DENATURATION OF MONOMERIC AND TRIMERIC PHYCOCYANINS STUDIED BY STATIC AND SPECTROSCOPY POLARIZED TIME-RESOLVED FLUORESCENCE
C-Phycocyanin (PC) and allophycocyanin (APC). as well as the a-subunit of PC. have been
isolated from the blue-green alga (cyanobacterium). Spirulina platensis. The effects of partial thermal
denaturation of PC and of its state of aggregation have been studied by ps time-resolved, polarized
fluorescence spectroscopy. All measurements have been performed under low photon fluxes (< 10ā ā
photonsipulse x cmā) to minimize singlet-singlet annihilation processes. A complex decay is obtained
under most conditions, which can be fitted satisfactorily with a bi-exponential (7ā = 70400 ps. T? =
1000-3000 ps) for both the isotropic and the polarized part, but with different intensities and time
constants for the two decay curves. The data are interpreted in the frameworkof the model first developed
by Teak and Dale (Biochern. J. 116, 161 (1970)], which divides the spectroscopically different
chromophores in (predominantly) sensitizing (s) and fluorescing U, ones. If one assumes temperature
dependent losses in the energy transfer from the s to the f and between f chromophores. both the
biexponential nature of the isotropic fluorescence decay and the polarization data can be rationalized. In
the isotropic emission (corresponding to the population of excited states) the short lifetime is related to the
s-,f transfer. the longer one to the āfreeā decay of the final acceptor(s) (= f). The polarized part is
dominated by an extremely short decay time. which is related to s+f transfer, as well as to resonance
transfer between the f-chromophores
Atomic-scale surface demixing in a eutectic liquid BiSn alloy
Resonant x-ray reflectivity of the surface of the liquid phase of the
BiSn eutectic alloy reveals atomic-scale demixing extending over
three near-surface atomic layers. Due to the absence of underlying atomic
lattice which typically defines adsorption in crystalline alloys, studies of
adsorption in liquid alloys provide unique insight on interatomic interactions
at the surface. The observed composition modulation could be accounted for
quantitatively by the Defay-Prigogine and Strohl-King multilayer extensions of
the single-layer Gibbs model, revealing a near-surface domination of the
attractive Bi-Sn interaction over the entropy.Comment: 4 pages (two-column), 3 figures, 1 table; Added a figure, updated
references, discussion; accepted at Phys. Rev. Let
FĆRSTER TRANSFER CALCULATIONS BASED ON CRYSTAL STRUCTURE DATA FROM Agmenellum quadruplicatum C-PHYCOCYANIN
Excitation energy transfer in C-phycocyanin is modeled using the Forster inductive resonance mechanism. Detailed calculations are carried out using coordinates and orientations of the chromophores derived from X-ray crystallographic studies of C-phycocyanin from two different species (Schirmer et al, J. Mol. Biol. 184, 257ā277 (1985) and ibid., 188, 651-677 (1986)). Spectral overlap integrals are estimated from absorption and fluorescence spectra of C-phycocyanin of Mastigocladus laminosus and its separated subunits. Calculations are carried out for the Ī²-subunit, Ī±Ī²-monomer, (Ī±Ī²)3-trimer and (Ī±Ī²)0-hexamer species with the following chromophore assignments: Ī²155 = 'sā(sensitizer), Ī²84 =āf (fluorescer) and Ī±84 =āmā(intermediate):]:. The calculations show that excitation transfer relaxation occurs to 3=98% within 200 ps in nearly every case; however, the rates increase as much as 10-fold for the higher aggregates. Comparison with experimental data on fluorescence decay and depolarization kinetics from the literature shows qualitative agreement with these calculations. We conclude that Forster transfer is sufficient to account for all of the observed fluorescence properties of C-phycocyanin in aggregation states up to the hexamer and in the absence of linker polypeptides
Structure of Lo'ihi Seamount, Hawai'i and lava flow morphology from high-resolution mapping.
Ā© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Clague, D. A., Paduan, J. B., Caress, D. W., Moyer, C. L., Glazer, B. T., & Yoerger, D. R. Structure of Lo'ihi Seamount, Hawai'i and lava flow morphology from high-resolution mapping. Frontiers in Earth Science, 7, (2019):58, doi:10.3389/feart.2019.00058.The early development and growth of oceanic volcanoes that eventually grow to become ocean islands are poorly known. In Hawaiāi, the submarine LÅāihi Seamount provides the opportunity to determine the structure and growth of such a nascent oceanic island. High-resolution bathymetric data were collected using AUV Sentry at the summit and at two hydrothermal vent fields on the deep south rift of LÅāihi Seamount. The summit records a nested series of caldera and pit crater collapse events, uplift of one resurgent block, and eruptions that formed at least five low lava shields that shaped the summit. The earliest and largest caldera, formed ā¼5900 years ago, bounds almost the entire summit plateau. The resurgent block was uplifted slightly more than 100 m and has a tilted surface with a dip of about 6.5Ā° toward the SE. The resurgent block was then modified by collapse of a pit crater centered in the block that formed West Pit. The shallowest point on LÅāihiās summit is 986 m deep and is located on the northwest edge of the resurgent block. Several collapse events culminated in formation of East Pit, and the final collapse formed Peleās Pit in 1996. The nine mapped collapse and resurgent structures indicate the presence of a shallow crustal magma chamber, ranging from depths of ā¼1 km to perhaps 2.5 km below the summit, and demonstrate that shallow sub-caldera magma reservoirs exist during the late pre-shield stage. On the deep south rift zone are young medium- to high-flux lava flows that likely erupted in 1996 and drained the shallow crustal magma chamber to trigger the collapse that formed Peleās Pit. These low hummocky and channelized flows had molten cores and now host the FeMO hydrothermal field. The Shinkai Deep hydrothermal site is located among steep-sided hummocky flows that formed during low-flux eruptions. The Shinkai Ridge is most likely a coherent landslide block that originated on the east flank of LÅāihi.Funding for the collection of the data was provided by the National Science Foundation OCE1155756 to CM and the Schmidt Ocean Institute to BG. Support for DC and JP to process the data and write the manuscript was provided by a grant from the David and Lucile Packard Foundation to MBARI
Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48,
have revealed a new monoclinic phase in the vicinity of the morphotropic phase
boundary (MPB), previously regarded as the the boundary separating the
rhombohedral and tetragonal regions of the PZT phase diagram. In the present
paper, the stability region of all three phases has been established from high
resolution synchrotron x-ray powder diffraction measurements on a series of
highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is
stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature
is increased. A first-order phase transition from tetragonal to rhombohedral
symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to
the tetragonal-rhombohedral phase boundary, but instead to the boundary between
the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides
important insight into the close relationship between the monoclinic phase and
the striking piezoelectric properties of PZT; in particular, investigations of
poled samples have shown that the monoclinic distortion is the origin of the
unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde
Redefining genomic privacy: trust and empowerment
Fulfilling the promise of the genetic revolution requires the analysis of large datasets containing information from thousands to millions of participants. However, sharing human genomic data requires protecting subjects from potential harm. Current models rely on de-identification techniques in which privacy versus data utility becomes a zero-sum game. Instead, we propose the use of trust-enabling techniques to create a solution in which researchers and participants both win. To do so we introduce three principles that facilitate trust in genetic research and outline one possible framework built upon those principles. Our hope is that such trust-centric frameworks provide a sustainable solution that reconciles genetic privacy with data sharing and facilitates genetic research
- ā¦