658 research outputs found
Structures and functions of carotenoids bound to reaction centers from purple photosynthetic bacteria
The photoprotective function of 15,15'-cis-carotenoids bound to the photosynthetic reaction centers (RCs) of purple bacteria has been studied using carotenoids reconstituted into carotenoidless RCs from Rhodobacter sphaeroides strain R26.1. The triplet-energy level of the carotenoid has been proposed to affect the quenching of the triplet state of special-pair bacteriochlorophyll (P). This was investigated using microsecond flash photolysis to detect the carotenoid triplets as a function of the number of conjugated double bonds, n. The carotenoid triplet signals were extracted by using singular-value decomposition (SVD) of the huge matrices data, and were confirmed for those having n = 8 to 11. This interpretation assumes that the reconstituted carotenoids occupy the same binding site in the RC. We have been able to confirm this assumption using X-ray crystallography to determine the structures of carotenoidless, wild-type carotenoid-containing, and 3,4-dihydro-spheroidene-reconstituted RCs. The X-ray study also emphasized the importance of the methoxy group of the carotenoids for binding to the RCs. Electroabsorption (Stark) spectroscopy was used to investigate the effect of the carotenoid on the electrostatic field around P. This electrostatic field changed by 10 % in the presence of the carotenoid
Binding of Polarons and Atoms at Threshold
If the polaron coupling constant is large enough, bipolarons or
multi-polarons will form. When passing through the critical from
above, does the radius of the system simply get arbitrarily large or does it
reach a maximum and then explodes? We prove that it is always the latter. We
also prove the analogous statement for the Pekar-Tomasevich (PT) approximation
to the energy, in which case there is a solution to the PT equation at
. Similarly, we show that the same phenomenon occurs for atoms, e.g.,
helium, at the critical value of the nuclear charge. Our proofs rely only on
energy estimates, not on a detailed analysis of the Schr\"odinger equation, and
are very general. They use the fact that the Coulomb repulsion decays like
, while `uncertainty principle' localization energies decay more rapidly,
as .Comment: 19 page
Fabrication of arrays of gold islands on self-assembled monolayers using pulsed laser deposition through nanosieves
Sandwich structures of gold-self-assembled monolayer-gold were prepared by deposition of gold on alkylthiolate self-assembled monolayers on polycrystalline gold, using pulsed laser deposition (PLD) through a nanosieve. The arrays of sandwiches, around 600 nm in diameter, approximately 10 nm high, and spaced 1.6 Ăm apart, were analyzed using tapping mode atomic force microscopy. Electrochemical copper deposition experiments showed that of the islands deposited on octadecanethiolate monolayers about 15% were electrically insulated from the bottom gold electrode. This means that PLD is a suitable technique for the fabrication of metal-SAM-metal sandwich structures
Endothelin-1 and blood pressure after inhibition of nitric oxide synthesis in human septic shock
BACKGROUND: The systemic hypotension during human sepsis has been ascribed
to increased production of nitric oxide (NO). Therefore, inhibitors of NO
synthesis have been used in the treatment of hypotension in patients with
septic shock. In addition, NO production may inhibit the synthesis and
vasoconstrictor effects of endothelin-1 (ET-1). In this study, we tested
whether ET-1 contributed to the vasopressor action of the NO synthase
inhibitor NG-nitro-L-arginine methyl ester (L-NAME) in patients with
severe septic shock. METHODS AND RESULTS: Compared with healthy
volunteers, patients with septic shock had increased plasma levels of
nitrite/nitrate (37+/-5 [SEM] versus 12+/-5 mmol/L, P<0.01), the stable
end products of NO metabolism, and ET-1 (45+/-7 versus 3+/-2 pg/mL,
P<0.001). Plasma ET-1 concentration was not related to plasma
nitrite/nitrate concentration or blood pressure. Continuous infusion of
L-NAME (1 mg. kg-1. h-1 IV) for 12 hours increased mean arterial pressure
by 43+/-5% and systemic vascular resistance by 64+/-10% (both P<0.01). The
increase in blood pressure and systemic vascular resistance correlated
positively with the level of ET-1 (both P<0. 005) but not with plasma
nitrite/nitrate level. L-NAME infusion did not result in significant
changes in the plasma concentrations of ET-1 or nitrite/nitrate.
CONCLUSIONS: NO and ET-1 may both play a role in the cardiovascular
derangements of human sepsis. Although L-NAME does not increase ET-1
concentration in patients with septic shock, the vasopressor response
induced by L-NAME depends on the plasma level of ET-1. These findings may
indicate that inhibitors of NO synthesis unmask a tonic pressor response
of ET-1 in human septic shock
Normal forms and entanglement measures for multipartite quantum states
A general mathematical framework is presented to describe local equivalence
classes of multipartite quantum states under the action of local unitary and
local filtering operations. This yields multipartite generalizations of the
singular value decomposition. The analysis naturally leads to the introduction
of entanglement measures quantifying the multipartite entanglement (as
generalizations of the concurrence and the 3-tangle), and the optimal local
filtering operations maximizing these entanglement monotones are obtained.
Moreover a natural extension of the definition of GHZ-states to e.g. systems is obtained.Comment: Proof of uniqueness of normal form adde
Alteration of postantibiotic effect during one dosing interval of tobramycin, simulated in an in vitro pharmacokinetic model
The kinetics of the postantibiotic effect (PAE) during one dosing interval
of tobramycin against Staphylococcus aureus and Pseudomonas aeruginosa was
investigated. We determined the PAE at different time points during this
dosing interval of 12 h in an in vitro pharmacokinetic model simulating
human pharmacokinetics in which the half-life of tobramycin was adjusted
to 2.4 +/- 0.2 h. Using an enzymatic method to inactivate tobramycin, we
determined PAEs in samples extracted from the model at 1, 5, 8, and 12 h,
corresponding with tobramycin concentrations of 20, 5, 2, and 1 times the
MIC for the test organism. The PAE decreased significantly from 2.5 h at 1
h to 0 h at 12 h. No change in MIC was observed for the strains during the
experiments. We conclude that the PAE decreases with decreasing tobramycin
concentrations during a 12-h dosing interval and completely disappears
after the concentration has reached the MIC for the test organism. On the
basis of these observations, the emphasis that is placed on the PAE in
discussions about the optimal dosing interval in aminoglycoside therapy is
questionable
Completeness and complexity of reasoning about call-by-value in Hoare logic
We provide a sound and relatively complete Hoare logic for reasoning about partial correctness of recursive procedures in presence of local variables and the call-by-value parameter mechanism and in which the correctness proofs support contracts and are linear in the length of the program. We argue that in spite of the fact that Hoare logics for recursive procedures were intensively studied, no such logic has been proposed in the literature
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