Comparative decay characteristics of the light generated free radical in chromatophores and chloroplasts

Abstract— Chromatophores of Rhodospirillum rubrum when illuminated exhibit a free radical which ordinarily shows a biphasic decay. When chromatophores are dehydrated at room temperature the time course of the appearance of the light-induced free radical is unaffected, but the decay pattern has been altered. Only the fast component remains, the slow component is no longer evident. Scanning the magnetic field reveals the presence of a dark signal which is stable as long as the preparation remains dehydrated. This signal has the same peak-to-peak line width of ? 10 G and the same g factor as the signal evident in the light. The amplitude of this signal is equal to the amplitude of the slow decay component seen in aqueous chromatophore suspensions. Chromatophores frozen in an aqueous medium at —150°C exhibit a behavior identical with dehydrated preparations. The effects produced by lyophilization or by freezing at low temperatures are entirely reversible. When a lyophilized preparation is re-hydrated, the stabilized portion of the signal now decays in the dark; the same is observed when preparations frozen at —150°C are thawed. When such thawed or re-hydrated preparations are illuminated again, they exhibit the usual light-induced ESR signal showing a biphasic dark decay. A comparison was made between the behavior of the light-induced ESR signal of chromatophores and that of system I of chloroplasts. This comparison revealed that there is a greater similarity in some of the decay characteristics of these signals than had been recognized previously. In chloroplasts, temperature insensitive, non-enzymatic back-reactions of the light-induced free radical appear to be nil, and in chromatophores a distinct portion of the light-induced free radicals exhibit the same characteristics. Another portion of the chromatophore free radicals must be able to back-react by electron tunneling, a mechanism which appears to be absent in the chloroplast system.     

Search for ZH → l+ l- bb production in 4.2 fb(-1) of pp collisions at sqrt[s] =1 .96 TeV

We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb(-1) of pp collisions, collected with the D0 detector at the Fermilab Tevatron at sqrt[s] =1 .96 TeV. Selected events contain one reconstructed Z → e+ e- or Z → μ+ μ- candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M(H) = 115 GeV is a factor of 5.9 larger than the standard model prediction.     

Proton acceleration to 40 MeV using a high intensity, high contrast optical parametric chirped-pulse amplification/Ti:sapphire hybrid laser system

Using a high-contrast (10(10):1) and high-intensity (10(21) W/cm(2)) laser pulse with the duration of 40 fs from an optical parametric chirped-pulse amplification/Ti:sapphire laser, a 40 MeV proton bunch is obtained, which is a record for laser pulse with energy less than 10 J. The efficiency for generation of protons with kinetic energy above 15 MeV is 0.1%.     

Assessing the validity of theoretical results

We present recent theoretical results for the V₃ and Au₄ clusters. Calculations of the V₃ doublet system indicate that the 6-311+G(d) basis set is sufficiently flexible to provide reliable minimum energy structures and vibrational frequencies, that these structures and frequencies are insensitive to spin contamination of the wave function when the BPW91 functional is used, and that changing to the B3LYP functional may result in very different structures and frequencies. A computationally less expensive scalar relativistic treatment of Au₄ clusters gives structural properties that are in good agreement with those obtained using a four-component method. Received 30 November 2000