PHOTOMODULATION OF AZOALDOLASE ACTIVITY

Abstract— Rabbit muscle aldolase, modified by azo groups covalently bound to cysteinyl residues 237 and 287, shows activation of the fructose-1,6 diphosphate cleavage reaction at 30T, after preirradiation with visible light. When the light treatment is carried out under an atmosphere of Ar to avoid oxidation, there is a reversible diminution of the Michaelis-Menten constant, with no change in the turnover number. The direct effect of the light is the E → Z geometric isomerization of the azochromophore; thereafter the Z isomer is thermally converted to the more stable E isomer. Both regulation at the level of the enzyme-substrate interaction and the involvement of the geometry of the chromophore have been demonstrated. The substrate, added to azoaldolase in the dark, causes E to Z isomerization of part of the extrinsic protein chromophores leading to a different isomeric composition at the thermal equilibrium. This result is consistent with higher binding energy of the enzymesubstrate complex when the modified enzyme contains azo chromophores in the Z configuration, as compared with the same chromophores in the E configuration. Azoaldolase has been discussed as a model for direct photoregulation of enzyme activity at the level of enzyme-substrate interaction.     

Measurement of the total proton-proton cross-section at the ISR

We present the first results of a measurement of the total cross-section σ_T in proton-proton collisions at equivalent laboratory momenta between 291 and 1480 GeV/c at the CERN Intersecting Storage Rings (ISR). The method is based on the measurement of the ratio of the total interaction rate and the machine luminosity. The data show an increase of about 10% in σ_T in this energy interval.     

Kinetic energy distributions around symmetric thermal fission of U234 and U236

The kinetic energy distributions have been measured at LOHENGRIN for symmetric fission of U²³⁴ and U²³⁶. The pre-neutron emission values for the average total kinetic energy and its rms width have been deduced using a Monte Carlo simulation for neutron evaporation. The total kinetic energy dip between symmetric and asymmetric divisions is 24 MeV for U²³⁴ and 29 MeV for U²³⁶. A strong peak in the initial rms width has been observed for masses 111–123 in U²³⁴ and for 111–125 in U²³⁶. A static scission point model is used to understand the origin of these features.     

Macroscopic dynamics as reporter of mesoscopic organization: the Belousov-Zhabotinsky reaction in aqueous layers of DPPC lamellar phases

The propagation of traveling chemical waves in the excitable Belousov-Zhabotinsky (BZ) system when performed in the presence of 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl choline (DPPC) bilayers responds sensitively to the phospholipid content. The characteristic features of wave propagation, such as spiral pitch, rotation period, and size of the spiral core region, show two regions of different behavior, one below and the other above a DPPC content of 12.5% (w/w) thus suggesting a transition in the organization of the lipid domains at a DPPC content of ～12.5% (w/w). This transition is supported by small-angle X-ray scattering data, which show pronounced changes in the coherence lengths of the lyotropic smectic domains. Thus, the dynamics of the chemical system occurring at a macroscopic length scale reflects the organization of the water/lipid domains which extend over mesoscopic lengths. These findings indicate that in the BZ/DPPC system, there is an interaction between processes that occurs at length scales differing by as much as 3 orders of magnitude.