Light-induced Aggregation Research Articles

Spectroscopy, chemistry and catalysis of metal atoms, metal dimers and metal clusters

This paper starts with the experimental realization of trapped silver atoms in various matrix supports, with emphasis placed on optical absorption, emission and e.s.r. spectroscopic properties. Special attention is devoted to ground- and excited-state silver atom interactions with the surrounding matrix cage of atoms, particularly with respect to the recently discovered phenomenon of light-induced diffusion and aggregation of silver atoms to small, precisely defined silver clusters. Various aspects and applications of photodiffusion methods are highlighted and some pertinent comparisons are made with metal concentration and bulk annealing approaches as alternatives to controlled metal nucleation. This will lead to the embryonic clusters Ag2 and Ag3 for which there now exists considerable spectroscopic, photochemical and theoretical information. Silver-containing bimetallic clusters, generated either by metal concentration deposition or photoaggregation methods, will be a natural extension to the Ag2,3 presentation; their relevance to bimetallic cluster catalysts will be briefly contemplated. The concept of generating anionic silver clusters by Na/Ag photoionization methods will be briefly described, with reference to the parent Ag– anion. In considering the higher silver clusters, one is now in a position to evaluate experimentally the genesis of silver nucleation from the stage of an isolated silver atom, through to a six-atom array and to higher, less well defined aggregates. With these data one can attempt to track the evolving optical and e.s.r. properties by reference to the build-up of cluster electronic states calculated by way of SCF-Xα-SW molecular-orbital procedures for assumed cluster geometries. The observation of rudimentary interband transitions of silver clusters in the range 6-13 atoms, that absorb in a similar energy region to the collective electronic excitations associated with plasmon absorption of silver microcrystallites, simultaneously with the evolution of conduction electron spin resonance absorption, whose observed linewidths and g-shifts conform to the theoretical predictions of Kubo and Kawabata, can in principle provide a valuable criterion on which to judge the atomic composition at which optical–electronic characteristics of silver aggregates transform from those of the molecular to the bulk state. Future directions in diatomic-metal and metal-cluster chemistry are briefly contemplated in the light of recent break-throughs with ambient-temperature metal-vapour–liquid-polymer techniques and the discovery of polymer-supported, very low nuclearity metal cluster, generated and stable at, or very close to, room temperature.

Changes in protein synthesis during alternate pathways of differentiation in the cellular slime mold Polysphondylium pallidum

Acrylamide gel electrophoresis in the presence of sodium lauryl sulfate and autoradiography of dried gels were used to detect incorporation of [ 14C]acetate into specific polypeptides of developing cells. Twenty-four polypeptides showed changes in net rate of synthesis during light-induced aggregation and sorogenesis, and 15 bands changed during KCl-induced encystment. Most of the changes during aggregation were common to three strains of P. pallidum, but different from those exhibited by D. discoideum. They failed to occur if the cells were maintained in darkness, and the pattern of changes was altered by sublethal high temperature. A few changes were common to both aggregating and encysting cells, and to stalkforming cells induced by cyclic AMP. A pulse-chase experiment indicated that all detectable increases in net synthesis were caused by alterations in actual synthesis rather than in degradation. It is suggested that the observed changes in synthesis may be essential to the correlated pathways of cellular differentiation.

Ultraviolet light modification of the prostaglandin receptor

Prostaglandin E 1 elicits minimal inhibition of ultraviolet lightinduced platelet aggregation. Exposure to ultraviolet light progressively inhibits prostaglandin-induced responses in both the ADP-aggregating platelet and in uterine smooth muscle. This effect, which is cumulative and lasting, may be prevented or fully reversed by a sulfhydryl-oxidizing agent. Ultraviolet light may, therefore, initiate reduction of critical disulfide groups in the vicinity of the prostaglandin receptor. The technique of ultraviolet irradiation may serve as a useful tool in the specific determination of prostaglandins in biological systems.

THE INFLUENCE OF LIGHT ON CELL AGGREGATION IN POLYSPHONDYLIUM PALLIDUM

Experiments have been performed on the effect of light on cell aggregation and aggregation density in Polysphondylium pallidum, strain WS-320. The results of these experiments may be summarized as follows:1 . Cell aggregation occurs more quickly and with a greater density of aggregation centers in cultures incubated in the light than in comparable cultures incubated in the dark.2. Myxamoebae grown in the dark in liquid culture and deposited on agar are most sensitive to light within the first few hours of incubation.3. Beyond the initial period of sensitivity, there is a diminution in response resulting in an almost complete absence of light-induced aggregation.4. Little variation in response occurred when myxamoebae were exposed to light for periods of from one to 15 minutes, indicating a fairly sensitive light response mechanism.5. In cultures grown in situ, exposure to the light following dark incubation induces aggregation in two- to three-day-old myxamoebae. Older myxamoebae fail to display any sign of morphogenetic activity in response to light; they do respond, however, in the presence of charcoal.6. Of certain physical and chemical agents tested for their effect on aggregation, charcoal and mineral oil were found to increase aggregation density in both light and dark, while CO2, in the light, had the opposite effect.The similarity in response achieved by charcoal and mineral oil, and by light, suggests that the action of light may be to change the relationship between aggregation and a center-suppressing factor that accumulates in the environment. In the light, aggregation occurs possibly because light limits the production of or destroys the center-suppressing factor, or renders the cells less sensitive to it. In the absence of light, aggregation may be inhibited by this substance while the cells are shunted in the direction of microcyst formation. If, in the latter situation, charcoal or mineral oil were added prior to actual cyst formation, the suppressor would be absorbed and aggregation could occur.