Properties of the skeleton of aggregates grown on a Cayley tree

We discuss and analyze a family of trees grown on a Cayley tree, that allows for a variable exponent in the expression for the mass as a function of chemical distance, M(l)l ^dl . For the suggested model, the corresponding exponent for the mass of the skeleton,d _l ^s , can be expressed in terms ofd _l asd _l ^s = 1,d _l d _l ^c = 2;d _l ^s = d _l –1,d ₁ d _l ^c = 2, which implies that the tree is finitely ramified ford _l 2 and infinitely ramified whend _l 2. Our results are derived using a recursion relation that takes advantage of the one-dimensional nature of the problem. We also present results for the diffusion exponents and probability of return to the origin of a random walk on these trees.     

Laser photolysis studies of spiropyran-merocyanine aggregate formation in solution

The photoinduced processes leading to formation of J-aggregate stacks of 1-(β-methacryloxyethyl)-3,3-dimethyl-6′- nitrospiro-(indoline-2,2′-[2H-[2H-1] benzopyran) d its associated ring opened merocyanine form B have been determined by N₂-laser transient spectroscopy. Detailed mechanisms for formation of complexes AB, A₂B, and J-aggregate stacks (A₂B) _n in aliphatic and aromatic solvents are presented.     

Mutagenesis Mediated by Triple Helix–Forming Oligonucleotides Conjugated to Psoralen: Effects of Linker Arm Length and Sequence Context

Targeted mutagenesis and gene knock-out can be mediated by triple helix-forming oligonucleotides (TFO) linked to mutagenic agents, such as psoralen. However, this strategy is limited by the availability of homopurine/ homopyrimidine stretches at or near the target site because such sequences are required for high-affinity triplex formation. To overcome this limitation, we have tested TFO conjugated to psoralen via linker arms of lengths varying from 2 to 86 bonds, thereby designed to deliver the psoralen at varying distances from the third strand binding site present at the 3'end of the supFG1 mutation reporter gene. Following triplex formation and UVA irradiation, mutations were detected using an SV40-based shuttle vector assay in human cells. The frequency and distribution of mutations depended on the length of the linker arm. Precise targeting was observed only for linker arms of length 2 and 6, which also yielded the highest mutation frequencies (3 and 14%, respectively). Psoralen–TFO with longer tethers yielded mutations at multiple sites, with the maximum distance from the triplex site limited by the linker length but with the distribution within that range influenced by the propensity for psoralen intercalation at A:T base-pair-rich sites. Thus, gene modification can be extended beyond the site of third strand binding but with a decrease in the precision of the targeting.     

Motion artifacts in quantitative magnetic resonance imaging

Several investigators have emphasized the potential value of quantitative relaxation times in the assessment of diseases. In performing such measurements using the spin-echo technique, we have encountered several anomalous results, whereby the intensity of the organ parenchyma on second-echo images is greater than on first echo images. This is most likely a result of respiratory motion, and it occurs only rarely. Several volunteers were studied before and after exercise to see if respiratory motion could reproduce the anomalous intensity reverse; a reversal of intensities in renal parenchyma was observed in two of five individuals. We conclude that respiratory motion artifacts will seriously limit quantitative magnetic resonance imaging of the upper abdomen if respiratory gating during imaging is not used.     

The complexity of identifying redundant and essential elements

In many optimization problems a solution is a subset of optimum number of elements satisfying some desired property. An element is redundant if it does not belong to any solution of the problem. An element is essential if it belongs to every solution of the problem. We consider the complexity of indentifying redundant and essential elements in a sample of NP-Hard optimization problems. It is shown that these identification problems are also NP-Hard. The proofs are based on an analysis of the original reductions of Cook [The complexity of theorem proving procedures, in “Proceedings, Third Annual Assoc. Comput. Mach. Symposium on Theory of Computing”, pp. 151–158, Assoc. Comput. Mach., New York 1971] and Karp [Reducibility among combinational problems, in “Complexity of Computer Computations” (R. E. Miller and J. W. Thatcher, Eds.), pp. 85–104, Plenum, New York 1972].     

A review of: “Ordering in strongly fluctuating condensed matter systems”

Abstract

Ed. T. Riste. (Nato Advanced Study Institutes Series, Vol. 50). Plenum, Price: $55 (U.S.)