Exact Algorithms for <Emphasis Type="Italic">L</Emphasis>(2,1)-Labeling of Graphs

The notion of distance constrained graph labelings, motivated by the Frequency Assignment Problem, reads as follows: A mapping from the vertex set of a graph G=(V,E) into an interval of integers {0,…,k} is an L(2,1)-labeling of G of span k if any two adjacent vertices are mapped onto integers that are at least 2 apart, and every two vertices with a common neighbor are mapped onto distinct integers. It is known that for any fixed k≥4, deciding the existence of such a labeling is an NP-complete problem. We present exact exponential time algorithms that are faster than the naive O ^*((k+1) ⁿ ) algorithm that would try all possible mappings. The improvement is best seen in the first NP-complete case of k=4, where the running time of our algorithm is O(1.3006 ⁿ ). Furthermore we show that dynamic programming can be used to establish an O(3.8730 ⁿ ) algorithm to compute an optimal L(2,1)-labeling.     

Structure-dependent relative toxic potencies of selected pyrrolizidine alkaloids

Pyrrolizidine alkaloids are naturally occurring secondary plant metabolites mainly found in plant families of Asteraceae, Boraginaceae, and Fabaceae. Chemically, Pas consist of a pyrrolizidine core bearing hydroxyl groups, the so-called necine base, and mono- or dicarboxylic necine acids bound to the pyrrolizidine core via ester linkages. 1,2-unsaturated PAs are hepatotoxic, genotoxic, and carcinogenic due to the highly reactive pyrrolic metabolites formed by cytochrome P450 monooxygenases (CYPs) primarily in the liver. The presence of PAs as frequent contaminants in the wide variety of food and feed products has to be considered a relevant safety issue. Based on the currently available data, the risk assessment of PAs was mainly approached using the two most toxic potent congeners, i.e., lasiocarpine and riddelliine. However, it is well recognized that toxicity is differing significantly between the congeners related to their structural features. The risk of PA-containing products is indeed overestimated, and a comprehensive risk assessment should take these differences into account. After analyzing the data of many PAs, Merz and Schrenk derived interim Relative Potency (iREP) factors to present the differences in their toxicity between the sub-groups of PA congeners concerning their structural features. The use of such iREP factors could probably provide a more scientific basis for PA risk assessment until sufficient experimental analysis of the toxicities of individual congeners is applied. To obtain a better understanding of the relationship between structure and toxicity of PA congeners and provide more evidence for further refinement of relative (toxic) potency factors, data of the in vitro cytotoxicity, genotoxicity, and mutagenicity of diverse individual PA congeners (lasiocarpine, monocrotalineφ, retrorsine, senecionine, seneciphyllineφ, echimidineφ, europineφ, heliotrineφ, indicine, and lycopsamine) has been generated in our project supported by Kooperation Phytopharmaka. Among them, lasiocarpine, retrorsine, senecionine, indicine and lycopsamine have been investigated on my part. Cytotoxicity was assessed using the Alamar blue assay in primary rat hepatocytes, HepG2 cells, and the HepG2 (CYP3A4) cell line. In HepG2 cells, none of the selected PAs exhibited cytotoxic effects, probably due to the lack of CYPs. In primary rat hepatocytes as well as in HepG2(CYP3A4) cells, a clear structure dependent cytotoxicity could be demonstrated. The role of CYP450 enzymes in metabolic activation was further confirmed using an inhibition assay. A kinetic assay analyzing 7-benzyloxyresorufin-O- dealkylation (BROD) was used for measuring the activity of CYP450 enzymes. Furthermore, the utilization of a glutathione-reductase-DTNB recycling assay indicated that glutathione might not play a critical role in PA-induced cytotoxicity. A micronucleus test was used for determining the PA-induced clastogenic genotoxicity. All selected PA congeners exhibited concentration-dependent toxicity in the HepG2 (CYP3A4) cells. The relative potencies of PA congeners estimated from Alamar blue assay and micronucleus assay are generally consistent with the following ranking: lasiocarpine > senecionine > seneciphylline > retrorsine > heliotrine (?) echimidine > europine = indicine = lycopsamine = monocrotaline. The relative toxic potencies evaluated based on our findings were not completely consistent with the iREP classification previously reported by Merz and Schrenk. Monocrotaline in both assays exhibited considerably lower toxic potency. Echimidine, however, was more toxic than expected. On the other hand, mutagenicity was measured in Ames fluctuation assay with Salmonella typhimurium strains TA98 and TA100. None of the selected PA congeners up to 300 μM showed mutagenic effects despite metabolic activation with S9-mix.     

Dislocation processes in the deformation of nanocrystalline aluminium by molecular-dynamics simulation

The mechanical behaviour of nanocrystalline materials (that is, polycrystals with a grain size of less than 100 nm) remains controversial. Although it is commonly accepted that the intrinsic deformation behaviour of these materials arises from the interplay between dislocation and grain-boundary processes, little is known about the specific deformation mechanisms. Here we use large-scale molecular-dynamics simulations to elucidate this intricate interplay during room-temperature plastic deformation of model nanocrystalline Al microstructures. We demonstrate that, in contrast to coarse-grained Al, mechanical twinning may play an important role in the deformation behaviour of nanocrystalline Al. Our results illustrate that this type of simulation has now advanced to a level where it provides a powerful new tool for elucidating and quantifying--in a degree of detail not possible experimentally--the atomic-level mechanisms controlling the complex dislocation and grain-boundary processes in heavily deformed materials with a submicrometre grain size.     

A Practical Attack on KeeLoq

KeeLoq is a lightweight block cipher with a 32-bit block size and a 64-bit key. Despite its short key size, it is used in remote keyless entry systems and other wireless authentication applications. For example, there are indications that authentication protocols based on KeeLoq are used, or were used by various car manufacturers in anti-theft mechanisms. This paper presents a practical key recovery attack against KeeLoq that requires 2¹⁶ known plaintexts and has a time complexity of 2^44.5 KeeLoq encryptions. It is based on the principle of slide attacks and a novel approach to meet-in-the-middle attacks.     

Photolytic hydrogenation of the polyynes C6H2, C8H2, C10H2, and C12H2 in n-hexane with 253.6 nm radiation

The polyynes C₆H₂, C₈H₂, C₁₀H₂, and C₁₂H₂ in n-hexane exposed to 253.6 nm radiation “disappear” much faster in air-free solutions than in solutions in equilibrium with air. These observations suggest that the disappearance is due to reaction of the polyyne molecules in the ground state with a photolytically produced chemical species which in solutions in equilibrium with air is overwhelmingly scavenged by dissolved O₂. Although no hydrogenation products were firmly established the most likely chemical is atomic hydrogen.     

Characterization of silica-based monoliths with bimodal pore size distribution

Band dispersion was studied and the retention thermodynamics addressed for insulin and angiotensin II on C18 silica monoliths with a bimodal pore size distribution, covering linear mobile-phase velocities up to 1 cm/s and different temperatures. These data suggest that the influence of average column pressure on retention (between 0 and 10 MPa) is not negligible. Plate height curves were interpreted with the van Deemter equation by assuming an independent contribution from mechanical and non-mechanical dispersion mechanisms. This analysis revealed diffusion-limited mass transfer in the mesoporous silica skeleton which, in turn, allowed us to calculate an equivalent dispersion particle diameter (d(disp) = 3 microm) using the C-term parameter of the van Deemter equation. The resulting superposition of reduced plate height curves for monolithic and particulate beds confirmed that this view presents an adequate analogy. The macroporous interskeleton network responsible for the hydraulic permeability of a monolith was translated to the interparticle pore space of particulate beds, and an equivalent permeability particle diameter (d(perm) = 15 microm) was obtained by scaling based on the Kozeny-Carman equation.     

Microcontact printing of DNA molecules

The controlled placement of DNA molecules onto solid surfaces is the first step in the fabrication of DNA arrays. The sequential deposition of tiny drops containing the probe DNA fragments using arrays of spotting needles or ink jet nozzles has become a standard. However, a caveat of liquid spotting is the drying of the deposited drop because this creates the typical inhomogeneities, i.e., rims around the spot. Another drawback is that each DNA array is an original and has to be fabricated individually. Microcontact printing is a versatile technique to place proteins onto different target surfaces in uniformly patterned monolayers with high lateral resolution. Here, we show for the first time that DNA can also be printed with equally high resolution in the submicrometer range using an elastomeric stamp with chemically tailored surface. Two regimes for the transfer of the molecules were observed. Finally, microcontact printing of an array of DNA probes onto a solid support and its use in a subsequent hybridization assay was demonstrated.