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Potential Artefacts (potential + artefact)

Distribution by Scientific Domains

Medical Sciences	75%

Selected Abstracts

Significant activity of a modified ribozyme with N7-deazaguanine at G10.1: the double-metal-ion mechanism of catalysis in reactions catalysed by hammerhead ribozymes

GENES TO CELLS, Issue 8 2000
Yuka Nakamatsu
Background Several reports have appeared recently of experimental evidence for a double-metal-ion mechanism of catalysis in reactions catalysed by hammerhead ribozymes. In one case, hammerhead ribozyme-mediated cleavage was analysed as a function of the concentration of La3+ ions in the presence of a fixed concentration of Mg2+ ions so that the role of metal ions that are directly involved in the cleavage reaction could be monitored. The resultant bell-shaped curve for activation of cleavage was used to support the proposed double-metal-ion mechanism of catalysis. However, other studies have demonstrated that the binding of a metal ion (the most conserved P9 metal ion) to the pro-Rp oxygen (P9 oxygen) of the phosphate moiety of nucleotide A9 and to the N7 of nucleotide G10.1 is critical for efficient catalysis, despite the large distance (,20 Å) between the P9 metal ion and the labile phosphodiester group in the ground state. In fact, it was demonstrated that an added Cd2+ ion binds first to the pro-Rp phosphoryl P9 oxygen but not with the pro-Rp phosphoryl oxygen at the cleavage site. Results In earlier discussions, it was difficult to completely exclude the possibility that La3+ ions might have replaced the P9 metal ion and, as a result, created conditions represented by the bell-shaped curve. In order to clarify this situation, we examined a chemically synthesized hammerhead ribozyme (7-deaza-R34) that included a minimal modification, namely, an N7-deazaguanine residue in place of G10.1. We compared the kinetic properties of this ribozyme with those of the parental ribozyme (R34). Kinetic analysis revealed that, unlike the cases of added Cd2+ ions, the added La3+ ions did not replace the pre-existing P9 metal ion, and that the replacement of N7 by C7 at G10.1 reduced the catalytic activity to a limited extent. This result indicates that the binding of a Mg2+ ion to N7 at G10.1 is catalytically important but not indispensable. Most importantly, 7-deaza-R34 also yielded a bell-shaped curve upon addition of La3+ ions to the reaction mixture. Conclusions Since the data based on our experiments with 7-deaza-R34 are completely free from potential artefacts, due to the binding of a La3+ ion to N7 at G10.1, our results, that 7-deaza-R34 yielded a bell-shaped curve following the addition of La3+ ions to the Mg2+ -background reaction mixture, strongly supports the proposal that a double-metal-ion mechanism is operative in the cleavage reaction which is catalysed by hammerhead ribozymes. [source]

Electron energy-loss near-edge structure , a tool for the investigation of electronic structure on the nanometre scale

JOURNAL OF MICROSCOPY, Issue 2 2001
V. J. Keast
Electron energy-loss near-edge structure (ELNES) is a technique that can be used to measure the electronic structure (i.e. bonding) in materials with subnanometre spatial resolution. This review covers the theoretical principles behind the technique, the experimental procedures necessary to acquire good ELNES spectra, including potential artefacts, and gives examples relevant to materials science. [source]

Transverse chromatic aberration and colour-defined motion

OPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 4 2000
Jocelyn Faubert
Summary A number of recent studies have explored the role of the chromatic system in motion processes using the isoluminance paradigm. A major concern when using such methodological procedures is potential artefacts produced by chromatic aberrations. In the present study we address the problem of optically induced luminance artefacts produced by transverse chromatic aberrations (TCA), which may contaminate the results obtained in chromatic motion-nulling experiments. Results show that different TCA levels artificially increase chromatic motion sensitivity values to varying degrees above 0.5 cpd for red/green gratings. The data also suggest the notion that naturally occurring TCA can decrease motion-nulling thresholds for chromatic gratings at high spatial frequencies. Furthermore, our data show that the motion-nulling paradigm for chromatic gratings may in fact be an efficient functional method for assessing the amount of TCA produced by optical factors. [source]

Ethnicity and gestational diabetes in New York City, 1995,2003

BJOG : AN INTERNATIONAL JOURNAL OF OBSTETRICS & GYNAECOLOGY, Issue 8 2008
DA Savitz
Objective, To characterise the patterns of occurrence of gestational diabetes among a wide range of ethnic groups that reside in New York City. Design, Birth records and hospital discharge data were linked to more accurately assess the risk of gestational diabetes by ethnicity, compare risk in US-born to foreign-born women, and assess time trends. Setting, New York City. Population, All singleton live births occurring between 1995 and 2003. Methods, Multivariable binomial regression analysis of ethnicity and gestational diabetes, yielding adjusted risk ratios with non-Hispanic white women as the referent. Main outcome measure, Diagnosis of gestational diabetes on birth certificate or in hospital discharge. Results, Adjusted relative risks (aRRs) were modestly elevated for African-Americans and sub-Saharan Africans and somewhat higher (<2.0) for non-Hispanic Caribbeans, Hispanic Caribbeans, Central Americans, and South Americans. The aRR was 4.7 (95% CI = 4.6,4.9) for South Central Asians (with an absolute gestational diabetes risk of 14.3%), 2.8 (95% CI = 2.7,3.0) among South-East Asian and Pacific Islanders, and 2.3 (95% CI = 2.2,2.4) among East Asians. Among South Central Asians, the greatest risks were found for women from Bangladesh (aRR = 7.1, 95% CI = 6.8,7.3). Foreign-born women consistently had higher risk than US-born women. Risk for gestational diabetes increased over time among South Central Asians, some Hispanic groups, and African-Americans. Conclusions, Risk of gestational diabetes appears to vary markedly among ethnic groups, subject to potential artefacts associated with screening and diagnosis. These differences would have direct implications for health care and may suggest aetiologic hypotheses. [source]