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E A Curtis

Publications and source records attributed to E A Curtis.

9 recordsLinked to original sources
PubMed2004

Absolute-frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser.

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond 1.8 microm. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology's optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the d/f crossover transition in 87Rb at 780 nm, and the methane v2 + 2v3 R(8) line at 1315 nm.

Journal Article↗
PubMed2001

An optical clock based on a single trapped 199Hg+ ion.

Microwave atomic clocks have been the de facto standards for precision time and frequency metrology over the past 50 years, finding widespread use in basic scientific studies, communications, and navigation. However, with its higher operating frequency, an atomic clock based on an optical transition can be much more stable. We demonstrate an all-optical atomic clock referenced to the 1.064-petahertz transition of a single trapped 199Hg+ ion. A clockwork based on a mode-locked femtosecond laser provides output pulses at a 1-gigahertz rate that are phase-coherently locked to the optical frequency. By comparison to a laser-cooled calcium optical standard, an upper limit for the fractional frequency instability of 7 x 10(-15) is measured in 1 second of averaging-a value substantially better than that of the world's best microwave atomic clocks.

Journal Article↗
PubMed2001

Absolute frequency measurements of the Hg+ and Ca optical clock transitions with a femtosecond laser.

The frequency comb created by a femtosecond mode-locked laser and a microstructured fiber is used to phase coherently measure the frequencies of both the Hg+ and Ca optical standards with respect to the SI second. We find the transition frequencies to be f(Hg) = 1 064 721 609 899 143(10) Hz and f(Ca) = 455 986 240 494 158(26) Hz, respectively. In addition to the unprecedented precision demonstrated here, this work is the precursor to all-optical atomic clocks based on the Hg+ and Ca standards. Furthermore, when combined with previous measurements, we find no time variations of these atomic frequencies within the uncertainties of the absolute value of( partial differential f(Ca)/ partial differential t)/f(Ca) < or =8 x 10(-14) yr(-1) and the absolute value of(partial differential f(Hg)/ partial differential t)/f(Hg) < or =30 x 10(-14) yr(-1).

Journal Article↗
PubMed2001

The hammerhead cleavage reaction in monovalent cations.

Recently, Murray et al. (Chem Biol, 1998, 5:587-595) found that the hammerhead ribozyme does not require divalent metal ions for activity if incubated in high (> or =1 M) concentrations of monovalent ions. We further characterized the hammerhead cleavage reaction in the absence of divalent metal. The hammerhead is active in a wide range of monovalent ions, and the rate enhancement in 4 M Li+ is only 20-fold less than that in 10 mM Mg2+. Among the Group I monovalent metals, rate correlates in a log-linear manner with ionic radius. The pH dependence of the reaction is similar in 10 mM Mg2+, 4 M Li+, and 4 M Na+. The exchange-inert metal complex Co(NH3)3+ also supports substantial hammerhead activity. These results suggest that a metal ion does not act as a base in the reaction, and that the effects of different metal ions on hammerhead cleavage rates primarily reflect structural contributions to catalysis.

Base Sequence↗
PubMed2000

Evolution and assembly of an extremely scrambled gene.

The process of gene unscrambling in hypotrichous ciliates represents one of nature's ingenious solutions to the problem of gene assembly. With some essential genes scrambled in as many as 51 pieces, these ciliates rely on sequence and structural cues to rebuild their fragmented genes and genomes. Here we report the complex pattern of scrambling in the DNA polymerase alpha gene of Stylonychia lemnae. The germline (micronuclear) copy of this gene is broken into 48 pieces with 47 dispersed over two loci, with no asymmetry in the placement of coding segments on either strand. Direct repeats present at the boundaries between coding and noncoding sequences provide pointers to help guide assembly of the functional (macronuclear) gene. We investigate the evolution of this complex gene in three hypotrichous species.

Animals↗
PubMed1971

A one-week introductory course in clinical biochemistry and research based on renal function testing in man.

A one-week introductory course in clinical biochemical and research techniques was devised to provide a brief but concentrated experience for interested persons. Urinary creatinine concentrations were measured to demonstrate acceptable degrees of precision of a method (co-efficients of variation, 0.5-2.4%), and to give confidence by favourable comparison with routine laboratory results (day 2, mean 0.95 vs 0.94 mg/ml). Subjects then submitted themselves to one-day renal function testing, and subsequently measured their own urinary acid-base parameters by autotitrimetry and glomerular filtration rates by spectrophotometry. The values obtained for the latter fell within the normal range (85-124% of normal, corrected for age and surface area). Finally, statistical methods and a small desk-top computer were employed to compute linear regression equations, correlation coefficients, and t tests from the week's data. The course contained a number of features which stimulated learning, including active participation by the learner, and the use of meaningful materials to induce both desire for success and tolerance of mistakes.

Australia↗