Showing papers by "North Carolina State University published in 1992"

GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System

Abstract: We present a new approach to remote sensing of water vapor based on the global positioning system (GPS). Geodesists and geophysicists have devised methods for estimating the extent to which signals propagating from GPS satellites to ground-based GPS receivers are delayed by atmospheric water vapor. This delay is parameterized in terms of a time-varying zenith wet delay (ZWD) which is retrieved by stochastic filtering of the GPS data. Given surface temperature and pressure readings at the GPS receiver, the retrieved ZWD can be transformed with very little additional uncertainty into an estimate of the integrated water vapor (IWV) overlying that receiver. Networks of continuously operating GPS receivers are being constructed by geodesists, geophysicists, government and military agencies, and others in order to implement a wide range of positioning capabilities. These emerging GPS networks offer the possibility of observing the horizontal distribution of IWV or, equivalently, precipitable water with unprecedented coverage and a temporal resolution of the order of 10 min. These measurements could be utilized in operational weather forecasting and in fundamental research into atmospheric storm systems, the hydrologic cycle, atmospheric chemistry, and global climate change. Specially designed, dense GPS networks could be used to sense the vertical distribution of water vapor in their immediate vicinity. Data from ground-based GPS networks could be analyzed in concert with observations of GPS satellite occultations by GPS receivers in low Earth orbit to characterize the atmosphere at planetary scale.

The Image Processing Handbook

Abstract: Consistently rated as the best overall introduction to computer-based image processing, The Image Processing Handbook covers two-dimensional (2D) and three-dimensional (3D) imaging techniques, image printing and storage methods, image processing algorithms, image and feature measurement, quantitative image measurement analysis, and more. Incorporating image processing and analysis examples at all scales, from nano- to astro-, this Seventh Edition: Features a greater range of computationally intensive algorithms than previous versions Provides better organization, more quantitative results, and new material on recent developments Includes completely rewritten chapters on 3D imaging and a thoroughly revamped chapter on statistical analysis Contains more than 1700 references to theory, methods, and applications in a wide variety of disciplines Presents 500+ entirely new figures and images, with more than two-thirds appearing in color The Image Processing Handbook, Seventh Edition delivers an accessible and up-to-date treatment of image processing, offering broad coverage and comparison of algorithms, approaches, and outcomes.

Comparing evolvability and variability of quantitative traits.

Abstract: There are two distinct reasons for making comparisons of genetic variation for quantitative characters. The first is to compare evolvabilities, or ability to respond to selection, and the second is to make inferences about the forces that maintain genetic variability. Measures of variation that are standardized by the trait mean, such as the additive genetic coefficient of variation, are appropriate for both purposes. Variation has usually been compared as narrow sense heritabilities, but this is almost always an inappropriate comparative measure of evolvability and variability. Coefficients of variation were calculated from 842 estimates of trait means, variances and heritabilities in the literature. Traits closely related to fitness have higher additive genetic and nongenetic variability by the coefficient of variation criterion than characters under weak selection. This is the reverse of the accepted conclusion based on comparisons of heritability. The low heritability of fitness components is best explained by their high residual variation. The high additive genetic and residual variability of fitness traits might be explained by the great number of genetic and environmental events they are affected by, or by a lack of stabilizing selection to reduce their phenotypic variance. Over one-third of the quantitative genetics papers reviewed did not report trait means or variances. Researchers should always report these statistics, so that measures of variation appropriate to a variety of situations may be calculated.

Stock Prices and Volume

Abstract: The authors undertake a comprehensive investigation of price and volume co-movement using daily New York Stock Exchange data from 1928 to 1987. They adjust the data to take into account well-known calendar effects and long-run trends. To describe the process, they use a seminonparametric estimate of the joint density of current price change and volume conditional on past price changes and volume. Four empirical regularities are found: (1) positive correlation between conditional volatility and volume; (2) large price movements are followed by high volume; (3) conditioning on lagged volume substantially attenuates the "leverage" effect, and (4) after conditioning on lagged volume, there is a positive risk-return relation. Article published by Oxford University Press on behalf of the Society for Financial Studies in its journal, The Review of Financial Studies.

Molecular-marker-facilitated investigations of quantitative trait loci in maize : 4. Analysis based on genome saturation with isozyme and restriction fragment length polymorphism markers.

Abstract: Restriction fragment length polymorphisms have become powerful tools for genetic investigations in plant species. They allow a much greater degree of genome saturation with neutral markers than has been possible with isozymes or morphological loci. A previous investigation employed isozymes as genetic markers to infer the location of genetic factors influencing the expression of quantitative traits in the maize population: (CO159×Tx303)F2. This investigation was conducted to examine the inferences that might be derived using a highly saturated map of RFLP markers and isozymes to detect quantitative trait loci (QTLs) in the same maize F2 population. Marker loci that were associated with QTL effects in this investigation generally corresponded well with previous information where such comparisons were possible. Additionally, a number of previously unmarked genomic regions were found to contain factors with large effects on some plant traits. Availability of numerous marker loci in some genomic regions allowed: more accurate localization of QTLs, resolution of linkage between QTLs affecting the same traits, and determination that some chromsome regions previously found to affect a number of traits are likely to be due to linkage of QTLs affecting different traits. Many of the factors that affected plant height quantitatively in this investigation were found to map to regions also including known sites of major genes influencing plant height. Although the data are not conclusive, they suggest that some of the identified QTLs may be allelic to known major genes affecting plant height.

Characterization of bias-enhanced nucleation of diamond on silicon by invacuo surface analysis and transmission electron microscopy.

Abstract: An in-depth study has been performed of the nucleation of diamond on silicon by bias-enhanced microwave plasma chemical vapor deposition. Substrates were pretreated by negative biasing in a 2% methane-hydrogen plasma. The bias pretreatment enhanced the nucleation density on unscratched silicon wafers up to ${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ as compared with ${10}^{7}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ on scratched wafers. In vacuo surface analysis including x-ray photoelecton spectroscopy (XPS), Auger electron spectroscopy, and combined XPS and electron-energy-loss spectroscopy were used to study systematically both the initial-nucleation and growth processes. High-resolution cross-sectional transmission electron microscopy (TEM) was used to study the physical and structural characteristics of the diamond-silicon interface as well as to complement and enhance the in vacuo surface-analytical results. Raman spectroscopy confirmed that diamond was actually nucleating during the bias pretreatment. Scanning electron microscopy has shown that once the bias is turned off, and conventional growth is conducted, diamond grows on the existing nuclei and no continued nucleation occurs. If the bias is left on throughout the entire deposition, the resulting film will be of much poorer quality than if the bias had been turned off and conventional growth allowed to begin. Intermittent surface analysis showed that a complete silicon carbide layer developed before diamond could be detected. High-resolution cross-sectional TEM confirmed that the interfacial layer was amorphous and varied in thickness from 10 to 100 \AA{}. A small amount of amorphous carbon is detected on the surface of the silicon carbide and it is believed to play a major role in the nucleation sequence. A model is proposed to help explain bias-enhanced nucleation on silicon, in hopes that this will improve the understanding of diamond nucleation, in general, and eventually result in the nucleation and growth of better-quality diamond films.

Development of a proton nuclear magnetic resonance spectroscopic method for determining plasma lipoprotein concentrations and subspecies distributions from a single, rapid measurement.

Abstract: We are developing a method for quantifying plasma lipoproteins by proton nuclear magnetic resonance (NMR) spectroscopy that offers advantages over existing clinical methods. We showed that the major lipoproteins have distinct NMR properties sufficient to permit their concentrations to be extracted from a computer lineshape analysis of the plasma lipid methyl resonance envelope (Clin Chem 1991; 37:377-86). We have now discovered that the spectra of the subspecies within each lipoprotein class are different enough to influence the composite spectrum of that class and hence the spectrum of whole plasma. By including spectra representative of these subspecies as additional components in the lineshape-fitting analysis, a complete concentration profile of very-low-density, low-density (LDL), and high-density (HDL) lipoproteins plus the subspecies distributions within these classes can be simultaneously generated. A pilot study of 30 plasma samples of widely varied composition demonstrated good agreement between NMR-derived values and lipoprotein lipid concentrations and LDL and HDL subspecies distributions determined by gradient-gel electrophoresis.

Topical vitamin C protects porcine skin from ultraviolet radiation‐induced damage

Abstract: Ultraviolet radiation damage to the skin is due, in part, to the generation of reactive oxygen species. Vitamin C (L-ascorbic acid) functions as a biological co-factor and antioxidant due to its reducing properties. Topical application of vitamin C has been shown to elevate significantly cutaneous levels of this vitamin in pigs, and this correlates with protection of the skin from UVB damage as measured by erythema and sunburn cell formation. This protection is biological and due to the reducing properties of the molecule. Further, we provide evidence that the vitamin C levels of the skin can be severely depleted after UV irradiation, which would lower this organ's innate protective mechanism as well as leaving it at risk of impaired healing after photoinduced damage. In addition, vitamin C protects porcine skin from UVA-mediated phototoxic reactions (PUVA) and therefore shows promise as a broad-spectrum photoprotectant.

New 'phantom' dinoflagellate is the causative agent of major estuarine fish kills

Abstract: A worldwide increase in toxic phytoplankton blooms over the past 20 years has coincided with increasing reports of fish diseases and deaths of unknown cause. Among estuaries that have been repeatedly associated with unexplained fish kills on the western Atlantic Coast are the Pamlico and Neuse Estuaries of the southeastern United States. Here we describe a new toxic dinoflagellate with 'phantom-like' behaviour that has been identified as the causative agent of a significant portion of the fish kills in these estuaries, and which may also be active in other geographic regions. The alga requires live finfish or their fresh excreta for excystment and release of a potent toxin. Low cell densities cause neurotoxic signs and fish death, followed by rapid algal encystment and dormancy unless live fish are added. This dinoflagellate was abundant in the water during major fish kills in local estuaries, but only while fish were dying; within several hours of death where carcasses were still present, the flagellated vegetative algal population had encysted and settled back to the sediments. Isolates from each event were highly lethal to finfish and shellfish in laboratory bioassays. Given its broad temperature and salinity tolerance, and its stimulation by phosphate enrichment, this toxic phytoplankter may be a widespread but undetected source of fish mortality in nutrient-enriched estuaries.

Sucrose Phosphate Synthase and Sucrose Accumulation at Low Temperature

Abstract: The influence of growth temperature on the free sugar and sucrose phosphate synthase content and activity of spinach (Spinacia oleracea) leaf tissue was studied. When plants were grown at 25 degrees C for 3 weeks and then transferred to a constant 5 degrees C, sucrose, glucose, and fructose accumulated to high levels during a 14-d period. Predawn sugar levels increased from 14- to 20-fold over the levels present at the outset of the low-temperature treatment. Sucrose was the most abundant free sugar before, during, and after exposure to 5 degrees C. Leaf sucrose phosphate synthase activity was significantly increased by the low-temperature treatment, whereas sucrose synthase and invertases were not. Synthesis of the sucrose phosphate synthase subunit was increased during and after low-temperature exposure and paralleled an increase in the steady-state level of the subunit. The increases in sucrose and its primary biosynthetic enzyme, sucrose phosphate synthase, are discussed in relation to adjustment of metabolism to low nonfreezing temperature and freezing stress tolerance.

Purification and Characterization of a Keratinase from a Feather-Degrading Bacillus licheniformis Strain

Abstract: A keratinase was isolated from the culture medium of feather-degrading Bacillus licheniformis PWD-1 by use of an assay of the hydrolysis of azokeratin. Membrane ultrafiltration and carboxymethyl cellulose ion-exchange and Sephadex G-75 gel chromatographies were used to purify the enzyme. The specific activity of the purified keratinase relative to that in the original medium was approximately 70-fold. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and Sephadex G-75 chromatography indicated that the purified keratinase is monomeric and has a molecular mass of 33 kDa. The optimum pH and the pI were determined to be 7.5 and 7.25, respectively. Under standard assay conditions, the apparent temperature optimum was 50°C. The enzyme is stable when stored at −20°C. The purified keratinase hydrolyzes a broad range of substrates and displays higher proteolytic activity than most proteases. In practical applications, keratinase is a useful enzyme for promoting the hydrolysis of feather keratin and improving the digestibility of feather meal. Images

How Maps Work

Abstract: This paper proposes a new definition of the map. It is argued that a map is a weapon in the fight for social dominion, one disguised as a representation of the world, but which in reality links territory with what comes with it (with voting rights or military service obligations, with rates of rainfall or types of vegetation). The map achieves this linkage by fusing onto a common plane (that of the map) multicoded images of the very world the map itself will bring into being. Cet article propose une nouvelle definition de la carte. On soutient que la carte est une arme dans le combat pour la domination sociale, arme deguisee en representation du monde, mais qui en realite lie territoire avec ce qui l'accompagne (avec le droit de vote ou le service militaire obligatoire, avec les taux de precipitations ou les types de vegetation). La carte accomplit cette liaison en fusionnant sur un meme plan — celui de la carte — des images multicodees de ce meme monde que la carte elle-meme va creer.s

Finding chaos in noisy systems

Abstract: In the past twenty years there has been much interest in the physical and biological sciences in nonlinear dynamical systems that appear to have random, unpredictable behavior. One important parameter of a dynamic system is the dominant Lyapunov exponent (LE). When the behavior of the system is compared for two similar initial conditions, this exponent is related to the rate at which the subsequent trajectories diverge. A bounded system with a positive LE is one operational definition of chaotic behavior. Most methods for determining the LE have assumed thousands of observations generated from carefully controlled physical experiments. Less attention has been given to estimating the LE for biological and economic systems that are subjected to random perturbations and observed over a limited amount of time. Using nonparametric regression techniques (Neural Networks and Thin Plate Splines) it is possible to consistently estimate the LE. The properties of these methods have been studied using simulated data and are applied to a biological time series: marten fur returns for the Hudson Bay Company (1820-1900). Based on a nonparametric analysis there is little evidence for lowdimensional chaos in these data. Although these methods appear to work well for systems perturbed by small amounts of noise, finding chaos in a system with a significant stochastic component may be difficult.

Aflatoxin in maize

Abstract: Aflatoxin B1 is a potent hepatacarcinogen that occurs in corn worldwide. The aflatoxin‐producing fungus Aspergillus flavus can grow and produce aflatoxin on corn preharvest and in storage. Within the U.S., aflatoxin contamination of preharvest corn has been reported in 23 states, and contamination is chronic in the southeastern U.S. where hot, drought conditions often favor the fungus and the production of aflatoxin. Management practices have been developed to reduce aflatoxin contamination, but in years when environmental conditions are extremely favorable for the fungus, no control strategy is effective. Resistance to aflatoxin accumulation appears to be heritable, but no commercial hybrids are available with adequate resistance. This review covers the epidemiology of A. flavus in preharvest corn from the infection process to the factors that influence aflatoxin formation. Also discussed are the problems associated with the development of resistant varieties and new strategies that are being de...

Broad-spectrum resistance to Bacillus thuringiensis toxins in Heliothis virescens.

Abstract: Evolution of pest resistance to insecticidal proteins produced by Bacillus thuringiensis (Bt) would decrease our ability to control agricultural pests with genetically engineered crops designed to express genes coding for these proteins. Previous genetic and biochemical analyses of insect strains with resistance to Bt toxins indicate that (i) resistance is restricted to single groups of related Bt toxins, (ii) decreased toxin sensitivity is associated with changes in Bt-toxin binding to sites in brush-border membrane vesicles of the larval midgut, and (iii) resistance is inherited as a partially or fully recessive trait. If these three characteristics were common to all resistant insects, specific crop-variety deployment strategies could significantly diminish problems associated with resistance in field populations of pests. We present data on Bt-toxin resistance in Heliothis virescens, a major agricultural pest targeted for control with Bt-toxin-producing crops. A laboratory strain of H. virescens developed resistance in response to selection with the Bt toxin CryIA(c). In contrast to other cases of Bt-toxin resistance, this H. virescens strain exhibits cross-resistance to Bt toxins that differ significantly in structure and activity. Furthermore, the resistance in this strain is not accompanied by significant changes in toxin binding, and resistance is inherited as an additive trait when larvae are treated with high doses of CryIA(c) toxin. These findings have important implications for Bt-toxin-based pest control.

A Longitudinal Study of Engineering Student Performance and Retention

Abstract: A cohort of chemical engineering students has been taught in an experimental sequence of five chemical engineering courses, beginning with the introductory course in the Fall 1990 semester. Differences in academic performance have been observed between students from rural and small town backgrounds (“rural students,” N=55) and students from urban and suburban backgrounds (“urban students,” N=65), with the urban students doing better on almost every measure investigated. In the introductory course, 80% of the urban students and 55% of the rural students passed with a grade of C or better, with average grades of 2.63 for the urban students and 1.80 for the rural students (A=4.0). The urban group continued to earn higher grades in subsequent chemical engineering courses. After four years, 79% of the urban students and 64% of the rural students had graduated or were still enrolled in chemical engineering; the others had either transferred out of engineering or were no longer attending the university. This paper presents data on the students’ home and school backgrounds and speculates on possible causes of observed performance differences between the two populations.

Relative rates of nucleotide substitution at the rbcL locus of monocotyledonous plants.

Abstract: We subjected 35 rbcL nucleotide sequences from monocotyledonous taxa to maximum likelihood relative rate tests and estimated relative differences in rates of nucleotide substitution between groups of sequences without relying on knowledge of divergence times between taxa. Rate tests revealed that there is a hierarchy of substitution rate at the rbcL locus within the monocots. Among the taxa analyzed the grasses have the most rapid substitution rate; they are followed in rate by the Orchidales, the Liliales, the Bromeliales, and the Arecales. The overall substitution rate for the rbcL locus of grasses is over 5 times the substitution rate in the rbcL of the palms. The substitution rate at the third codon positions in the rbcL of the grasses is over 8 times the third position rate in the palms. The pattern of rate variation is consistent with the generation-time-effect hypothesis. Heterogenous rates of substitution have important implications for phylogenetic reconstruction.

Optical Signal Processing

Abstract: : We studied optical processing techniques to detect and track frequency hopped radio signals in a dense electromagnetic environment by using cross-spectrum of two signals obtained from antenna elements spaced by one-half an RF wavelength. We decimate the array by retaining only every Mth element and scan the cross-spectrum past the decimated array. We therefore reduce the circuit complexity, but suffer some loss in system performance because we require more photodetector bandwidth to accommodate the scanning action. An extension of the decimated array concept is to decimate the reference waveform in the heterodyne spectrum analyzer. In this case, we generate only 64 optical probes in the Fourier domain instead of the 2048 that are normally required. The advantage is that all of the optical power in the reference beam can be concentrated into the reduced number of optical probes, thereby offsetting some of the intrinsic loss in performance experienced by the cross-spectrum analyzer. (JHD)

Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition

Abstract: We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x‐ray diffraction, Rutherford backscattering, four‐point‐probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four‐point‐probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low‐resistivity epitaxial TiN in silicon device fabrication are discussed.

The role of benthic foraminifera in deep-sea food webs and carbon cycling

Abstract: Benthic foraminifers are a major element in deep-sea sediment and hard-substrate communities, sometimes accounting for 50% or more of eukaryotic biomass. They feed at a low trophic level, consuming mainly planktonic and other detritus and bacteria. Some species have metabolic adaptations enabling them to respond quickly to pulsed detrital inputs with rapid rates of reproduction and growth. These foraminifers probably assist microorganisms in the breakdown of fresh detrital material, while others are deposit feeders which convert more refractory organic substances into biomass. DOM uptake may be important, although no data exist as yet to substantiate this. Foraminifers are consumed by a wide variety of organisms, including selective and non-selective deposit feeders and specialised predators, and probably represent an important link between lower and higher levels of deep-sea food webs. A variety of non-trophic interactions between metazoans and foraminifers, for example, the provision of physical substrates, may facilitate access to enhanced food supplies. Thus, foraminifera playa largely unquantified but potentially significant role in deep-sea carbon cycling

Myths and Science about the Chemistry and Fertility of Soils in the Tropics

Abstract: In many scientific and popular publications, soils o! the tropics are consi­ dered to be universally acid, infertile, and often incapable of sustained agricul­ tural production (Gourou, 1966; McNeil, 1964; Goodland & Irwin, 1975; Friedman, 1977; Irion, 1978; Reiss et al., 1980; Jordan, 1985). The soil science literature shows that universal tropical soil infertility is a myth devoid of scien­ tific validity. This myth has generated major misconceptions relevant to cur­ rent global issues such as rural poverty, land degradation, deforestation, biodiversity, and climate change. The historical development of this misconception has been recently lyzed by Richter and Babbar (199i) ana­ who traced it from the initial explora­ tions in the tropics in the early 19th century (Buchanan, 1807), through the prevalence of broad soil genesis theories during the first half of the 20th cen­ tury (Sibirtzev, 1914; Jenny, 1941), apd finally to the lack of utilization of quantitative data about the diversity and management of soils in the tropics, g,.-._ted largely during the second half of this century. Richter and babbar cite telli.,. examples of how major ecological texts still use obsolete concepts about soils, and conclude that the myth is a consequence of a major com­ munications gap between soil scientists and other environmental scientists. Newer books, products of multidisciplinary efforts, put this misconception aside (Leith & Werger, 1989; Coleman et al., 1989). The myth about universal soil infertility in the tropics is readily coun­ teracted by two kinds of evidence. First, the vast diversity of soils in the tropics (Sanchez & Buol, 1975; Moormann & Van Wambeke, 1978; Drosdoff et al., 1978) which is now systematized according to quantitative soil taxonomy (Soil Survey Staff, 1975), a world soil map (FAO, 1971-1979), and numerous and Copyright © 1992 Soil Science Society of America and American Society of Agronomy, 677 S. Segoe Rd., Madison, WI 53711, USA. Myths andScience ofSoils ofthe Tropics.SSSA Spe­ cial Publication no. 29.

Prognosis for dogs with appendicular osteosarcoma treated by amputation alone: 162 cases (1978-1988).

Abstract: Long-term follow-up information pertaining to 162 dogs with appendicular osteosarcoma treated by amputation alone was collected from 17 veterinary institutions. The majority (72.5%) of dogs died or were euthanatized because of problems documented to be related to metastases. The first clinically apparent sites of metastasis were the lungs (60.8% of total), the skeleton (5.2%), or both (4.6%). A Kaplan-Meier survivorship distribution was plotted on the basis of available survival time data in all 162 dogs. The mean and median survival times were estimated to be 19.8 and 19.2 weeks, respectively, and the 1- and 2-year survival rates were estimated to be 11.5 and 2.0% respectively. Statistically significant relationships were not found between survival time and reporting institution, gender, site of primary tumor, whether the primary tumor was proximally or distally located, whether the primary tumor was located in the forelimb or hind limb, whether presurgical biopsy was performed, and whether death was tumor related. A significant (P less than 0.01) quadratic relationship was found between age and survival time. Survival time was longest in dogs 7 to 10 years old and was shorter in older and younger dogs.

Cation-dependent transition between the quadruplex and Watson-Crick hairpin forms of d(CGCG3GCG).

Abstract: The DNA oligonucleotide d(CGCG3GCG) can form either a Watson-Crick (WC) hairpin or a parallel-stranded quadruplex structure containing six G-quartet base pair assemblies. The exchange between these forms and single strands can be monitored using circular dichroism (CD). NMR results verified the assignment of specific CD bands to quadruplex and hairpin species, respectively. Cations stabilize the quadruplex in the order K+ greater than Ca2+ greater than Na+ greater than Mg2+ greater than Li+ and K+ greater than Rb+ greater than Cs+, indicating that K+ has an optimum ionic radius for complex formation and that ionic charge affects the extent of ion-induced stabilization. The quadruplex is stable in the presence of 40 mM K+ at micromolar DNA concentration and can be kinetically trapped as a metastable form when prepared at millimolar DNA concentration and then diluted into buffer containing 40 mM Na+. The concentration of K+ required to reverse the equilibrium from the hairpin to the quadruplex decreases sharply with increased DNA concentration. The quadruplex has an unusual pKa of ca. 6.8, indicating that C.C+ base pairs are probably forming. This system provides insights into some of the detailed structural characteristics of a ["G4-DNA".ion] complex and an experimental model for the recently proposed "sodium-potassium conformational switch" [Sen, D., & Gilbert, W. (1988) Nature 334, 364-366; Sen, D., & Gilbert, W. (1990) Nature 344, 410-414]. These results may help to explain the lack of cytidine residues in G-rich telomeric DNAs and suggest that methylation of GC-rich duplex DNAs in "GpC islands" may induce quadruplex formation within heterochromatin domains, resulting in reversible chromosomal condensation.