Harnessing nanotechnology to expand the toolbox of chemical biology

Although nanotechnology often addresses biomedical needs, nanoscale tools can also facilitate broad biological discovery. Nanoscale delivery, imaging, biosensing, and bioreactor technologies may address unmet questions at the interface between chemistry and biology. Currently, many chemical biologists do not include nanomaterials in their toolbox, and few investigators develop nanomaterials in the context of chemical tools to answer biological questions. We reason that the two fields are ripe with opportunity for greater synergy. Nanotechnologies can expand the utility of chemical tools in the hands of chemical biologists, for example, through controlled delivery of reactive and/or toxic compounds or signal-binding events of small molecules in living systems. Conversely, chemical biologists can work with nanotechnologists to address challenging biological questions that are inaccessible to both communities. This Perspective aims to introduce the chemical biology community to nanotechnologies that may expand their methodologies while inspiring nanotechnologists to address questions relevant to chemical biology.

Glutathione-S-transferase Fusion Protein Nanosensor

Fusion protein tags are widely used to capture and track proteins in research and industrial bioreactor processes. Quantifying fusion-tagged proteins normally requires several purification steps coupled with classical protein assays. Here, we developed a broadly applicable nanosensor platform that quantifies glutathione-S-transferase (GST) fusion proteins in real-time. We synthesized a glutathione-DNA-carbon nanotube system to investigate glutathione-GST interactions via semiconducting single-walled carbon nanotube (SWCNT) photoluminescence. We found that SWCNT fluorescence wavelength and intensity modulation occurred specifically in response to GST and GST-fusions. The sensor response was dependent on SWCNT structure, wherein mod(n - m, 3) = 1 nanotube wavelength and intensity responses correlated with nanotube diameter distinctly from mod(n - m, 3) = 2 SWCNT responses. We also found broad functionality of this sensor to diverse GST-tagged proteins. This work comprises the first label-free optical sensor for GST and has implications for the assessment of protein expression in situ, including in imaging and industrial bioreactor settings.

Synthetic molecular recognition nanosensor paint for microalbuminuria

Microalbuminuria is an important clinical marker of several cardiovascular, metabolic, and other diseases such as diabetes, hypertension, atherosclerosis, and cancer. The accurate detection of microalbuminuria relies on albumin quantification in the urine, usually via an immunoturbidity assay; however, like many antibody-based assessments, this method may not be robust enough to function in global health applications, point-of-care assays, or wearable devices. Here, we develop an antibody-free approach using synthetic molecular recognition by constructing a polymer to mimic fatty acid binding to the albumin, informed by the albumin crystal structure. A single-walled carbon nanotube, encapsulated by the polymer, as the transduction element produces a hypsochromic (blue) shift in photoluminescence upon the binding of albumin in clinical urine samples. This complex, incorporated into an acrylic material, results in a nanosensor paint that enables the detection of microalbuminuria in patient samples and comprises a rapid point-of-care sensor robust enough to be deployed in resource-limited settings.

An in Vivo Nanosensor Measures Compartmental Doxorubicin Exposure

Preclinical measurements of drug exposure to specific organs and tissues is normally performed by destructive methods. Tissue-specific measurements are important, especially for drugs with intractable dose-limiting toxicities, such as doxorubicin-mediated cardiotoxicity. We developed a method to rapidly quantify doxorubicin exposure to tissues within living organisms using an implantable optical nanosensor that can be interrogated noninvasively following surgical implantation. The near-infrared fluorescence of single-walled carbon nanotubes functionalized with DNA was found to respond to doxorubicin via a large and uniform red-shift. We found this to be common to DNA-intercalating agents, including anthracycline compounds such as doxorubicin. Doxorubicin was measured in buffer and serum, intracellularly, and from single nanotubes on a surface. Doxorubicin adsorption to the DNA-suspended nanotubes did not displace DNA but bound irreversibly. We incorporated the nanosensors into an implantable membrane which allowed cumulative detection of doxorubicin exposure in vivo. On implanting the devices into different compartments, such as subcutaneously and within the peritoneal cavity, we achieved real-time, minimally invasive detection of doxorubicin injected into the peritoneal cavity, as well as compartment-specific measurements. We measured doxorubicin translocation across the peritoneal membrane in vivo. Robust, minimally invasive pharmacokinetic measurements in vivo suggest the suitability of this technology for preclinical drug discovery applications.

HIV Detection via a Carbon Nanotube RNA Sensor

Viral illnesses remain a significant concern in global health. Rapid and quantitative early detection of viral oligonucleotides without the need for purification, amplification, or labeling would be valuable in guiding successful treatment strategies. Single-walled carbon nanotube-based sensors recently demonstrated optical detection of small, free oligonucleotides in biofluids and in vivo, although proteins diminished sensitivity. Here, we discovered an unexpected phenomenon wherein the carbon nanotube optical response to nucleic acids can be enhanced by denatured proteins. Mechanistic studies found that hydrophobic patches of the denatured protein chain interact with the freed nanotube surface after hybridization, resulting in enhanced shifting of the nanotube emission. We employed this mechanism to detect an intact HIV in serum, resulting in specific responses within minutes. This work portends a route toward point-of-care optical detection of viruses or other nucleic acid-based analytes.

Noninvasive ovarian cancer biomarker detection via an optical nanosensor implant

Patients with high-grade serous ovarian carcinoma (HGSC) exhibit poor 5-year survival rates, which may be significantly improved by early-stage detection. The U.S. Food and Drug Administration-approved biomarkers for HGSC-CA-125 (cancer antigen 125) and HE4 (human epididymis protein 4)-do not generally appear at detectable levels in the serum until advanced stages of the disease. An implantable device placed proximal to disease sites, such as in or near the fallopian tube, ovary, uterine cavity, or peritoneal cavity, may constitute a feasible strategy to improve detection of HGSC. We engineered a prototype optical sensor composed of an antibody-functionalized carbon nanotube complex, which responds quantitatively to HE4 via modulation of the nanotube optical bandgap. The complexes measured HE4 with nanomolar sensitivity to differentiate disease from benign patient biofluids. The sensors were implanted into four models of ovarian cancer, within a semipermeable membrane, enabling the optical detection of HE4 within the live animals. We present the first in vivo optical nanosensor capable of noninvasive cancer biomarker detection in orthotopic models of disease.

A Carbon Nanotube Optical Reporter Maps Endolysosomal Lipid Flux

Lipid accumulation within the lumen of endolysosomal vesicles is observed in various pathologies including atherosclerosis, liver disease, neurological disorders, lysosomal storage disorders, and cancer. Current methods cannot measure lipid flux specifically within the lysosomal lumen of live cells. We developed an optical reporter, composed of a photoluminescent carbon nanotube of a single chirality, that responds to lipid accumulation via modulation of the nanotube's optical band gap. The engineered nanomaterial, composed of short, single-stranded DNA and a single nanotube chirality, localizes exclusively to the lumen of endolysosomal organelles without adversely affecting cell viability or proliferation or organelle morphology, integrity, or function. The emission wavelength of the reporter can be spatially resolved from within the endolysosomal lumen to generate quantitative maps of lipid content in live cells. Endolysosomal lipid accumulation in cell lines, an example of drug-induced phospholipidosis, was observed for multiple drugs in macrophages, and measurements of patient-derived Niemann-Pick type C fibroblasts identified lipid accumulation and phenotypic reversal of this lysosomal storage disease. Single-cell measurements using the reporter discerned subcellular differences in equilibrium lipid content, illuminating significant intracellular heterogeneity among endolysosomal organelles of differentiating bone-marrow-derived monocytes. Single-cell kinetics of lipoprotein-derived cholesterol accumulation within macrophages revealed rates that differed among cells by an order of magnitude. This carbon nanotube optical reporter of endolysosomal lipid content in live cells confers additional capabilities for drug development processes and the investigation of lipid-linked diseases.

Control of Carbon Nanotube Solvatochromic Response to Chemotherapeutic Agents

Alkylating agents such as cisplatin play an essential role in chemotherapy regimens, but initial and acquired resistance in many cancer types often dampen therapeutic response. The poor understanding of the mechanisms of resistance highlight the need for quantitative measurements of alkylating agent distribution at both the tissue and subcellular levels. Sensors for use in live animals and cells would allow for more effective study of drug action and resistance. Toward this end, single-walled carbon nanotubes suspended with single-stranded DNA have suitable optical properties for in vivo sensors, such as near-infrared emission and sensitivity to the local environment via solvatochromic responses. Currently, solvatochromic changes of such sensors have been limited by the chemical nature of the analyte, making it impossible to control the direction of energy emission changes. Here, we describe a new approach to control the direction and magnitude of solvatochromic responses of carbon nanotubes. We found that the alkylation of DNA on the nanotube surface can result in small changes in DNA conformation that allow the adsorption of amphiphiles to produce large differences (>14 nm) in response to different drugs. The technique surprisingly revealed differences among drugs upon alkylation. The ability to control carbon nanotube solvatochromism as desired may potentially expand the application of nanotube-based optical sensors for new classes of analytes.

Abstract LB-222: A nanoscale optical reporter implant for miRNA biomarkers in vivo

MicroRNAs and other small oligonucleotides in biofluids are promising biomarkers, but conventional assays require complex processing steps unsuitable for point-of-care assays or implantable/wearable sensors. Single-walled carbon nanotubes are an ideal material for implantable sensors due to emission in the near-infrared spectral region, photostability, and exquisite sensitivity. We engineered a carbon nanotube-based platform capable of real-time optical quantification of hybridization events of microRNA and other oligonucleotides for use in vivo. The sensor mechanism derived from competitive effects between displacement of both oligonucleotide charge groups and water from the nanotube surface, resulting in a solvatochromism-like response. The platform allowed detection via single molecule sensor elements and multiplexing using multiple nanotube species. The sensor monitored toehold-based strand displacement events, reversing the sensor response and regenerating the sensor complex. The sensor functioned in whole urine and serum, and it non-invasively measured DNA and microRNA biomarkers after implantation into live mice.

Citation Format: Daniel A. Heller, Jackson D. Harvey, Prakrit V. Jena, Ryan M. Williams, Thomas V. Galassi, Hanan A. Baker, Daniel Roxbury, Gül Zerze, Jeetain Mittal. A nanoscale optical reporter implant for miRNA biomarkers in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-222. doi:10.1158/1538-7445.AM2017-LB-222

Polymer cloaking modulates the carbon nanotube protein corona and delivery into cancer cells

Polycarbodiimide cloaking of photoluminescent single-walled carbon nanotubes modulates their surface chemistry, protein corona, and uptake in cancer cells.

ADCC develops over time during persistent infection with live-attenuated SIV and is associated with complete protection against SIV(mac)251 challenge

Live-attenuated strains of simian immunodeficiency virus (SIV) routinely confer apparent sterilizing immunity against pathogenic SIV challenge in rhesus macaques. Understanding the mechanisms of protection by live-attenuated SIV may provide important insights into the immune responses needed for protection against HIV-1. Here we investigated the development of antibodies that are functional against neutralization-resistant SIV challenge strains, and tested the hypothesis that these antibodies are associated with protection. In the absence of detectable neutralizing antibodies, Env-specific antibody-dependent cell-mediated cytotoxicity (ADCC) emerged by three weeks after inoculation with SIVΔnef, increased progressively over time, and was proportional to SIVΔnef replication. Persistent infection with SIVΔnef elicited significantly higher ADCC titers than immunization with a non-persistent SIV strain that is limited to a single cycle of infection. ADCC titers were higher against viruses matched to the vaccine strain in Env, but were measurable against viruses expressing heterologous Env proteins. In two separate experiments, which took advantage of either the strain-specificity or the time-dependent maturation of immunity to overcome complete protection against SIV(mac)251 challenge, measures of ADCC activity were higher among the SIVΔnef-inoculated macaques that remained uninfected than among those that became infected. These observations show that features of the antibody response elicited by SIVΔnef are consistent with hallmarks of protection by live-attenuated SIV, and reveal an association between Env-specific antibodies that direct ADCC and apparent sterilizing protection by SIVΔnef.

Bragg scattering in a positive β4 fiber

The phase-matching curves for the four-wave mixing effect of Bragg scattering in two fibers with opposite sign β(4) dispersion coefficients have been measured experimentally. The measured phase-matching curves are in good agreement with theoretical expectations, and their dependence on several key parameters has been determined.

Widely tunable photonic crystal fiber Fabry-Perot optical parametric oscillator

We present a widely tunable low-threshold chi(3) optical parametric oscillator. The oscillator cavity is formed by butt coupling dichroic mirrors to either end of a highly nonlinear index-guiding photonic crystal fiber. This yields a singly resonant Fabry-Perot oscillator with a high feedback fraction for the resonant parametric sideband. The tuning range of the output parametric sideband stretches from 23 to 164 THz above the pump frequency. The threshold power of the oscillator is only 15 W.

Strong signal suppression in single-pump optical parametric amplifiers

We show that the combined action of parametric gain and Raman scattering can lead to the complete suppression of an input optical signal in a single-pump parametric amplifier. This suppression is due to an interference between the two parametric gain modes. The interference occurs only at a set of discrete combinations of pump power, phase mismatch, and frequency detuning. Experimentally we are able to demonstrate over 95% (13 dB) suppression of an input signal in an amplifier with a peak parametric gain of only 6 dB.

Combined effect of Raman and parametric gain on single-pump parametric amplifiers

We investigate the combined effect of Raman and parametric gain on single-pump parametric amplifiers. The phasematched parametric gain is shown to depend strongly on the real part of the complex Raman susceptibility. In fused silica fibers this results in a significant reduction in the available parametric gain for signal detunings beyond 10 THz. We are able to experimentally measure this effect for signal detunings ranging from 7 to 22 THz. Finally we discuss the implications of these results for the design of broadband optical parametric amplifiers.

High-conversion-efficiency widely-tunable all-fiber optical parametric oscillator

A high-conversion-efficiency widely-tunable all-fiber optical parametric oscillator is described. It is based on modulation instability in the normal dispersion regime near the fiber's zero-dispersion wavelength. A 40 m long dispersion-shifted fiber is used in a synchronously pumped ring cavity. We demonstrate continuous sideband tuning from 1300 to 1500 nm and 1600 to 1860 nm by tuning the pump wavelength between 1532 and 1556 nm. Internal conversion efficiencies of up to 40% are achieved.

Influence of Raman susceptibility on optical parametric amplification in optical fibers

The real part of the Raman susceptibility is shown to have a strong influence on the peak parametric gain of single-pump parametric amplifiers. This results in a 35% variation in the peak parametric gain over the frequency range 0-30 THz. We are able to experimentally demonstrate this effect in a photonic crystal fiber and obtain good agreement between the experimentally measured and theoretically predicted gains.

180-nm wavelength conversion based on Bragg scattering in an optical fiber

Efficient, wideband and tunable optical wavelength conversion over 180 nm by four-wave mixing (Bragg scattering) in a fiber is demonstrated experimentally. This process has the potential to translate optical data (states of light) without the noise pollution associated with parametric amplification and spontaneous Raman scattering.

Effect of dispersion fluctuations on widely tunable optical parametric amplification in photonic crystal fibers

The effect of dispersion fluctuations on the conversion efficiency of large frequency shift parametric sidebands is studied by numerical simulation and experiment. Numerical results based on periodic and random dispersion models are used to fit the experimental results. The fitting parameters provide a measure of the uniformity of the photonic crystal fiber used in the experiment. This allows us to place limits on the required uniformity of a photonic crystal fiber for strong frequency conversion.

Polarization modulation instability in photonic crystal fibers

Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of 64 THz between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be 5.3 x 10(-5) at a pump wavelength of 647.1 nm. This birefringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a 70 THz range. These results could lead to amplifiers and oscillators based on PMI.

Cross-phase modulation instability in photonic crystal fibers

We report on the observation of cross-phase modulation instability in a highly nonlinear photonic crystal fiber. In such fibers the presence of higher orders of dispersion results in a complex phase-matching curve. We are able to observe this behavior experimentally and obtain excellent agreement between the measured and predicted shifts.

Exact solutions of the generalized nonlinear Schrödinger equation with distributed coefficients

A broad class of exact self-similar solutions to the nonlinear Schrödinger equation (NLSE) with distributed dispersion, nonlinearity, and gain or loss has been found describing both periodic and solitary waves. Appropriate solitary wave solutions applying to propagation in optical fibers and optical fiber amplifiers with these distributed parameters have also been studied in detail. These solutions exist for physically realistic dispersion and nonlinearity profiles. They correspond either to compressing or spreading solitary pulses which maintain a linear chirp or to chirped oscillatory solutions. The stability of these solutions has been confirmed by numerical simulations of the NLSE with perturbed initial conditions. These self-similar propagation regimes are expected to find practical application in both optical fiber amplifier systems and in fiber compressors.

Widely tunable optical parametric generation in a photonic crystal fiber

We report on the observation of widely tunable optical parametric generation in a photonic crystal fiber. The frequency shift of the generated sidebands that arise from modulational instability is strongly dependent on the detuning of the pump from the fiber's zero-dispersion wavelength. We are able to demonstrate experimentally more than 450 nm of sideband tunability as we tune the pump wavelength over 10 nm. Excellent agreement has been found between the experimentally measured and theoretically predicted shifts.

Exact self-similar solutions of the generalized nonlinear Schrödinger equation with distributed coefficients

A broad class of exact self-similar solutions to the nonlinear Schrödinger equation (NLSE) with distributed dispersion, nonlinearity, and gain or loss has been found. Appropriate solitary wave solutions applying to propagation in optical fibers and optical fiber amplifiers with these distributed parameters have also been studied. These solutions exist for physically realistic dispersion and nonlinearity profiles in a fiber with anomalous group velocity dispersion. They correspond either to compressing or spreading solitary pulses which maintain a linear chirp or to chirped oscillatory solutions. The stability of these solutions has been confirmed by numerical simulations of the NLSE.

Intramural multisite recording of transmembrane potential in the heart

Heart surface optical mapping of transmembrane potentials has been widely used in studies of normal and pathological heart rhythms and defibrillation. In these studies, three-dimensional spatio-temporal events can only be inferred from two-dimensional surface potential maps. We present a novel optical system that enables high fidelity transmural recording of transmembrane potentials. A probe constructed from optical fibers is used to deliver excitation light and collect fluorescence from seven positions, each 1 mm apart, through the left ventricle wall of the rabbit heart. Excitation is provided by the 488-nm line of a water-cooled argon-ion laser. The fluorescence of the voltage-sensitive dye di-4-ANEPPS from each tissue site is split at 600 nm and imaged onto separate photodiodes for later signal ratioing. The optics and electronics are easily expandable to accommodate multiple optical probes. The system is used to record the first simultaneous measurements of transmembrane potential at a number of sites through the intact heart wall.

White-light supercontinuum generation with 60-ps pump pulses in a photonic crystal fiber

The generation of a spatially single-mode white-light supercontinuum has been observed in a photonic crystal fiber pumped with 60-ps pulses of subkilowatt peak power. The spectral broadening is identified as being due to the combined action of stimulated Raman scattering and parametric four-wave-mixing generation, with a negligible contribution from the self-phase modulation of the pump pulses. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate that ultrafast femtosecond pulses are not needed for efficient supercontinuum generation in photonic crystal fibers.

Self-similar propagation of high-power parabolic pulses in optical fiber amplifiers

Self-similarity techniques are used to study pulse propagation in a normal-dispersion optical fiber amplifier with an arbitrary longitudinal gain profile. Analysis of the nonlinear Schrödinger equation that describes such an amplifier leads to an exact solution in the high-power limit that corresponds to a linearly chirped parabolic pulse. The self-similar scaling of the propagating pulse in the amplifier is found to be determined by the functional form of the gain profile, and the solution is confirmed by numerical simulations. The implications for achieving chirp-free pulses after compression of the amplifier output are discussed.

Self-similar propagation and amplification of parabolic pulses in optical fibers

Ultrashort pulse propagation in high gain optical fiber amplifiers with normal dispersion is studied by self-similarity analysis of the nonlinear Schrödinger equation with gain. An exact asymptotic solution is found, corresponding to a linearly chirped parabolic pulse which propagates self-similarly subject to simple scaling rules. The solution has been confirmed by numerical simulations and experiments studying propagation in a Yb-doped fiber amplifier. Additional experiments show that the pulses remain parabolic after propagation through standard single mode fiber with normal dispersion.

Complete characterization of a self-mode-locked ti:sapphire laser in the vicinity of zero group-delay dispersion by frequency-resolved optical gating

The intensity and the phase of ultrashort pulses from a self-mode-locked Ti:sapphire laser operating in the vicinity of zero group-delay dispersion (GDD) have been completely characterized by the technique of frequency-resolved optical gating (FROG). For small values of negative GDD, the appearance of a dispersive wave in the pulse spectrum is manifested in the measured FROG trace, and pulse retrieval directly shows its association with a broad leading-edge pedestal. For positive GDD, we confirm previous experimental observations of picosecond pulses with large positive chirp and report a new operating regime in which the output pulses are of picosecond duration but are intensity modulated at 20 THz. The physical origin of this modulation is discussed by analogy with similar effects observed during pulse propagation in optical fibers, and the experimental results are compared with a model of intracavity four-wave mixing about the cavity zero GDD wavelength.

Complete pulse characterization at 1.5 mum by cross-phase modulation in optical fibers

Cross-phase modulation in optical fibers has been used for complete characterization of ultrashort pulses by a modified frequency-resolved optical gating (FROG) measurement technique. This technique has been used for characterization of picosecond pulses at 1.5mum with energy as low as 24 pJ, and the results are in excellent agreement with second-harmonic generation (SHG)-FROG characterization. The use of an optical waveguide gives measurement sensitivity comparable with that of SHG-FROG but without any temporal ambiguity in the retrieved pulse.

Quasi-phase-matched modulation instability in birefringent fibers

The phase-sensitive nature of polarization modulation instability has been demonstrated in optical fibers whose birefringence has been manipulated to generate phase mismatches. Quasi-phase-matched modulation instability has been demonstrated, and the gain of the quasi-phase-matched sidebands has been investigated. The results are in good agreement with experiment.

Direct measurement of pulse distortion near the zero-dispersion wavelength in an optical fiber by frequency-resolved optical gating

The technique of frequency-resolved optical gating is used to characterize the intensity and the phase of picosecond pulses after propagation through 700 m of fiber at close to the zero-dispersion wavelength. Using the frequency-resolved optical gating technique, we directly measure the severe temporal distortion resulting from the interplay between self-phase modulation and higher-order dispersion in this regime. The measured intensity and phase of the pulses after propagation are found to be in good agreement with the predictions of numerical simulations with the nonlinear Schrödinger equation.

Polarization modulation instability in weakly birefringent fibers

Modulation instability generated by the coherent interaction of two polarization modes in a weakly birefringent optical fiber has been observed in the normal dispersion regime. In contrast to previous observations of crossphase-modulation instability in the visible, the two generated sidebands have the same polarization, which is orthogonal to that of the pump, and their frequency shift is readily controlled by variation of the birefringence of the fiber. The frequency of the modulation and its qualitative features agree with those theoretically predicted.

Coherent effects in a self-mode-locked Ti:sapphire laser

The spectral and temporal characteristics of approximately 10-fs pulses from a self-mode-locked Ti:sapphire laser were found to exhibit large deviations from the predictions of current theories of mode locking. A simple model that takes into account coherent coupling between the circulating pulse and the gain medium gives results in good agreement with experimental observations.

Towards a user-friendly future. The impact of information technology within primary health care

The use of computers in primary care in the United Kingdom has developed without coordination. The present situation is described. The proposal is that policies for improving primary care will be enhanced by appropriate application of information technology. Benefits and obstacles are identified; practical proposals for effective development are listed.

Aerobic and anaerobic swimming speeds of spermatozoa investigated by twin beam laser velocimetry

The motility of bovine and ovine spermatozoa has been studied under aerobic and anaerobic conditions, using a dual beam laser velocimeter. Cells swimming under aerobic conditions were found to be characterized by a translational swimming speed and a rotation rate that were approximately double those of cells swimming in an anaerobic environment. Both types of spermatozoa have been found to exhibit a sudden coordinated transition between fast and slow swimming states when the available oxygen is exhausted. This transition from aerobic to anaerobic swimming states has also been shown to be reversible. Studies of the duration of aerobic motility using the same apparatus have shown that the cells have a constant motile efficiency over the temperature range 32 degrees-42 degrees C.

Electron donor properties of the antitumour drug amsacrine as studied by fluorescence quenching of DNA-bound ethidium

The effect of the antitumour acridine derivative amsacrine [4'-(9-acridinylamino)methanesulphon-m-anisidide] on the fluorescence lifetime of DNA-bound ethidium has been investigated using a synchronously pumped cavity dumped dye laser producing picosecond pulses for sample excitation and a time-correlated single photon counting detection system. As the proportion of DNA-bound amsacrine on the synthetic DNA polymer poly[deoxyadenylic-thymidylic acid] is increased, the fluorescence decay curve of ethidium can be accurately resolved into two exponential components. The short lifetime component, whose proportion increases with increasing proportions of DNA-bound amsacrine, has a lifetime of between 3 and 4 ns, significantly longer than that of ethidium in aqueous solution (1.63 ns). The magnitude of the long lifetime component decreases from 25.4 to 14 ns with increasing proportions of bound amsacrine. It is concluded that a new fluorescence state of ethidium (lifetime 3-4 ns) is present, probably resulting from reversible electron transfer between ethidium and amsacrine. The ability of various 9-anilinoacridine derivatives to quench the fluorescence of DNA-bound ethidium appears to be related to the electron donor properties of the substituents on the anilino ring, as well as to experimental antitumour activity. The electron donor properties of DNA-bound amsacrine may therefore be relevant to its antitumour action.

National leadership in healthcare: the role of the hospital CEO

For CEOs, leadership may be exercised in three contexts. However, only two avenues to practice leadership are open to most CEOs: leadership in the CEO's organization and participation as a member of a leadership group. Managers who are successful while functioning as a part of the leadership group generally are those who perform as "transforming" leaders in their own organizations. Transforming leaders hold to strong values and set about to practice them. They do it best when they do for others before doing for themselves. Such behavior draws followers, which in turn confers leadership status on a manager.

Reovirus RNA transcriptase: evidence for a conformational change during activation of the core particle

Reovirus cores contain an RNA transcriptase capable of synthesizing messenger RNA. When cores are suspended in 1 X SSC at 37 degrees they are quiescent and synthesize no product, but in the presence of the components of an RNA transcriptase reaction mixture they actively synthesize mRNA. Photochemical crosslinking has been used to investigate the arrangement of RNA and protein in both "quiescent" and "active" cores. Irradiation induces the formation of a noncovalent RNA:protein complex in "quiescent" but not in "active" cores. This difference is attributed to a conformational change in the reovirus core which results from the transition between the "quiescent" and "active" states of the particle.

Twin-beam laser velocimeter for the investigation of spermatozoon motility

Previous laser light-scattering studies of spermatozoon motility have been hampered by the large, asymmetric shape of spermatozoa, which causes difficulties in the interpretation of intensity fluctuations in the light scattered from a single laser beam. This paper describes an experimental arrangement for measuring the distribution of transit times for swimming spermatozoa using two slightly separated, focused laser beams. The theory of operation of the instrument is developed to enable the analysis of the experimentally obtained cross-correlation functions. The effects of the pronounced spermatozoon asymmetry and associated intensity modulation in the scattered light are also investigated and shown to be negligible for the twin beam experimental arrangement, provided that the swimming speed distribution has a coefficient of variation (sigma/upsilon greater than 0.1. Results obtained using this apparatus are presented for the velocity distribution of spermatozoa from a variety of bulls.

Light-scattering studies of bull spermatozoa. II. Interaction and concentration effects

The complete autocorrelation function of the intensity fluctuations of laser light scattered from motile bull spermatozoa is shown to depend upon several factors not previously considered. Samples of bull spermatozoa generally contain a substantial proportion of dead cells, which give rise to slowly decaying components of the autocorrelation function. Whereas previous work has concentrated on the form of the fast decaying autocorrelation component, we are concerned here with the relative amplitude and shape of the slow autocorrelation component and the general form of the composite function. In principle, the relative amplitudes of the fast and slow components of the autocorrelation function can be used as an assay of the proportion of swimming cells. We show that this amplitude ratio depends upon cell concentration, scattering cell geometry, and scattering angle. A simple model is developed to explain these results on the basis of the asymmetry of light scattered from these cells, motile/immotile cell interactions, wall-swimming effects, and geotactic reorientation of dead cells.

The relationship between the size of mitochondria and the intensity of light that they scatter in different energetic states

The intensity of light scattered at 90 degrees to the incident beam and the effective hydrodynamic radii of mitochondria incubated under a variety of conditions have been measured. Addition of high concentrations of uncouplers to respiring mitochondria resulted in a decrease in scatter which was not due to swelling. Addition of valinomycin to mitochondria depleted to substrate in K+-free medium produced an increase in scatter that was not due to shrinking. It is concluded that changes in the intensity of scattered light are not reliable indices of changes of volume of mitochondria, and the changes in conformation with changes in metabolic state dominate changes in light scatter. A molecular mechanism for the effect of metabolic state upon the scattered intensity is suggested.