Exciton dynamics in carbon nanotubes: from the Luttinger liquid to harmonic oscillators.

We show that the absorption spectrum in semiconducting nanotubes can be determined using the bosonization technique combined with mean-field theory and a harmonic approximation. In this framework, a multiple band semiconducting nanotube reduces to a system of weakly coupled harmonic oscillators. We also find that gaps of any size destroy the Luttinger liquid phase in single-walled carbon nanotubes and give rise to electron density fluctuations that resemble excitons built from quasiparticles. Interband processes, such as multiple exciton generation, become progressively weaker as the length to diameter ratio grows larger.

Population pharmacokinetics of atovaquone in patients with acute malaria caused by Plasmodium falciparum.

The population pharmacokinetics of atovaquone were examined in 458 black, Oriental, and Malay patients with acute Plasmodium falciparum malaria receiving atovaquone alone or concomitantly with other drugs. Oral clearance (CL/F) showed a 0.674 power relationship with weight and is similar in Oriental and Malay subjects but 58.5% lower in black subjects. On the basis of mean body weight, the population estimate of CL/F is 3.28, 8.49, and 9.13 L/hr in black, Oriental, and Malay subjects, respectively. The relationship between apparent volume of distribution (V area/F) and weight was linear and similar in all three races at 7.98 L/kg. The population estimate of V area/F is 345, 383, and 428 L in black, Oriental, and Malay subjects, respectively. The bioavailability of the high and low doses of atovaquone was similar. Neither CL/F nor V area/F were significantly affected by age, gender, and the coadministration with chloroguanide (proguanil), pyrimethamine, and tetracycline. Half-life (t1/2) showed a 0.326 power relationship with weight; thus, the population estimate of t1/2 in black, Oriental, and Malay subjects is 72.9, 31.3, and 32.5 hours, respectively. The final magnitudes of interpatient variability in CL/F and V area/F were 68% and 49%, respectively.

Hydrogen bonds in liquid water are broken only fleetingly.

Although it is widely accepted that the local structure of liquid water has tetrahedral arrangements of molecules ordered by hydrogen bonds, the mechanism by which water molecules switch hydrogen-bonded partners remains unclear. In this mechanism, the role of nonhydrogen-bonded configurations (NHBs) between adjacent molecules is of particular importance. A molecule may switch hydrogen-bonding partners either (i) through thermally activated breaking of a hydrogen bond that creates a dangling hydrogen bond before finding a new partner or (ii) by infrequent but rapid switching events in which the NHB is a transition state. Here, we report a combination of femtosecond 2D IR spectroscopy and molecular dynamics simulations to investigate the stability of NHB species in an isotopically dilute mixture of HOD in D2O. Measured 2D IR spectra reveal that hydrogen-bonded configurations and NHBs undergo qualitatively different relaxation dynamics, with NHBs returning to hydrogen-bonded frequencies on the time scale of water's fastest intermolecular motions. Simulations of an atomistic model for the OH vibrational spectroscopy of water yield qualitatively similar 2D IR spectra to those measured experimentally. Analysis of NHBs in simulations by quenching demonstrates that the vast majority of NHBs are in fact part of a hydrogen-bonded well of attraction and that virtually all molecules return to a hydrogen-bonding partner within 200 fs. The results from experiment and simulation demonstrate that NHBs are intrinsically unstable and that dangling hydrogen bonds are an insignificant species in liquid water.

Studies on Neisseria gonorrhoeae cultured in liquid medium.

Experiments were carried out to determine the minimum amount of inoculum of gonococci that is needed to produce growth in ANM liquid medium. This was found to be less than had previously been thought. Minimum inocula were used to study penicillin sensitivity in liquid medium. This work showed that growth occurred at higher concentrations of penicillin with larger inocula. Evidence was also obtained which suggested that in some circumstances penicillin acts bacteriostatically rather than bacteriocidally. It was also shown that growth of gonococci can be inhibited in liquid medium by the presence of hyperimmune serum, the inhibition being more marked in the presence of fresh complement. This work is continuing in the hope that it may provide a basis for a much-needed serological test for gonorrhoea. This would be especially valuable in chronic assymptomatic infections.

Impaired mitogen responses of the non-leukaemic B cells from patients with chronic lymphocytic leukaemia.

We investigated the immunological mechanism of the low level of circulating immunoglobulin and depressed primary and secondary responses of patients with chronic lymphocytic leukaemia (B-CLL) using purified non-leukaemic B cells in vitro. To assess the function of the non-leukaemic B cells we separated them from the much larger leukaemic population, which expresses the pan-T cell marker CD5, by immunoabsorption using anti-CD5 antibodies and Dynabeads. Immunoglobulin production was measured after the cells had been cultured with the B cell mitogens, pokeweed mitogen (PWM) and Staphylococcus aureus Cowan strain 1 (SAC). Autologous T cells that were found to function normally in our systems were added to cultures containing PWM. Non-leukaemic B cells from 15 B-CLL patients produced 539 ng/ml, immunoglobulin (mean value) with SAC and 162 ng/ml with PWM compared with 14,182 and 5513 ng/ml, respectively, from B cells from normal, age-matched control patients. Most of the immunoglobulin produced in the non-leukaemic B cell cultures carried the light chain associated with the leukaemic clone. We conclude that even at early stages in the disease (12 patients were Rai stage 0 patients) when the total serum immunoglobulin levels are still near normal, the B cells respond poorly to B cell mitogens.

Ultrafast hydrogen-bond dynamics in the infrared spectroscopy of water.

We investigated rearrangements of the hydrogen-bond network in water by measuring fluctuations in the OH-stretching frequency of HOD in liquid D2O with femtosecond infrared spectroscopy. Using simulations of an atomistic model of water, we relate these frequency fluctuations to intermolecular dynamics. The model reveals that OH frequency shifts arise from changes in the molecular electric field that acts on the proton. At short times, vibrational dephasing reflects an underdamped oscillation of the hydrogen bond with a period of 170 femtoseconds. At longer times, vibrational correlations decay on a 1.2-picosecond time scale because of collective structural reorganizations.