The surface plasmon polariton-enhanced Raman spectra of size-selected C 16 , C 18 , and C 20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C 20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm Ϫ1 region for C 16 , C 18 , and C 20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C 20 . The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions.

Intravaginal rings releasing tenofovir (TFV) or its prodrug, tenofovir disoproxil fumarate (TDF), are being evaluated for HIV and herpes simplex virus (HSV) prevention. The current studies were designed to determine the mechanisms of drug accumulation in human vaginal and immune cells. The exposure of vaginal epithelial or T cells to equimolar concentrations of radiolabeled TDF resulted in over 10-fold higher intracellular drug levels than exposure to TFV. Permeability studies demonstrated that TDF, but not TFV, entered cells by passive diffusion. TDF uptake was energy independent but its accumulation followed nonlinear kinetics, and excess unlabeled TDF inhibited radiolabeled TDF uptake in competition studies. The carboxylesterase inhibitor bis-nitrophenyl phosphate reduced TDF uptake, suggesting saturability of intracellular carboxylesterases. In contrast, although TFV uptake was energy dependent, no competition between unlabeled and radiolabeled TFV was observed, and the previously identified transporters, organic anion transporters ( T opical preexposure prophylaxis (PrEP) with tenofovir (TFV)-based drugs could provide a female-initiated method for the prevention of HIV and genital herpes simplex virus (HSV) infections in women (1). Pericoital dosing with 1% vaginal TFV gel reduced HIV acquisition by 39% overall and by 54% in highly adherent women (2, 3) and reduced HSV-2 acquisition by 51% in the CAPRISA 004 trial (4). However, the same gel did not provide protection against HIV in the Vaginal and Oral Interventions to Control the Epidemic (VOICE) and Follow-On Consortium for Tenofovir Studies (FACTS) 001 phase 3 trials, presumably reflecting low adherence (5, 6). Women with detectable TFV in their plasma at the first quarterly visit were less likely to acquire HIV than women with no drug detected (adjusted hazard ratio, 0.34; 95% confidence interval [CI], 0.13 to 0.87; P ϭ 0.02) in a secondary analysis of VOICE data, but the interpretation of this is complex, as women at highest risk for HIV acquisition were less likely to be adherent (5). These findings underscore the need for drugs and delivery systems that mitigate the difficulties associated with adherence.Intravaginal rings (IVRs) designed to deliver TFV and tenofovir disoproxil fumarate (TDF) are currently in early clinical development (7,8). A TDF ring completely protected macaques against 16 weekly intravaginal challenges with simian HIV (8) and also was highly protective in more susceptible, medroxyprogesteronetreated animals (9). Nonhuman primate challenge studies with the TFV IVR have not been published. Consistent with in vitro antiviral studies, 0.3% TDF gel provided significantly greater protection than 1% TFV gel in mice challenged intravaginally with HSV-2 (10). Moreover, the 0.3% TDF gel demonstrated significantly greater protection than 1% TFV gel in mice transgenic for human CD4, CCR5, and cyclin T1 against HIV and HSV-2 (11). These findings may reflect differences in drug pharmacokinetics (PK).The mechanisms of TFV and TDF cellular tran...

The surface plasmon polariton-enhanced Raman spectra of size-selected, matrix-isolated C 14 neutral clusters are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra show resonance enhancement over a range of excitation wavelengths from 457.9 to 514.5 nm. The measured vibrational spectra are most consistent with the linear chain isomer. In addition, fluorescence spectra of neutral, mass-selected C n (n ) 14 and 18) clusters are presented. The isolated C 14 clusters display strong fluorescence with vibrational structure between 520 and 700 nm. The origins of the observed fluorescence for both C 14 and C 18 are investigated using the ZINDO/S, CIS, and TD-B3LYP methods.

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Transmission electron microscopy and reflected high-energy electron-diffraction investigation of plastic relaxation in doped and undoped ZnSe/GaAs (001) Arsenic incorporation during GaAs/GaAs͑100͒ molecular beam epitaxy is studied in situ with laser single-photon ionization time-of-flight mass spectrometry and reflection high-energy electron diffraction ͑RHEED͒. Incident and scattered fluxes of Ga and As n species in front of the growing GaAs wafer are ionized repetitively by a pulsed laser beam of 118 nm ͑10.5 eV͒ photons. The methods to obtain and interpret time-of-flight mass spectra and the simultaneous RHEED measurements are described. The real time behaviors of incident Ga and desorbing As 2 and As 4 , obtained without mass spectral cracking, are studied during growth of GaAs layers with As 4 and when growth is arrested as a function of substrate temperature and Ga/As 4 flux ratio. During growth only with As 4 , both As 2 and As 4 are desorbed or scattered in varying amounts depending on flux and substrate temperature conditions. Without an incident gallium flux, desorbing As 4 decreases while desorbing As 2 increases with increasing surface temperature. During gallium deposition and GaAs growth, the amounts of desorbing arsenic fluxes decrease linearly with increasing Ga/As 4 flux ratio, but the arsenic incorporation rate saturates at a Ga/As 4 flux ratio у2, i.e. ͑Ga/Asу 1 2 ͒. The total integrated incorporation of arsenic increases linearly with increasing Ga/As 4 flux ratio when the surface is allowed to recover with an incident arsenic flux after the gallium flux is terminated. In the range of substrate temperatures optimum for layer-by-layer GaAs growth with As 4 , As 4 incorporation dominates at low temperatures, while As 4 and As 2 incorporations contribute equally at high temperatures. Surface reaction sequences and mechanisms of arsenic incorporation are discussed and compared with measured RHEED results and previous experimental and theoretical results.

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Single-photon ionization time-of-flight mass spectrometry (SPI-TOFMS) is used in situ to monitor desorbing species and surface reactions during molecular beam epitaxy (MBE) of GaAs. In this method, the 1064 nm fundamental output of a Nd:YAG laser is tripled twice to produce 118 nm (10.5 eV) photons. The pulsed light is passed in front of a growing substrate, giving gaseous scattered molecules sufficient energy to ionize, but not fragment, them. Ionized species are detected with time-of-flight mass spectrometry. Arrangement of the experiment also allows for simultaneous real time monitoring with reflection high-energy electron diffraction (RHEED).Mass spectra are examined and analyzed to quantify fluxes and relative ionization cross sections of growth species. The real time behavior of arsenic and gallium mass signals during epitaxy is presented as a function of substrate temperature and incident gallium flux. Surface reactions are proposed to elucidate mechanisms of arsenic incorporation and compared to measured RHEED results.