Complete characterization of the (D2O)2 ground state: high Ka rotation-tunneling levels

We report the observation of extensive a- and c-type rotation-tunneling (RT) spectra of (D2O)2 for Ka = 0-4. These data allow quantification of molecular constants and tunneling splittings for a number of previously unobserved RT states of (D2O)2. The vibrational ground state has thus been characterized to energies as high as those of some of the intermolecular vibrations, and we present the first test of the VRT(ASP-W) potential at these high Ka states.     

Rotationally resolved absorption cross sections of formaldehyde in the 28100-28500 cm(-1) (351-356 nm) spectral region: implications for in situ LIF measurements

The rotationally resolved ultraviolet absorption cross sections for the 2(0)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(1) electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm(-1) (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm(-1) (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here. Accurate rotationally resolved cross sections are important for the development of in situ HCHO laser-induced fluorescence (LIF) instruments and for atmospheric monitoring. Pressure dependence of the cross sections between 75 and 400 Torr at 298 K was explored, and an average pressure broadening coefficient in dry air of 1.8 x 10(-4) cm(-1) Torr(-1) for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micron-sized droplets of HCHO solution, using a novel microinjector technique. The condensed-phase concentrations of HCHO were determined by iodometric titrations to an accuracy of <1%. Accuracy of the measured absorption cross sections is estimated to be better than +/-5%. Integrated and differential cross sections over the entire band at low resolution (approximately 1 cm(-1)) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 x 10(-19) cm(2) molecule(-1) was observed at high resolution-almost an order of magnitude greater than any previously reported data at low resolution.     

Terahertz vibration-rotation-tunneling spectroscopy of the water tetramer-d8: combined analysis of vibrational bands at 4.1 and 2.0 THz

We report the measurement of terahertz vibration-rotation-tunneling spectra of (D(2)O)(4) in the spectral region near 4.13 THz. The characterization of this perpendicular band extends a previously reported study [J. Chem. Phys. 111, 7801 (1999)]. We observed 239 new transitions, each being split into a doublet of constant (approximately 192 MHz) spacing. These are included in a combined fit with the 113 previously measured transitions of the 2.03 THz parallel band using an effective Hamiltonian similar to that used in the global fit of the water trimer. The detailed understanding of the water tetramer evolving from this work underlies our efforts to quantify the contribution of many-body forces to the hydrogen bonding interactions in condensed phase water.     

Hydrogen bond breaking dynamics of the water trimer in the translational and librational band region of liquid water.

The effect of exciting each of the three classes of intermolecular vibrations on the hydrogen bond lifetime (tau(H)) of the isolated water trimer is investigated by far-infrared laser spectroscopy. Single excitation of a librational vibration decreases tau(H) by 3 orders of magnitude to tau(H) = 1-6 ps, comparable to the time scale of a number of important bulk water dynamical relaxation processes. In contrast, excitation of translational or torsional vibrations has no significant effect (tau(H) = 1-2 ns). Although such a dependence of tau(H) on intermolecular motions has also been proposed for liquid water via computer simulations, these are the first experiments that provide a detailed molecular picture of the respective motions without extensive interpretation.     

Design considerations in high-sensitivity off-axis integrated cavity output spectroscopy

Off-axis integrated cavity output spectroscopy (OA-ICOS) has generated much interest because it potentially allows highly sensitive field measurements with robust optical alignment. We discuss here design choices involved in design of an OA-ICOS instrument and how these choices impact instrument sensitivity, using as our example the design of the Harvard ICOS isotope instrument, which demonstrates the highest reported sensitivity for mid-IR OA-ICOS (2.4×10^-11 cm^-1Hz^-1/2 at 6.7 μm, obtained during measurements of water vapor isotopologues H₂O, HDO, and H₂  ¹⁸O in the laboratory and onboard NASA’s WB-57 high-altitude research aircraft). We compare the sensitivity of several OA-ICOS instruments with differing design parameters, show how comparisons are hindered by differing definitions of instrument performance metrics, and suggest a common metric of MDA_meas, the fractional absorption equivalent to 1σ uncertainty in an actual measurement, normalized to 1 s integration. We also note that despite an emphasis on sensitivity in the literature, in the Harvard ICOS isotope instrument and likely also similar instruments, systematic errors associated with fitting of the baseline laser power are of equal importance to total measurement uncertainty.     

The role of the UPS in cystic fibrosis

CF is an inherited autosomal recessive disease whose lethality arises from malfunction of CFTR, a single chloride (Cl-) ion channel protein. CF patients harbor mutations in the CFTR gene that lead to misfolding of the resulting CFTR protein, rendering it inactive and mislocalized. Hundreds of CF-related mutations have been identified, many of which abrogate CFTR folding in the endoplasmic reticulum (ER). More than 70% of patients harbor the DeltaF508 CFTR mutation that causes misfolding of the CFTR proteins. Consequently, mutant CFTR is unable to reach the apical plasma membrane of epithelial cells that line the lungs and gut, and is instead targeted for degradation by the UPS. Proteins located in both the cytoplasm and ER membrane are believed to identify misfolded CFTR for UPS-mediated degradation. The aberrantly folded CFTR protein then undergoes polyubiquitylation, carried out by an E1-E2-E3 ubiquitin ligase system, leading to degradation by the 26S proteasome. This ubiquitin-dependent loss of misfolded CFTR protein can be inhibited by the application of 'corrector' drugs that aid CFTR folding, shielding it from the UPS machinery. Corrector molecules elevate cellular CFTR protein levels by protecting the protein from degradation and aiding folding, promoting its maturation and localization to the apical plasma membrane. Combinatory application of corrector drugs with activator molecules that enhance CFTR Cl- ion channel activity offers significant potential for treatment of CF patients. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).     

Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)

The C(5)-hydroperoxyenals (C(5)-HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closely-related, custom-synthesized C(6)-HPALD. The photolysis quantum yield is 1.0 ± 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 ± 0.8. The OH reaction rate constant is (5.1 ± 1.8) × 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The ozone reaction rate constant is (1.2 ± 0.2) × 10(-18) cm(3) molecule(-1) s(-1) at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C(5)-HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C(5)-HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C(5)-HPALD photo-oxidation products also photolyze to yield additional OH or if the C(5)-HPALD production rate is faster than has been observed.     

Infrared laser spectroscopy of jet-cooled carbon clusters: the nu 5 band of linear C9

The nu 5 antisymmetric stretching vibration of 1 sigma+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079-2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported nu 6 transitions yielded the following molecular constants for the nu 5 band: nu 0 = 2 079.673 58(17) cm-1, B"= 0.014 321 4(10) cm-1, and B'=0.014 288 9(10) cm-1. The IR intensity of the nu 5 band relative to nu 6 was found to be 0.108 +/- 0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD).