Low-frequency Shoulder Research Articles

Photoactivation of rhodopsin involves alterations in cysteine side chains: detection of an S-H band in the Meta I-->Meta II FTIR difference spectrum

FTIR difference spectroscopy has been used to study the role of cysteine residues in the photoactivation of rhodopsin. A positive band near 2550 cm-1 with a low frequency shoulder is detected during rhodopsin photobleaching, which is assigned on the basis of its frequency and isotope shift to the S-H stretching mode of one or more cysteine residues. Time-resolved studies at low temperature show that the intensity of this band correlates with the formation and decay kinetics of the Meta II intermediate. Modification of rhodopsin with the reagent NEM, which selectively reacts with the SH groups of Cys-140 and Cys-316 on the cytoplasmic surface of rhodopsin, has no effect on the appearance of this band. Four other cysteine residues are also unlikely to contribute to this band because they are either thio-palmitylated (Cys-322 and Cys-323) or form a disulfide bond (Cys-110 and Cys-187). On this basis, it is likely that at least one of the four remaining cysteine residues in rhodopsin is structurally active during rhodopsin photoactivation. The possibility is also considered that this band arises from a transient cleavage of the disulfide bond between cysteine residues 110 and 187.

An optical cell for Raman spectroscopic studies of supercritical fluids and its application to the study of water to 500°C and 2000 bar

A high-temperature, high-pressure optical cell has been developed for the study of aqueous solutions by Raman spectroscopy. The disk-shaped cell has a sample volume of < 1 ml and utilizes diamond or sapphire windows set at 90° to one-another. Temperatures to 700°C and pressures to 4000 bar have been attained as measured using an internal thermocouple and a strain gauge. The apparatus was employed in the study of water to 500°C and 2000 bar with spectra of the OH stretching mode being collected at intervals of 50°C and 250 bar. A low-frequency shoulder between 3250 and 3300 cm −1 was found to persist to the maximum temperatures to at least 450°C at pressures above those of the liquid vapor curve; its intensity decreased with increasing temperature and decreasing pressure. The frequency of the maximum intensity of the spectral envelope increased dramatically with temperature to above 250°C and was found to be linear with respect to density and independent of temperature at constant density above 250°C. Similar behavior is seen for the viscosity, dielectric constant and the limiting equivalent conductances of pure water. The data indicate the presence of intermolecular hydrogen bonding to temperatures to well above 300°C at densities above the critical density.

Third normal stress difference and component relaxation spectra for bidisperse melts under oscillatory shear

A recently developed rheooptical technique that simultaneously measures infrared dichroism and birefringence is used to study bidisperse, linear polymer melts in an oscillatory shear. The method was used to individually measure the contribution of the high and low molecular weight components to the third normal stress difference. This was accomplished by measuring the infrared dichroism due to deuterium-labeled polymers. Binary blends are prepared from nearly monodisperse pairs of hydrogenated and deuteriated polyisoprenes of molecular weights 53 000 and 370 000 with volume fractions of long polymer, Φ_L, of 0.10, 0.20, 0.30, 0.50, and 0.75. The long-chain relaxation in the blends shows the appearance of a peak for compositions with Φ_L ≤ 0.50 at approximately the same frequency as a similar peak in the pure short polymer. The response of the short-chain relaxation in the blends shows the appearance of a low-frequency shoulder that becomes surprisingly large as the volume fraction of long chains increases. The results are used to test a modified reptation model that includes constraint release and a short-range orientational coupling between chain segments.

Tubuloglomerular feedback and autoregulation in spontaneously hypertensive rats

Experiments were conducted in 8-wk-old spontaneously hypertensive rats to determine whether tubuloglomerular feedback is essential for the autoregulation of renal blood flow. Autoregulation curves were obtained by measuring mean renal arterial blood pressure and flow during graded aortic occlusion. Renal vascular admittance was calculated from recordings of pulsatile renal arterial blood pressure and flow during induced atrial fibrillation. After a control period, acute ureteral obstruction was used to suppress tubuloglomerular feedback, as confirmed by measuring stop-flow pressure responses to rapid perfusion of Henle's loop. Ureteral obstruction did not impair steady-state autoregulation. During both the control and obstruction periods, the admittance gain was less than 1 at frequencies below 0.2 Hz, indicating dynamic autoregulatory activity. The control admittance contained two gain shoulders and two phase maxima, suggesting the presence of two control systems with response half-times of 1 and 10 s. During ureteral obstruction, the low-frequency shoulder and maximum disappeared, indicating that the slower system was no longer active. However, the high-frequency shoulder and maximum persisted, suggesting continued activity of the faster system. Collectively, these observations indicate the existence of a rapidly acting intrarenal control mechanism, in young spontaneously hypertensive rats, that may provide efficient autoregulation without assistance from tubuloglomerular feedback.

Laser-assisted deposition of iron on Si(111)-(7×7): The mechanism and energetics of Fe(CO)5 decomposition

The adsorption and decomposition of Fe(CO)5 on the Si(111)-(7×7) surface has been investigated using multiple internal reflection Fourier transform-infrared, Auger electron, and temperature programmed desorption spectroscopies and low energy electron diffraction under ultrahigh vacuum conditions at 120 K. Resonant decomposition of adsorbed Fe(CO)5 via a multiphoton electronic excitation of the molecule was observed using laser photolysis. Iron deposition was induced efficiently by ultraviolet but not visible photons with high cross sections, in other words, only in the wavelength region where excitation of metal–ligand charge transfer bands and d–d transitions occur. In addition, Fe(CO)5 decomposition was induced using 1.6 keV electrons. No thermal reaction was apparent in temperature programmed desorption experiments. Only molecular Fe(CO)5 desorption was observed at temperatures of 150 and 170 K. Significant amounts of carbon were deposited from the electron induced decomposition while no residual oxygen or carbon were detected in the photodeposited Fe. No partially decarbonylated Fe(CO)x, x&amp;lt;5, fragments were detected subsequent to exposure to ultraviolet photons using infrared spectroscopy. In addition, no new features were observed in a temperature programmed desorption experiment after laser photolysis. These data suggest that there are no surface stable Fe(CO)x, x&amp;lt;5, species in the photodeposition process. Instead, photolysis yields adsorbed Fe atoms without trapping of iron carbonyl fragments, even at low temperatures. Multiple photons were required to induce the Fe(CO)5 decomposition based on the fluence dependence of the photodecomposition yield. These data are consistent with gas phase photodecomposition energetics. The two mechanisms proposed are rapid sequential ejection of CO into the gas phase with loss of the first CO being rate limiting or simultaneous ejection of all CO. An unresolved low frequency shoulder did appear in the infrared spectrum after exposing the Fe(CO)5 covered Si(111)-(7×7) crystal to the electron beam, possibly due to formation of a distribution of surface stable Fe(CO)x, x&amp;lt;5, fragments. The relative Fe:Si Auger peak intensities after photolysis and annealing to 300 and 1000 K were different by a factor of 2 indicative of diffusion of Fe into the crystal and probable silicide formation. Molecular carbon monoxide could not be readsorbed on a surface where Fe(CO)5 had been photolytically decomposed and annealed to 300 or 1000 K, further evidence that isolated iron is not formed in the laser deposition process.

Spectroscopic evidence for spatial correlations of hydrogen bonds in liquid water

The low-frequency shoulder of the OH stretching Raman spectrum is developed as a probe of in-phase collective motions in liquid water. Its relative intensity approaches that of ice I as the supercooled liquid temperature tends toward the conjectured thermodynamic singularity in the vicinity of -46/sup 0/C. The collective band appears despite the large disorder of the OH stretching frequencies in the liquid compared to the strength of the resonance coupling. The resonance condition required for collective OH motions leads us to conjecture that patches of water molecules with similar hydrogen bond energies, which are capable of sustaining the resonance, appear as water is supercooled toward T/sub s/.

Deposition of Iron on Si(111)-(7×7): Photo- and Electron-Assisted Decomposition of Fe(CO)5

AbstractLaser- and electron-assisted deposition of Fe on Si(111)-(7×7) surfaces using decomposition of Fe(CO)5 has been investigated with multiple internal reflection Fourier transform infrared, Auger electron and temperature programmed desorption spectroscopies and low energy electron diffraction under ultra-high vacuum conditions. No thermal reaction was apparent in temperature programmed desorption experiments: only molecular Fe(CO)5 desorption was observed at temperatures of 150 and 170 K, corresponding to desorption energies in the range of 7–10 kcal./mole. Fe(CO)5 decomposition could be induced using either incident 1.6 keV electrons or ultraviolet photons. Significant amounts of carbon were deposited from the electron induced decomposition, consistent with earlier reports on the Si(100) surface. In contrast, ultraviolet photolysis did not result in any detectable incorporation of carbon or oxygen into the iron deposits. No partially decarbonylated Fe(CO)x, x&lt;5, fragments were detected subsequent to exposure to photons using infrared spectroscopy. However, a new, unresolved low frequency shoulder did appear in the infrared spectrum after exposing the Fe(CO)5 covered Si(111)-(7×7) crystal to the electron beam. Iron photodeposition was evident in the Auger electron spectra obtained subsequent to photolysis and annealing of the surface to either 300 K or 1000 K in order to desorb unreacted Fe(CO)5. These data suggest that there are no surface stable Fe(CO)x, x&lt;5, species in the photodeposition process. Instead, photolysis yields Fe atoms directly, even at low temperatures. Annealing to temperatures on the order of 1000 K subsequent to iron deposition resulted in a significant decrease in the Fe:Si ratio as measured by Auger electron spectroscopy. In addition, CO could not be readsorbed on a surface where the Fe(CO)5 had been decomposed. This is attributed to dissolution of Fe into the bulk silicon crystal.

Normal mode assignments in vitreous silica, germania and beryllium fluoride

Detailed assignments are given for normal modes of vibration in vitreous silica, germania and beryllium fluoride. They have been obtained by calculating the atomic displacement eigenvectors for selected modes throughout the spectrum, and separating the kinetic energy of these modes into contributions from cation motion, bond-stretching, bond- bending and bond-rocking motions of the anions. Bands in the 1050, 500-800 and 400 cm-1 regions of the silica spectrum can be attributed largely to bond-stretching, bond-bending and bond-rocking vibrations respectively, while a low frequency shoulder below about 350 cm-1 is found to be mainly acoustic in character. In the vibrational spectrum of vitreous germania, bands which occur at about 850, 500, 370 and 200 cm-1 are similar in their respective assignments to those listed above for silica. In the case of beryllium fluoride, the highest frequency band, at around 700 cm-1, is dominated by cation motion.