FT-IR spectra of proteins at low temperatures

Infrared spectra of several globular proteins have been recorded in fluid solutions (hydro-alcoholic mixtures) in the temperature range –110° C to 30° C. In the case of hemoglobin, which has a highly ordered secondary structure, smaller spectral changes are observed which are compatible with distortions of its helices. In contrast to this,-casein, a protein with mostly unordered structure, undergoes a much larger change whereby low temperature induces folding of the protein chains.Issued as NRCC No. 28929     

The IR spectra of DCOOH and HCOOD crystals at low temperatures

We show the IR spectra of HCOOD and DCOOH crystals in the region 1000–3000 cm⁻¹ at 7, 77 and 125 K. This article is a complement to previous articles which described spectra of HCOOH and DCOOD crystals recorded in the same conditions. In this article we also compare the results obtained on the four isotopic species and specially focus our attention on a comparison of the moments or order 0, 1 and 2 of the v_O---H…O (O---H…O) and v_O---D…O bands. Furthermore, we precise some experimental aspects, particularly those concerning polarization effects. In the following paper, we shall deduce from the first moments the relative importance of the different anharmonicities governing the stretching motion of the H or D atoms in the H- or D---bonds.     

Luminescence spectra and Raman excitation profiles in small CdS particles

The “edge”-luminescence spectra and the Raman excitation profiles of small CdS particles chemically deposited on transparent substrates show size effects. The peaks are shifted to higher frequencies as the size of particles decreases. Aggregated samples show maxima at lower frequencies than those of the bulk material and behave as amorphous films. After the samples are warmed, a new peak appears in the luminescence spectra whose position depends on the size and aggregation of the particles. An explanation of the shifts based on the size-quantization of the band width or of the localized electronic levels is given.     

Homogeneous pyrolysis of acetylacetone at high temperatures in shock waves

The kinetics of the thermal decomposition of acetylacetone has been studied in a shock tube in the temperature range of 1120–1660 K. Detailed analyses of CO and H₂O formation data indicate that H₂O is formed by a four-center molecular channel, whereas CO is formed by the rapid dissociation of CH₃CO produced by the C? C bond dissociation of acetylacetone. The Arrhenius equations for H₂O and CH₃CO formation channels are ??₂ = 10^14.24±0.21 exp(?60,800 ± 1,220/RT)sec^?1 and ??₃ = 10^17.05±0.28 exp(?74,600 ± 1,680/RT) sec^?1, respectively. The results of the study suggest that the six-center molecular channel for the production of acetone and ketene is not important under the condition used in this investigation.     

Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures

Many of the spectroscopic features and photophysical properties of carotenoids are explained using a three-state model in which the strong visible absorption of the molecules is associated with an S0 (1(1)Ag-) --> S2 (1(1)Bu+) transition, and the lowest lying singlet state, S1 (2(1)Ag-), is a state into which absorption from the ground state is forbidden by symmetry. However, semiempirical and ab initio quantum calculations have suggested additional excited singlet states may lie either between or in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+), and some ultrafast spectroscopic studies have reported evidence for these states. One such state, denoted S*, has been implicated as an intermediate in the depopulation of S2 (1(1)Bu+) and as a pathway for the formation of carotenoid triplet states in light-harvesting complexes. In this work, we present the results of an ultrafast, time-resolved spectroscopic investigation of a series of open-chain carotenoids derived from photosynthetic bacteria and systematically increasing in their number of pi-electron carbon-carbon double bonds (n). The molecules are neurosporene (n = 9), spheroidene (n = 10), rhodopin glucoside (n = 11), rhodovibrin (n = 12), and spirilloxanthin (n = 13). The molecules were studied in acetone and CS2 solvents at room temperature. These experiments explore the effect of solvent polarity and polarizability on the spectroscopic and kinetic behavior of the molecules. The molecules were also studied in ether/isopentane/ethanol (EPA) glasses at 77 K, in which the spectral resolution is greatly enhanced. Analysis of the data using global fitting techniques has revealed the ultrafast dynamics of the excited states and spectral changes associated with their decay, including spectroscopic features not previously reported. The data are consistent with S* being identified with a twisted conformational structure, the yield of which is increased in molecules having longer pi-electron conjugations. In particular, for the longest molecule in the series, spirilloxanthin, the experiments and a detailed quantum computational analysis reveal the presence of two S* states associated with relaxed S1 (2(1)Ag-) conformations involving nearly planar 6-s-cis and 6-s-trans geometries. We propose that in polar solvents, the ground state of spirilloxanthin takes on a corkscrew conformation that generates a net solute dipole moment while decreasing the cavity formation energy. Upon excitation and relaxation into the S1 (2(1)Ag-) state, the polyene unravels and flattens into a more planar geometry with comparable populations of 6-s-trans and 6-s-cis conformations.     

Resonance Raman spectra of n-pi* singlet-triplet transition of p-benzoquinone at low concentrations

A weak visible absorption spectrum of p-benzoquinone (p-BQ) in CS2 due to n-pi* singlet-triplet transition was measured. Using the resonance Raman (RR) effect in liquid-core optical fiber (LCOF), we have obtained the 514.5 nm excited RR spectra of p-benzoquinone near 1445 cm(-1) and have demonstrated that the new characteristic RR band is attributed to the symmetric C=O stretch (nu(C=O)) of n-pi* singlet-triplet transition of p-BQ. The effect of solution concentration on the RR band was investigated at very low concentrations. The RR peak spreads toward short wavelength side with decreasing solution concentration ranging from 10(-7) to 10(-11) mol L(-1), whereas the blue-shift isn't obvious when the concentration is, at single molecule level, lower than 10(-11) mol L(-1). Our result is useful for single molecule detection to some extent.     

Raman spectroscopy of vapors at elevated temperatures

The most effective way to obtain high quality vapor-phase Raman spectra is to heat the samples to increase their vapor pressure. Many samples can be heated to 350 °C and higher without decomposition. We have designed a simple Raman cell to allow these high temperature studies to be carried out. The high-temperature Raman spectra of nine molecules will be presented and discussed. Most of these are non-rigid molecules containing aromatic rings for which vibrational potential energy surfaces have been determined from their spectra. Two molecules (p-cresol and 3-methylindole) are model compounds for amino acids and their vapor-phase spectra are characteristic of environments with no hydrogen bonding.     

Infrared spectra of molecular chlorine complexes with acetylene,ethylene and propylene at low temperatures

Infrared spectra of molecular complexes of C₂H₂Cl₂, C₂D₂Cl₂, C₂H₄Cl₂, C₂D₄Cl₂, and C₃H₆Cl₂ were obtained after simultaneous slow deposition of both components on the cooled window of an optical cryostat at 75°K. Chlorine addition reactions to the multiple bond start in these systems at 90–100°K.     

Chlorination of lignin at low temperatures

A high reactivity of the olefinic double bonds of the lignin (Lg) macromolecule in the reaction with molecular chlorine has been found which has permitted the chlorination of Lg to be carried out under cryogenic conditions at 150–250 K. The process takes place by a radical-chain mechanism with a length of the kinetic chain of approximately 1000 units. The bulk of the chlorine is consumed in the olefinic chains of the Lg. Other reaction products besides chlorolignin are hydrogen chloride and phenoxyl radicals, which have singlet ESR spectrum (ΔH=1.0−1.2 mT) and are retained in the chlorinated samples of Lg to ∼250 K. Institute of Chemical Physics, Academy of Sciences of the USSR, Chernogolovka, Moscow Province. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 671–676, September–October, 1990.     

Raman spectra of ammonia borane: low frequency lattice modes

We have measured the Raman spectrum of ammonia borane at low temperature (T = 15 K) and across the orthorhombic-to-tetragonal phase transition at T = 225 K. A comprehensive study of the low frequency lattice modes using Raman spectroscopy has been carried out. Data analysis has been complemented by a density functional theory calculation of which the results have been used for a detailed assignment of the Raman active modes. The analysis of the spectroscopic measurements taken across the phase transition seems to be consistent with the increasing orientational disorder of the molecular components and seems to be compatible with the equalization of the a and b lattice constants characteristic of the tetragonal phase.     

Monodisperse CdSe nanorods at low temperatures

A new synthetic method is presented that allows the preparation of highly monodisperse CdSe nanorods (so called quantum rods) at relatively low temperatures (160 degrees C). This method is characterized by a high aspect ratio of the particles and affords good reproducibility. The morphology of the resulting nanorods was examined by means of transmission electron microscopy (TEM) and the electro-optic properties by means of fluorescence spectroscopy. The conditions of the reaction of nanoparticle growth were examined by varying the concentration of the organometallic precursors, the growth temperature, and the growth time. The experimental findings correspond well with previously published semiempirical pseudo-potential calculations.     

Electronic structure and absorption spectra of acetylacetone and its analogs

The molecules of acetylacetone, benzoylacetone, dibenzoylmethane, and thiodibenzoylmethane have been calculated by the simple MO LCAO method and their absorption spectra recorded. The chelation of acetylacetone with a hydrogen atom does not affect the energy of the first allowed transition ¹A₁-¹B₁. Calculations correctly predict the change in the energy for this transition in the -diketone series: acetylacetone > benzoylacetone > dibenzoylmethane > thiodibenzoylmethane; the intensity reaches a maximum value for dibenzoylmethane and dropping sharply for thiodibenzoylmethane. The secondary absorption bands for benzoylacetone and dibenzoylmethane are examined experimentally and interpreted theoretically.     

Resonance Raman spectroscopic studies of hydroperoxo-myoglobin at cryogenic temperatures

In agreement with previous reports (Gasyna, Z. FEBS Lett. 1979, 106, 213-218 and Leibl, W.; Nitschke, W.; Huettermann, J. Biochim. Biophys. Acta 1986, 870, 20-30) radiolytically reduced samples of oxygenated myoglobin at cryogenic temperatures have been shown by optical absorption and EPR studies to produce directly the peroxo-bound myoglobin at 77 K. Annealing to temperatures near 185 K induces proton transfer, resulting in the formation of the hydroperoxo heme derivative. Resonance Raman studies of the annealed samples has permitted, for the first time, the direct observation of the key nu(Fe-O) stretching mode of the physiologically important Fe-OOH fragment of this ubiquitous intermediate. The assignment of this mode to a feature appearing at 617 cm(-1) is strongly supported by documentation of a 25 cm(-1) shift to lower energy upon substitution with (18)O(2) and by a 5 cm(-1) shift to lower energy for samples prepared in solutions of deuterated solvent.     

Vibrational relaxation of DF at low temperatures

Temperature dependence of the probability of the deexcitation process DF(ν = 1) + DF(ν = 0) → DF(ν = 0) + DF(ν = 0) has been investigated near and below room temperature based on the model that the vibrational energy is transferred to the hindered rotational (oscillatory) motion as well as the translational motion. The deexcitation probability increases sharply with decreasing temperature; this inverse temperature dependence is attributed to the important contribution of hydrogen-bond attraction.     

Post-irradiation grafting of tetrafluoroethylene at low temperatures

Low-temperature postirradiation grafting of tetrafluoroethylene (TFE) to polyolefines and silicone elastomers was the subject of study. After preliminary dissolution of TFE in the polymer at 273°K the system was slowly cooled to 77°K. In this process a certain part of TFE in polyolefines (ethylene-propylene copolymer and polypropylene) is retained by the vitreous polymer matrix. When slowly heated after radiolysis at 77°K this system shows graft polymerization of dissolved TFE after T_g. The graft copolymer is soluble and its IR spectra contain absorption bands characteristic of polytetrafluoroethylene. In polydimethylsiloxane rubber (SKT) the dissolved TFE when frozen to 77°K remains sorbed between the SKT crystallites rather than in a separate phase. When these radiolyzed samples are heated the graft-polymerization occurs primarily over the temperature range between the TFE and SKT melting points. The technique provedes for 100–150% grafting of TFE. This method also permits grafting to silicone rubbers and to several other polymers and elastomers.     

Radiation polymerization of acrolein at low temperatures

Radiation polymerization of acrolein in the presence of the additives such as 2-hydroxyethyl methacrylate, acrylic acid, and water at low temperatures (0–78°C) was studied. The polyacrolein resulting from the polymerization gave a small particlelike microsphere (0.5–5 μm in diameter). The rate of the polymerization reaction of acrolein appeared to be accelerated by the addition of a small amount of 2-hydroxyethyl methacrylate or acrylic acid. The particle size of the polyacrolein particle varied with the condition of the polymerization and increased or decreased with an increase in the addition concentration of 2-hydroxyethyl methacrylate or acrylic acid.     

Thermodynamic behaviors of polyampholytes at low temperatures

The thermodynamic properties of 6-mer polyampholyte chains were simulated with the multicanonical Monte Carlo method in a three-dimensional off-lattice model. The results demonstrate that the number of the charged monomers on the chain and the sequence significantly affect the transition behaviors of polyampholyte chains. In the case of the random distribution of the charged monomers, two oppositely charged monomers act as two stickers during a collapse process, due to which the movement of monomers is suppressed and the chains can more directly reach their solid state. Interestingly, in a proper arrangement of the charged monomers, the chains may have a proteinlike nondegenerate state, i.e., the chain is designable. The transition of such polyampholyte chains appears to be of a two-state, all-or-none type.     

Solution photochemistry of o-hydroxybenzophenone at low temperatures

The decay of triplet o-hydroxybenzophenone takes place with a lifetime of 24 ns in methanol at 175 K and is proposed to involve the interconversion of the intermolecularly hydrogen bonded (anti) conformer into a short-lived syn species.