Low Temperature Optical Spectroscopy Research Articles

5] Low-Temperature optical spectroscopy: Metalloprotein structure and dynamics

Publisher Summary This chapter recounts outlines the history of the topic and presents an appropriate context to the value and proper status of low-temperature studies in revealing secrets of the structure-function relationships of metalloproteins. The values of low-temperature optical studies are many. Some of the obvious advantages these investigations often have over those at ambient temperatures are that they allow (1) the trapping or isolation of reactive species, (2) increased resolution of spectral bands, (3) probing of conformational dynamics and broadening mechanisms based on temperature-dependent changes in optical bands, and (4) examination of electron-nuclear coupling strengths by comparison of optical and resonance Raman theory and experiment. This chapter presents examples of the way both historical and current experiments capitalize on these four points and suggests some suitable areas for future research. Developments in the application of the technique have been evidenced by the expanding utilization of low-temperature optical spectroscopy to probe metailoprotein structure and dynamics. Specifically, analysis of low-temperature optical data to understand structure in heme proteins has seen an acceleration of interest. The use of the “optical method” in biophysics has been reviewed, and this chapter discusses these new applications and attempts to trace some of their historical origins.

Dynamics of adsorbed molecules in zeolites

Dye molecules adsorbed or embedded in the cages of a microporous zeolite have unique degrees of freedom which are not available to chromophores in e.g. molecular crystals or glasses. We have used low temperature optical spectroscopy and spectral hole burning to study the dynamics of a selected system, namely thionine molecules in a faujasite structure. The spectral properties depend strongly on the water content of the faujasite. Thionine in a hydrated sample shows considerable electron-phonon coupling comparable to the behaviour in a glassy matrix. In a dehydrated sample there exist two stable thionine conformations which can interconvert at low temperatures through ground state as well as excited state reactions. This reversible photocycle explains the appearance of antiholes at the burning wavelength. The electron-phonon coupling in the dehydrated sample is weak, similar to the characteristics of adsorbate/substrate systems.

Deuteration of pentacene in benzoic acid: monitoring the reaction kinetics via low-temperature optical spectroscopy

For pentacene dissolved in molten deuterated benzoic acid a proton-deuterium exchange between the solvent and solute molecules is observed. The reaction is monitored by recording at low temperature the visible absorption spectra of pentacene in the crystallized benzoic acid. The rate of exchange at 125°C is 6×10 -6 s -1 for six of the 14 protons of pentacene and is larger by more than 2 orders of magnitude than the exchange rate of the remaining eight protons

Photochemical and radiation-chemical aspects of matrix acidity effects on some organic systems

The role of matrix effects in radiolysis and photolysis is illustrated using two systems: organosulphur compounds and benzenediazonium salts. Their intermediates as detected by low temperature ESR and optical spectroscopy or FAB-MS give evidence that the main reaction pathways depend strongly on these effects. Changes in matrix acidity can control the formation of neutral radical, ion-radical or ionic species which are crucial to the character of the final products of irradiation of organosulphur compounds, which are of great importance in medicine, biology, ecology and industry. Microenvironmental influences determine whether the triplet aryl cation or radical species are detected as the principal or sole intermediates in the decomposition of diazonium salts, a process leading to different stable products with industrial application.

Magnetic observations of carrier quantization and dimensional crossover in diluted magnetic semiconductor superlattices

Diluted magnetic semiconductors have been shown to be excellent hosts in which to study the fundamental exchange interaction of a carrier with localized fluctuating magnetic spins. New low-temperature optical spectroscopy techniques using integrated DC SQUIDs have allowed sensitive magnetization studies in Cd 1− x Mn x Te superlattices as the magnetic layer thickness was systematically reduced towards the two-dimensional limit. Static and picosecond measurements reveal the magnetic manifestations of carrier localization in these electronic quantum wells.

Spectral hole burning of model photosynthetic systems

Basic problems in the primary processes of photosynthesis, interactions between pigments, proteins and quinones, as well as processes of energy transfer and electron transport in the primary events were studied using low temperature optical spectroscopy (hole burning technique). The obtained results (homogeneous widths of optical transitions and hole burning kinetics) were correlated on both model and in vivo photosynthetic systems.

X-ray absorption and low-temperature optical spectroscopy of coenzyme B-12

X-ray absorption and low-temperature optical spectroscopy of coenzyme B-12

Interaction between external medium and haem pocket in myoglobin probed by low-temperature optical spectroscopy

The visible absorption spectra of carbonmonoxymyoglobin in the temperature range 300 to 20 K are reported and compared with the analogous spectra of carbonomonoxyhaemoglobin. The temperature dependence of the zeroth, first and second moment of the observed bands is analysed to obtain information on the local dynamics in the proximity of the haem. Contrary to haemoglobin, the first moment of the observed bands in myoglobin is markedly affected by the solvent composition and its value saturates at temperatures at which the solvent undergoes the glass transition. These data indicate that solvent properties influence the haem pocket stereodynamics in myoglobin; moreover, the different behaviour between myoglobin and haemoglobin suggests that the process should involve the surfaces that are buried in the haemoglobin tetramer and exposed to the solvent in myoglobin, and/or the different protein compressibility.

Crystal field spectroscopy of the crystalline precipitates in NaCl:Co2+

The formation of three precipitated phases of Co2+ in NaCl, observed in as grown samples and after quenching from different temperatures, has been put in evidence by means of low temperature optical absorption and Raman spectroscopy. As grown samples show the presence of CoCl2 precipitates; and in quenched samples a tetrahedrally coordinated phase (probably Na2CoCl4) and the octahedrally coordinated Suzuki phase CoNa6⧠Cl8 have been found. Crystal field assignments of the Co2+ ions in the different crystalline phases, together with the B and Dq values, are also given. Finally, a comparative discussion on the different behavior of Co2+ and Ni2+ in their respective precipitates is made.

Closed-cycle refrigerator solution cell for low-temperature optical spectroscopy

The design and construction of a solution cell for low-temperature optical spectroscopy with a closed-cycle refrigerator is discussed.

A study of magnetic field effects upon triplet-triplet absorptions

A study of triplet-triplet absorptions in the presence and absence of an external magnetic field is presented. Some theoretical aspects are discussed. It is demonstrated that the application of the external field has no effect upon the absorption bands if the zero field spin-dipolar and spin-orbit interactions are identical for the triplet states involved in the transition. If the zero field interactions are different, however, the external field is expected to induce changes in the absorption band profile. Some experimental results of high resolution, low temperature optical Zeeman spectroscopy using mixed crystals of diphenylmethylene in benzophenone are reported. Field-induced broadening of the fwhm intensity of an electronic—vibrational band of the guest carbene is observed.

Low temperature photoacoustic spectroscopy of the purple membrane of Halobacterium halobium

The photoacoustic spectra of light- and dark-adapted bacteriorhodopsin in aqueous and mixed water-glyc-erol (1:2) suspensions are studied in the 90-300 K temperature range. Photochemical transients are observed and interpreted in terms of the known photocycles of trans- and 13-cis-bacteriorhodopsin. The spectra differ from those obtained by low temperature optical absorption spectroscopy of glassy glycerol-water suspensions.

Molecular units in ionic crystals via abstraction of matrix atoms by implanted metal ions

Abstract Heavy metal ions (*M = *Re, *Os, *Ir, *Pt) were implanted into halogen containing ionic crystals KX and K2MX6 (X = CI, Br; M = Sn, Re, Os, Ir, Pt) with energies ranging from 40 to 400 keV. They were generated via ion acceleration or recoil processes as a consequence of fast neutron capture. Optical spectroscopy and radiochemical methods were applied to analyse the chemical state of the projectiles after thermalization. Depending on the geometrical arrangement of the halogens in the different target materials up to 84% of the implanted ions were recovered in form of octahedral complexes *MX2− 6. In K2MX6 the amount of halogen ligand abstraction by *M showed a linear dependence on the relative strength of the broken M–X to that of the new *M–X bond. The beam experiments combined with low temperature optical spectroscopy proved a complex formation even for implantations at 5 K. The experiments demonstrated that implanted ions may undergo chemical reactions which are both spontaneous and selective.

Studies on the charge transfer band in high spin state of ferric myoglobin and hemoglobin by low temperature optical and magnetic circular dichroism spectroscopy

The behavior of charge transfer band, appearing at 600–650 nm in ferric high spin derivatives of myoglobin and hemoglobin, was studied under various conditions by low temperature optical and magnetic circular dichroism spectroscopy. Optical absorption spectra have demonstrated that: 1. (1)|The charge transfer band at 630 nm of myoglobin (Fe 3+) · H 2O (pH 7.0) at room temperature split into three bands, 627 nm, 645 nm and 664 nm (shoulder) at 77 °K, whereas that of hemoglobin (Fe 3+) · H 2O showed no splitting. 2. (2)|By lowering the pH value from 7.5 to 4.3 this splitting in myoglobin was observed to disappear only in the presence of a small amount of phosphate ion, accompanying a midpoint at pH 6.7 ± 0.1. This does not originate from the released hemin. 3. (3)|Hemin (pH 7.55) showed no splitting of the charge transfer band at 77 °K. 4. (4)|This splitting depended on the species of 6th ligand. For myoglobin · F − the splitting could scarcely be observed, whereas the proton-donating ligands such as HCOOH and CH 3OH exhibit the splitting as well as H 2O. Magnetic circular dichroism spectra have demonstrated that: 5. (5)|The charge transfer band at 600–650 nm indicated Faraday A term and B term. 6. (6)|A negative B term band was observed at 650 nm for myoglobin · H 2O in the glassic solvent of potassium glycerophosphate-glycerol, whereas it was not observed for hemoglobin · H 2O. Several discussions were performed on the origin of splitting of the charge transfer band in myoglobin · H 2O. It is now concluded that the hydrogen bond between the 6th ligand and the distal histidine contributes to the splitting of the charge transfer band around 630 nm for myoglobin (Fe 3+) · H 2O at low temperature and that disappearance of the splitting at low pH is originated from the presence of phosphate ion.

Myoglobin-facilitated oxygen diffusion: role of myoglobin in oxygen entry into muscle.

Myoglobin-facilitated oxygen diffusion: role of myoglobin in oxygen entry into muscle.

The radiation chemistry of liquid and glassy methanol

The methods of pulse radiolysis, low temperature optical and electron spin resonance spectroscopy and chemical analysis of stable products have been applied to an investigation of theγ-radiolysis of liquid and glassy methanol. Processes induced by generation of presumed intermediates by photoionization of solutes have been studied using the same techniques. Electrons are trapped in glassy methanol afterγ-irradiation at 77 K with a yieldG(e¯t)= 2.7 ± 0.3 and on warming are rendered mobile, ultimately forming an amount of hydrogen equal to1/2G(e¯t). In contrast, illumination of the irradiated sample at 77 K with visible light decomposes the trapped electron, forming hydrogen and CH2OH or CH2O-in yield equal toG(e¯t). The solvated electron in liquid methanol also reacts with the solvent, at a rate faster than the corresponding reaction in water. The addition of sodium methoxide to liquid methanol results in a marked increase in both the yield and lifetime of the solvated electron observed after 0.2μs irradiation pulse. The optical absorptions and decay parameters of CH2OH and CH2O-in liquid methanol were obtained; the ultraviolet absorption observed in irradiated methanol glass after removal of the trapped electrons by photo-bleaching is probably due to these two species. Solutions of methanol glass containing silver perchlorate exhibit complex optical and e. s. r. spectra, which provide evidence for several different species, but a complete analysis has not been possible. The radicals expected from the initial ionization event are considered. The results of this work together with published data are used to predict the reaction of the free radicals CH3O. , CH2OH, CH2O-, H. ande¯sor e¯tin the systems of interest and the relevance of these reactions to some aspects of the radiation chemistry of methanol is discussed.