Spin Barrier Research Articles

Heme enzyme structure and function.

Metalloporphyrins are widely used throughout the biosphere and of these heme (iron protoporphyrin IX, Fig. 1) is one of the most abundant and widely used. Heme shuttles electrons between proteins as in mitochondrial respiration or transports and stores O2 as with the globins. The role of heme in more active enzymatic chemical transformation began to be appreciated just after the discovery by Mason1 and Hayaishi2 that O2 O atoms can be enzymatically incorporated into organic substrates which represented the seminal discovery of oxygenases. While the enzymes used in these studies did not contain heme, it was not too long before heme-containing oxygenases also were discovered. In 1958 Klingenberg3 and Garfinkel4 found an unusual pigment in microsomes that when reduced in the presence of CO generated a spectrum with a peak at 450 nm instead of the expected 420 nm peak. Hence the name P450 was born. In 1964 Omura and Sato5,6 showed that this “pigment” is actually a protein and the function of this strange heme protein became clear in a seminal study by Estabrook et al.7 that demonstrated the involvement of the 450 nm pigment in steroid hydroxylation. Thus by the mid-1960s it was established that heme plays an active role in biology by somehow catalyzing the hydroxylation of organic substrates. While these discoveries certainly mark the beginning of modern approaches to studying heme enzyme oxygenases, the enzymatic role of heme dates much earlier to 1903 when horseradish peroxidase (HRP) was described.8 Indeed, owing to the ease of purification and stability of the various intermediates, HRP dominated heme enzyme studies until P450 was discovered. Figure 1 Structure of iron protoporphyrin IX. Heme enzymes can catalyze both reductive and oxidative chemistry but here we focus on those that catalyze oxidation reactions, and especially those for which crystal structures are available. There are two broad classes of heme enzyme oxidants: oxygenases that use O2 to oxidize, usually oxygenate, substrates and peroxidases that use H2O2 to oxidize, but not normally oxygenate, substrates. Of the two oxidants molecular oxygen is the most unusual because even though the oxidation of nearly all biological molecules by O2 is a thermodynamically favorable process, O2 is not a reactive molecule. The reason, of course, is that there is a large kinetic barrier to these reactions owing to O2 being a paramagnetic molecule so the reaction between a majority of biological molecules that have paired spins is a spin forbidden process. Overcoming this barrier is why Nature recruited transition metals and heme into enzyme active sites. As shown in Fig. 2, heme oxygenases bind O2 and store the O2 oxidizing equivalents in the iron, porphyrin, and/or amino acid side chains for further selective oxidation of substrates. Peroxidases use H2O2 as the oxidant and while not having the O2 spin barrier, H2O2 presents its own problems. The reaction between H2O2 and transition metals generates toxic hydroxyl radicals in the well known Fenton chemistry9 which would be highly destructive to enzyme active sites. As illustrated in Fig. 2, all heme oxidases are at some point in the catalytic cycle peroxidases. Molecular oxygen must first be reduced by two electrons to the peroxide level before the interesting chemistry starts: cleavage of the O-O bond. This bond can cleave either homolytically, which gives two hydroxyl radicals, or heterolytically to effectively give H2O and a naked O atom with only 6 valence electrons. Since the release of hydroxyl radicals in the active site must, in most cases, be avoided Nature has engineered heme enzyme active sites to ensure that the heterolytic pathway dominates. Figure 2 Oxygen and peroxide activation by heme enzymes. Oxygenases like P450 must have the iron reduced to ferrous (Fe(II) or Fe2+) before O2 can bind. The oxy complex is best described as ferric-superoxide, Fe(III)-OO−. A second electron transfer results ... The list of heme enzymes is substantial and thus it is necessary to be selective on which to discuss in detail. It may appear that a disproportionate amount of space is devoted to peroxidases and P450s. This is true and admittedly reflects the author’s own interests and area of expertise. Additionally, however, peroxidases are the most extensively studied heme enzymes and have provided fundamental insights into the chemistry and structure shared by many other enzymes. The other enzymes to be discussed were selected owing to both subtle variations on common themes and novel features that Nature selected for specific biological function.

Quantum-well states in Cu/Fe/Cu(111) coupled to the bulk band through thebarrier

The quantum-well state (QWS) has been observed on the surface of Cu/Fe/Cu(111). Theconfinement of the states on the top Cu layers is due to the minority spin barrier of the Feunderlayer. This QWS coexists with the Shockley surface state, which is observedon a clean Cu(111) surface. The resonant behavior of this QWS versus photonenergy results from the vertical transition to the unoccupied bulk band, which ispossibly due to the coupling between the overlayer Cu and the substrate Cu(111).

The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump-probe transient absorption spectroscopy, which directly measured FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-C5(CH3)5) were studied by the steady-state method. The pump-probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.

NEA rotations and binaries

AbstractOf the nearly 3900 near-Earth asteroids (NEAs) known as of June 2006, 325 have estimated rotation periods, with most of those determined by lightcurve analysis led by a few dedicated programs. NEAs with diameters down to 10 meters have been sampled. Observed spin distribution shows a major changing point around diameter of 200 meters. Larger NEAs show a barrier against spins faster than 11 d−1(period about 2.2 h) that shifts to slower rates (longer periods) with increasing lightcurve amplitude (i.e., with increasing equatorial elongation). The spin barrier is interpreted as a critical spin rate for bodies in a gravity regime; NEAs larger than 200 meters are predominantly bodies with tensile strength too low to withstand a centrifugal acceleration for rotation faster than the critical spin rate. The cohesionless spin barrier disappears at sizes less than 200 meters where most objects rotate too fast to be held together by self-gravitation only, so a cohesion is implied in the smaller NEAs.The distribution of NEA spin rates in the cohesionless size range (D0.2 km) is highly non-Maxwellian, suggesting that mechanisms other than just collisions have been at work. There is a pile up just in front of the barrier, at periods 2–3 h. It may be related to a spin up mechanism crowding asteroids to the barrier. An excess of slow rotators is observed at periods longer than 30 hours. A spin-down mechanism has no obvious lower limit on spin rate; periods as long as tens of days have been observed.Most NEAs appear to be in their basic spin states with rotation around principal axis with maximum moment of inertia. Tumbling objects (i.e., bodies in excited, non-principal axis rotation) are present and actually predominate among slow rotators with estimated damping timescales longer than the age of the solar system. A few tumblers observed among fast rotating coherent objects appear to be either more rigid or younger than the larger (cohesionless) tumblers.An abundant population of binary systems has been found among NEAs. The fraction of binaries among NEAs larger than 0.3 km has been estimated to be 15 ± 4%. Primaries of binary systems concentrate at fast spin rates (periods 2–3 h) and low amplitudes, i.e., they lie just below the cohesionless spin barrier. The total angular momentum content in binary systems suggests that they formed from parent bodies spinning at the critical rate. The fact that a very similar population of binaries has been found among small main belt asteroids suggests a binary formation mechanism that may not be related to close encounters with the terrestrial planets.

Rotational properties of asteroids, comets and TNOs

Over the past 25 years the number of reliably determined rotation rates of asteroids has increased by an order of magnitude, from 157 in 1979 to 1686 in 2005. As the numbers have increased, various special classes and features have emerged. Asteroids larger than diameter have a dispersion of spin rates that is well represented by a single Maxwellian distribution. Smaller asteroids have a more dispersed distribution, with both slow and fast spinning populations. We see a “spin rate barrier” in the size range of 1–10 km diameter that suggests that even rather small asteroids are “rubble piles”. Among the very slow rotators are some (but not all) that are “tumbling” in non-principal axis rotation states. Among the smallest asteroids (less than a few hundred m diameter) are some that spin dramatically faster than the “spin barrier”, indicating that they must have some tensile strength rather than consisting of loose regolith. In the last few years it has been recognized that the spins of asteroids smaller than a few tens of km diameter are affected by radiation pressure torques that tend to either speed up or slow down asteroid spin rates, thus providing an explanation for the dispersion of small asteroid spins, and also their non-random axis orientations. Lightcurves have also revealed the presence of binary asteroids among both Near-Earth and Main-Belt populations. Automated robotic observatories and next-generation survey instruments promise to increase the rate of production of asteroid lightcurves so that we may soon have tens of thousands of lightcurve results, extending down to even smaller sizes. In contrast, there are only about 20 rotation rates known for comets, and 15 for TNOs. Very little can be said from such meager statistics; the mean spin rate of TNOs appears to be comparable to that of asteroids, without extremes of fast or slow rotation; the mean spin rate of comets appears to be a bit slower than asteroids, perhaps due to lower mean density, and there may be an excess of slow rotators, probably due to gas jetting effects. The future is promising for studies of these objects as larger telescopes become available to do photometry to fainter magnitudes, so that comet nuclei can be studied at greater heliocentric distance with less coma interference, and more TNOs can be observed.

Tunnel magnetoresistance in GaMnAs: Going beyond Jullière formula

The relation between tunnel magnetoresistance (TMR) and spin polarization is explored for GaMnAs∕GaAlAs∕GaMnAs structures where the carriers experience strong spin–orbit interactions. TMR is calculated using the Landauer approach. The materials are described in the 6 band k∙p model which includes spin–orbit interaction. Ferromagnetism is described in the virtual crystal mean field approximations. Our results indicate that TMR is a function of spin polarization and barrier thickness. As a result of the stong spin–orbit interactions, TMR also depends on the the angle between current flow direction and the electrode magnetization. These results compromise the validity of Julliere formula.

Oxygen and I

Oxygen is a remarkable element and without it in its triplet, more stable state, complex life on earth would not be possible. The singlet carbon-hydrogen bond does not react readily with oxygen because of the spin barrier. Nature has, however, solved this problem by using iron based enzymes either of the heme-type (porphyrin ligand) or of the non-heme type. The P 450 enzymes are illustrative of the first kind. Methane monooxygenase (MMO) is a good example of the second type. Recent work on the selective functionalization of saturated hydrocarbons has provided a system which, like MMO, has a μ-oxo-di-iron center with obligatory carboxylate complexation. In this system, two manifolds, Fe IIFe IV and Fe IIIFe V provide two different processes for the selective conversion of saturated hydrocarbons into ketones.

Discrete versus continuous antiferromagnetic domain wall

Some specific features of the domain-wall (DW) solution for an antiferromagnetic chain of classical spins are discussed. It is shown that the Peierls-Nabarro barrier is absent. However, there is a specific spin barrier (i.e. the total spin of the chain depends on the position of the centre of the wall). The value of this barrier is calculated analytically. Existence of the spin barrier leads to the conclusion that the DW is unmovable for a discrete AFM chain, in contrast to the continuous case. The energy barrier is restored in the presence of an external magnetic field.

Function, mechanism and regulation of cytochrome P-450 enzymes in plants.

The spontaneous reaction of organic molecules with atmospheric oxygen is a spin-forbidden, and therefore slow. process. In aerobic organisms a few proteins, bearing a prosthetic metal or flavin to overcome this spin barrier, catalyse this crucial interaction. Prominent among these are the cytochromes 1’-450, specialized in the oxidation of lipophilic physiological substrates and in the activatioddetoxification of foreign compounds. Cytochrome 1’-450 enzymes are ubiquitous in the plant kingdom and play paramount roles in the biosynthesis of plant hormones, sterols, flavonoids, phytoalexins, of key intermediates in the synthesis of lignins and plant cuticles, and in the metabolism of xenobiotics [ 1, 21. The substrates of over 40 reactions already characterized are mainly phenylpropanoids, fatty acids, terpenes and foreign compounds. Over the past two decades, i t mas found that the plant mono-oxygenases have many similarities to the microsonial cytochrome 1’-450 enzymes from animals; and that the minimal active system is composed of the flavoprotein NAI>PI I-cytochrome P 450 reductase and o f cytochrome P-450 per se. The multienzynie complex is located in the endoplasmic reticulum (E.K.) and mitochondria1 enzymes similar to the mammalian steroid hydroxylases have not been detected in plants. Recently, the purification of NAIII’I I-cytochrome 1’-450 reductase [ 3-51 and of three cytochrome 1’-450 isoforms have been reported 10-8 opening the way to moleculacloning of these enzymes. Another haemoprotein, a peroxygenase which catalyses the transfer of oxygen to unsaturated fatty acids and sulphurcontaining chemicals by a different mechanism, has been discovered and is being characterized in our laboratory 10, 10 1.

Possible explanation of susceptibility experiments on dilute systems of random magnetic dipoles

Qualitative fits to low frequency susceptibility measurements of a random system of magnetic dipoles is consistent with the following picture. As we increase the concentration, larger spin clusters will appear which will have available a greater variety of spin configurations and barrier heights. Since the lowest barrier height dominates the dynamics of a cluster, the higher concentration samples have lower effective barrier heights.

ChemInform Abstract: CONJUGATED BIRADICAL INTERMEDIATES: SPECTROSCOPIC, KINETIC, AND TRAPPING STUDIES OF 2,2‐DIMETHYL‐1,3‐PERINAPHTHADIYL

Abstract(I) und (III), die aus dem entsprechenden Dihydrophenalendion über (II) gewonnen werden, ergeben beim thermischen Zerfall oder durch Blitzphotolyse das sehr kurzlebige Singulett‐Biradikal (1IV) (Lebensdauer τ < 1 ps), das kein "intersystem crossing" zeigt, sondern (I) bildet bzw. zurückbildet.

Conjugated Biradical Intermediates: Spectroscopic, Kinetic, and Trapping Studies of 2,2‐Dimethyl‐1,3‐perinaphthadiyl

AbstractThe biradical 2,2‐dimethyl‐1,3‐perinaphthadiyl (a) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2, and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low‐temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (3a) and rather long‐lived (400 μs) at room temperature. When formed in its lowest singlet state (1a), the biradical is too short‐lived to undergo intersystem crossing to 3a or bimolecular reactions. Thus, 3a is formed exclusively from the excited triplet state of the precursor compounds, 31* or 33*. The monomolecular decay of 3a is retarded by the spin barrier; 3a initiates the polymerization of acrylonitrile and is trapped by 3O2.

New reactions of triplet oxygen which avoid the spin barrier

The cation triphenylmethylium and the cation radical tris-(p-bromophenyl)ammoniumyl are effective catalysts for oxygenation of ergosteryl acetate (1; R = Ac) to the peroxide (2; R = Ac); the triphenylmethylium-catalysed reaction is a photo-oxygenation, whereas the cation radical reaction is thermal. Several other ‘forbidden’ additions of triplet oxygen to cisoid dienes have been demonstrated. A number of cation radicals have been examined for catalytic oxygenation ability. Ergosteryl acetate can also be converted into its peroxide by triplet oxygen in the presence of various Lewis acids.Two mechanisms for overcoming the spin barrier for triplet oxygen addition have been defined by experimental work. A mechanistic interpretation of the facts is proposed.

Prompt Gamma Rays fromU235(n,f),Pu239(n,f), and Spontaneous Fission ofCf252

The spectra of prompt $\ensuremath{\gamma}$ rays from $^{235}\mathrm{U}(n,f)$, $^{239}\mathrm{Pu}(n,f)$, and $^{252}\mathrm{Cf}$(s.f.) emitted at 0-10 nsec after fission were measured with 4-nsec time resolution. A $\ensuremath{\gamma}$-ray spectrometer with a near-Gaussian response was used over the entire energy region, and the prompt neutrons from fission were positively rejected by time of flight. The measured $\ensuremath{\gamma}$-ray spectra show a systematic softening with increasing mass number for $^{235}\mathrm{U}$, $^{239}\mathrm{Pu}$, and $^{252}\mathrm{Cf}$. The average photon energy above 0.14 MeV is 0.97 \ifmmode\pm\else\textpm\fi{} 0.05, 0.94 \ifmmode\pm\else\textpm\fi{} 0.05, and 0.88 \ifmmode\pm\else\textpm\fi{} 0.04 MeV/photon for thermal-neutron fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$, and spontaneous fission of $^{252}\mathrm{Cf}$, respectively. This behavior is discussed in terms of nearness of $Z$ and $N$ of the average fission fragment to closed-shell values, of the corresponding changes in level density expected, and of corroborative evidence from x-ray measurements. The total $\ensuremath{\gamma}$-ray energy released, ${\overline{E}}_{\ensuremath{\gamma},\mathrm{tot}}$, is 6.51, 6.81, and 6.84 MeV/fission for $^{235}\mathrm{U}(n,f)$, $^{239}\mathrm{Pu}(n,f)$, and $^{252}\mathrm{Cf}$ (s.f.), respectively, for photons above 0.14 MeV and for 0-10 nsec after fission. The calculations of Thomas and Grover are seen to be in agreement with these data in that they predict the correct partition of fission-fragment deexcitation energy between neutron and photon emission. This partition is related to the separation energy required to emit an additional neutron, to the pairing energy, and to the spin barrier for neutron emission. Two theoretical $\ensuremath{\gamma}$-ray spectra were compared to the measured spectrum for $^{235}\mathrm{U}(n,f)$, and each was found to fit the measured spectrum well over only a limited portion of the total energy range.