Raymond Y. N. Ho

An FeIV=O complex of a tetradentate tripodal nonheme ligand.

The reaction of [FeII(tris(2-pyridylmethyl)amine, TPA)(NCCH3)2]2+ with 1 equiv. peracetic acid in CH3CN at −40°C results in the nearly quantitative formation of a pale green intermediate with λmax at 724 nm (ɛ ≈ 300 M−1⋅cm−1) formulated as [FeIV(O)(TPA)]2+ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [FeIV(O)(TPA)(ClO4)]+ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔEQ = 0.92(2) mm/s and δ = 0.01(2) mm/s; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 FeIVO complexes. The presence of an FeIVO unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s → 3-d transition and the requirement for an O/N scatterer at 1.67 Å to fit the extended x-ray absorption fine structure region. The [FeIV(O)(TPA)]2+ intermediate is stable at −40°C for several days but decays quantitatively on warming to [Fe2(μ-O)(μ-OAc)(TPA)2]3+. Addition of thioanisole or cyclooctene at −40°C results in the formation of thioanisole oxide (100% yield) or cyclooctene oxide (30% yield), respectively; thus [FeIV(O)(TPA)]2+ is an effective oxygen-atom transfer agent. It is proposed that the FeIVO species derives from O—O bond heterolysis of an unobserved FeII(TPA)-acyl peroxide complex. The characterization of [FeIV(O)(TPA)]2+ as having a reactive terminal FeIVO unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.

AN FEIVO COMPLEX COMPLEX OF A TETRADENTATE TRIPODAL NONHEME LIGAND

The reaction of [FeII(tris(2-pyridylmethyl)amine, TPA)(NCCH3)2]2+ with 1 equiv. peracetic acid in CH3CN at −40°C results in the nearly quantitative formation of a pale green intermediate with λmax at 724 nm (ɛ ≈ 300 M−1⋅cm−1) formulated as [FeIV(O)(TPA)]2+ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [FeIV(O)(TPA)(ClO4)]+ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔEQ = 0.92(2) mm/s and δ = 0.01(2) mm/s; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 FeIVO complexes. The presence of an FeIVO unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s → 3-d transition and the requirement for an O/N scatterer at 1.67 Å to fit the extended x-ray absorption fine structure region. The [FeIV(O)(TPA)]2+ intermediate is stable at −40°C for several days but decays quantitatively on warming to [Fe2(μ-O)(μ-OAc)(TPA)2]3+. Addition of thioanisole or cyclooctene at −40°C results in the formation of thioanisole oxide (100% yield) or cyclooctene oxide (30% yield), respectively; thus [FeIV(O)(TPA)]2+ is an effective oxygen-atom transfer agent. It is proposed that the FeIVO species derives from O—O bond heterolysis of an unobserved FeII(TPA)-acyl peroxide complex. The characterization of [FeIV(O)(TPA)]2+ as having a reactive terminal FeIVO unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.

Interspecies conservation and variation in peptidergic neurons

Abstract The degree of variation in structure of neurohypophyseal nonapeptides is reviewed and contrasted with the striking conservation in the structure of somatostatin across a range of vertebrate phylogeny. The distribution of neuropeptides has been most thoroughly studied in the central nervous system of the rat. Preliminary interspecies comparisons of the distribution of neuropeptides suggest that certain peptidergic elements found to be abundant in some species are poorly represented in the rat. Other peptidergic elements seem not to vary significantly between species.

Immunohistochemical Studies of Central and Peripheral Peptidergic Neurons

Publisher Summary This chapter reviews some aspects of the distribution of presently characterized neuropeptides revealed by immunohistochemical techniques. The distribution of neuropeptides as revealed by immunohistochemical techniques suggests two varieties of neural circuits in which the peptides play a role. The first of these generalized systems is thought to participate in neuroendocrine regulation, as the peptides are found in high concentrations in terminals adjacent to vascular elements that drain either to the anterior pituitary or to systemic circulation. Peptides released from such neurohemal sites may act as hormones upon target cells. Alternatively, they may act via axo-axonic interactions with other peptidergic terminals that liberate “true” hormones. Neuropeptides are also found in circuits of neurons engaged in interneuronal communication.

Overview of the Energetics and Reactivity of Oxygen

Oxygen, the most abundant element in the Earth’s crust (approximately 49.5% by weight), is believed to have been discovered first around 1774, by Carl Wilhelm Scheele, a Swedish pharmacist, who observed that heating silver carbonate produced a gas which would support respiration. Publication of Scheele’s manuscript on this discovery was delayed, however (Scheele, 1777), allowing Joseph Priestley, an English clergyman who made similar observations upon heating mercuric oxide, to publish his findings first (Priestley, 1776). Regardless of the true chronology of the discovery of this element, it was not until 1787 that it was given the name “oxygen”, meaning acid-former, by Antoine Laurent Lavoisier, who believed at the time that all acids contained oxygen (Jaffe, 1949). Since those early studies, a wealth of information on the chemistry and biochemistry of oxygen has been discovered. It is now known that oxygen can form compounds with all of the elements except helium, neon, argon, and probably krypton. Oxygen, in the form of dioxygen, is widely used in industry in the production of steel and other metals, the manufacture of chemicals, rocket propulsion, and the production of stone- and glass-containing products (Francis, 1992).

Biological Reactions of Dioxygen: An Introduction

Life on earth originated during a time when the atmosphere contained little or no gaseous oxygen. Primitive cells obtained the energy for their metabolism from glycolysis rather than respiration, and the fact that they were rich in thiols and other reducing agents did not present a problem because they were not normally confronted with appreciable levels of dioxygen (O2) or other strong oxidants. The advent of photosynthesis changed this situation dramatically by introducing gaseous dioxygen into the atmosphere, initiating what has been referred to as the most dramatic example of environmental pollution that has ever occurred on earth (Levine, 1988). Ultimately, the level of dioxygen reached its modern level of 21%, an environment that is toxic to strict anaerobic bacteria, the modern-day descendants of those first primitive organisms. By contrast, modern aerobic organisms, which, like anaerobic organisms, also consist of cells rich in reducing agents, evolved to use the powerful oxidizing potential of dioxygen to their benefit by developing respiration and, at the same time, elaborate systems to protect, repair, or replace their components that might be damaged by the oxidation reactions that are the inevitable by-product of dioxygen metabolism (Bilinski, 1991). The fact that aerobic organisms need dioxygen to survive and yet must constantly guard against its toxicity is frequently referred to as the oxygen paradox(Koppenol, 1988).