Mini-Reviews in Organic Chemistry - Volume 8, Issue 4, 2011

The links between cigarette smoking and diseases such as lung cancer, chronic obstructive pulmonary disease and cardiovascular disease are now well established. Significant efforts have been made over the last 50 years to establish links between specific smoke constituents and the onset of these smoking related diseases. In recent years risk assessment approaches have also been developed to prioritise key toxicants amongst the thousands of identified species in cigarette smoke. The main focus of these models has been on stable, easily measured smoke constituents which have documented toxicity data. However, on a quantitative population basis, these models fail to predict the observed incidence of disease. A number of researchers have suggested alternative aspects of smoke which may also contribute to the risk of cigarette smoking. Amongst these, cigarette smoke free radicals have a long history of research findings which point to their potential to induce oxidative stress in smokers. However, due to the analytical challenges in identifying and quantifying these species, and the lack of quantitative risk data, they are not amenable to analysis using current risk assessment models. This special issue therefore reviews recent advances in our understanding of cigarette smoke free radicals with an emphasis on examining evidence for their involvement in smoking related diseases. Investigation of cigarette smoke chemistry is a specialised field, and this issue begins with an overview of recent advances in understanding of cigarette combustion and smoke formation, with focus on the sensitivity of smoke composition and chemistry to the methods used for smoke generation, trapping and analysis. Regulatory interest in smoke composition and the current generation of cigarette smoke risk assessment models are also discussed (Liu et al.). Interest in the chemistry of cigarette smoke free radicals is driven primarily by evidence for the participation of in vivo free radical species in disease processes, particularly oxidative stress. Current understanding of the contribution of oxidative stress to smoking related diseases is summarised by Fearon et al., together with evidence for the involvement of reactive oxygen species, reactive nitrogen species and biological free radicals. A review of the role of these reactive species in vivo leads to the conclusion that reactive oxygen species are major species involved in damage at molecular, cell and organism levels. An important biological mechanism for the generation of oxygen free radicals is decomposition of hydrogen peroxide to hydroxyl radicals via the Fenton reaction (Sharan et al.). Biomarkers of oxidative stress have been used as a surrogate measure of radical damage in smokers; these biomarkers are critically reviewed in light of the consistency of their relationship with smoking status (Lowe and Cemeli)......

This short review summarises some fundamental aspects of cigarette combustion and smoke formation, including how cigarettes burn, how cigarette smoke is formed and the complex and reactive nature of it's composition. Particular emphasis has been placed on important factors which have to be controlled when generating, trapping and analyzing cigarette smoke. Examples are provided which demonstrate the sensitivity of cigarette smoke composition to the way it is produced and measured, a subject of particular importance for redox sensitive species such as free radicals and multiple valency-state metals. Recent regulatory interest in smoke constituent yields is summarized, as well as risk assessment approaches which have sought to identify the smoke constituents which make the greatest contribution to smoking related diseases. Limitations of these approaches are discussed, and a number of other aspects of cigarette smoke that have been suggested to contribute to the incidence of smoking related diseases are highlighted.

Oxidative stress, the accumulation of oxygen free radicals (reactive oxygen species) above and beyond the capacity of a cell to utilise antioxidant systems to detoxify these potentially damaging molecules, is a common feature of many human disorders. Cigarette smoke is not only a source of free radicals but is also a potent stimulator of the intracellular production of free radicals, by the mitochondrial electron transport chain and the plasmalemmal NADPH oxidase. Adding to this free radical burden is the reduction, by cigarette smoke, of the cellular antioxidant capacity. Together, the increased production and reduced detoxification of free radicals has been strongly linked to smoking-induced diseases including atherosclerotic cardiovascular disease, cancer and chronic obstructive pulmonary disease (COPD). In this review, we discuss the mechanisms underlying cellular free radical production, relate this to the three major smoking-related human diseases listed above and present potential mechanisms by which cigarette smoke may increase the oxidative burden on cells and contribute to disease.

Free radicals (FR) are chemical species of significant importance to biological systems. FRs are generated by endo- as well as exogenous factors. Biological systems are equipped with appropriate metabolic pathways to remove cellular FR as well as repair the damages caused by them. Their cellular/physiological load profoundly influences the metabolism, physiology and overall well being of biological systems. Therefore, they are implicated in cellular degenerative processes and in patho-physiology, including carcinogenesis and ageing. The review shall attempt to give a generalized overview of FR chemistry, especially to the biologically important oxygen free radicals (OFR) and reactive oxygen species (ROS). The review shall also discus the OFR/ROS biology to get an overview of the induced damage at molecular, cellular and organismal levels in order to give a perspective of their influences on the genomic integrity, cellular microenvironment and physiology with special reference to human health.

Oxidative stress has been implicated in the development of smoking-related diseases such as lung cancer, cardiovascular disease and chronic obstructive pulmonary disease. Damage to biological tissues from interactions with free radicals found within the smoke and induced within cells by smoke exposure, is thought to contribute to smoking-related disease development. Many of these radical components are very short-lived in vivo due to their highly reactive nature and the highly efficient detoxification mechanisms possessed by the body to counteract their effects. Hence, biomarkers are needed to assess the extent of radical exposure and subsequent oxidative damage in humans. Oxidised lipids, proteins and DNA bases persist longer in vivo, and have been extensively investigated as surrogate measures of radical damage. Furthermore, assessment of the body's antioxidant defence mechanisms such as antioxidant enzyme activity and antioxidant compounds can help to understand the extent of radical exposure. This mini review critically evaluates various biomarkers falling into these categories and the consistency of their relationship with smoking status, as a preliminary evaluation of their usefulness in dissecting disease pathways in smokers. The future use of such biomarkers is also briefly discussed.

Tobacco smoke contains many thousands of chemicals including a large number of carcinogens. The exposure of human tissues and organs to these carcinogens and their metabolism in relation to smoking-related cancer has been reviewed. The assault on DNA and RNA as well as other cellular components by the ROS is the base for oxidation. Free radicals (ROS) are implicated in chemical carcinogenesis via various metabolic pathways. The participation of free radicals in tobacco smoke in the process of carcinogenesis is mainly due to the effect of oxidative substances on the signal transduction pathways which lead to the cell replication by transforming the signaling proteins. However, the exact mechanism through which free radicals function in this process is not completely understood.

Hydrogen peroxide (H2O2) is one of the most important constituents in the metabolic chain transforming cigarette smoke in the human organism. The present analytical review provides a survey of the state-of-art in the study of H2O2 chemistry and biochemistry (both as a reactive and signaling species) in the context of cigarette smoking, taking into account both endogenous and exogenous (environmental, dietary, etc.) sources of H2O2. Particular attention is given to the methodological problems of monitoring smoke-associated H2O2. Our analysis is of prime interest for understanding the various mechanisms of smoke-induced oxidative stress and for developing rational approaches for diminishing the risk factors for human health associated with cigarette smoking.

Free radicals in the particulate phase of cigarette smoke were first measured by direct electron paramagnetic resonance (EPR) spectroscopy over 60 years ago. Early efforts to measure free radicals in cigarette smoke were prompted by the theory that radicals could be involved in carcinogenesis. It was thought that free radicals could be either direct acting or produced by other components of cigarette smoke such as polycyclic aromatic hydrocarbons. Even today, it is uncertain which of these routes of action is the most important. Ultimately, the development of a strategy to minimize potential biological damage from free radicals is dependent on the extent to which free radicals delivered in cigarette smoke are directly involved in disease processes. In recent years, the primary instrumental means for identifying and studying free radicals in smoke have been both EPR and mass spectrometry (MS) techniques using spin trapping. The spin trapping technique allows stabilization of short-lived species. When coupled with MS, spin trapping allows complete structural characterization of free radicals. When coupled with EPR, spin trapping allows structural characterization by comparison to EPR spectra of known chemical species. Advances in the development of spin traps and spin trapping techniques, along with EPR and MS methods used for the study of cigarette smoke free radicals are presented in this review.

Cigarette smoke free radicals have been the subject of many years of investigation. Recently, individual acyl and carbamoyl radicals in fresh whole cigarette smoke were isolated, identified, and quantified by HPLC-mass spectrometry for the first time. These carbon-centered radicals do not conform to the established “steady-state model” for radical formation in gas-phase cigarette smoke. In separate studies, NO2 in fresh whole mainstream smoke has been measured on a puff-to-puff basis by highly precise and rapid tunable infrared diode laser spectroscopy (TILDAS). Only the first puff contained a significant amount of NO2 and its overall yield was substantially lower than previously reported. Nevertheless, when the smoke was passed through a Cambridge filter pad, NO2 appeared in the filtered gas-phase smoke during every puff, suggesting that NO2 forms directly on the pad. For fresh smoke, these results also challenge the applicability of the steady-state mechanism, which postulates that NO2 in the smoke itself is an essential intermediate for gas-phase radical formation. In this review, the historical investigations of cigarette smoke radicals are re-examined and the implications of the new spectroscopic and radical trapping data are discussed. Possible alternative mechanisms for radical formation in cigarette smoke are proposed. When combined with data from previous corroborating reports, these recent results raise serious issues about the use of the Cambridge pad, which is conventionally used to separate gas smoke constituents from whole smoke, but may introduce more artifacts of measurement for certain reactive smoke constituents than previously appreciated.

A critical examination of the literature on gas-phase free radicals and TPM-associated EPFRs in cigarette smoke is presented. These radicals participate in catalytic cycles that generate ROS that can result in oxidative stress. It seems that primary radicals originate from the direct decomposition of tobacco constituents and are linked to the chemical structure of the constituents and the temperature of pyrolysis. On the other hand, secondary radicals are generated during aging of smoke. Their EPR spectra are indicative of hydroquinone and catechol. Lignin, chlorogenic acid, and proteins are major precursors of EPFRs in TPM, but combinations of components form more secondary radicals than the sum of the individual components, suggesting a synergistic role of TPM in the formation and the stabilization of secondary radicals. Matrix isolation EPR has identified cyclopentadienyl, phenoxyl, and semiquinone radicals as additional free radicals formed from phenolic precursors that may be responsible for gas-phase alkoxy radicals previously identified using spin-trapping techniques. EPFRs are found to initiate catalytic cycles that generate ROS, and the overall concentrations of ROS generated by EPFRS are expected to be much higher than that generated by hydroquinones/quinones.

The hydroxyl radical (˙OH) has been considered to be one of the most reactive oxygen species that are produced in biological systems. Frequently, ˙OH formation is ascribed to the transition metal-catalyzed Fenton reaction. During the study of the molecular mechanism for the genotoxicity of the wood preservative pentachlorophenol (PCP), we found that ˙OH could be produced by H2O2 and PCP metabolite tetrachloro-1,4-benzoquinone (TCBQ) independent of transition metal ions. Further studies showed that TCBQ, but not its corresponding semiquinone radical, is essential for ˙OH production. Metal-independent production of ˙OH could also be observed with other halogenated quinones and H2O2. Based on these data, we propose that ˙OH production by TCBQ and H2O2 is not through a semiquinone-dependent organic Fenton reaction, but rather through a novel nucleophilic substitution and homolytical decomposition mechanism. This represents a novel mechanism for ˙OH production not requiring the catalysis of redox-active transition metal ions, and may partly explain the potential carcinogenicity of the widely used biocides such as PCP and other polyhalogenated aromatic compounds.

In this review, the features of electron paramagnetic resonance (EPR) spectra of biomass chars prepared from single biopolymers (cellulose, pectin, lignin, etc.) to complex plant materials (tobacco) are summarized, and the interaction of biomass chars with air, O2 and/or H2O as detected by EPR is critically examined. The effects of inorganic components are emphasized for understanding the creation and annihilation of free radicals in biomass chars in the presence of O2 or air. While the EPR spectra can be difficult to interpret, the interactions observed reflect fundamental processes occurring in biomass chars during pyrolysis and combustion that may affect the chemical composition of biomass conversion products as well as cigarette emission.

In the synthesis of primary (hetero)aryl amines through copper-catalysis, the ammonia surrogates or aqueous ammonia nucleophilic partners can be replaced by sodium azide. Recent efforts in copper-mediated transformation of (hetero)aryl halides to (hetero) aryl amines using azide anion as the amino source are discussed in these highlights.

Recently an aqueous medium has attracted the interest of chemists for many organic synthesis, and sometimes, surprising discoveries have been made using water. Among the various organic reactions investigated in aqueous medium, the synthesis of heterocyclic compounds has been the most widely studied because heterocyclic compounds are rich sources of diverse physical, chemical, and biological properties. They are commonly used as templates to design biologically active agents in medicinal chemistry. Moreover, in the past 20 years, the drug-discovery process has undergone extraordinary changes, and high-throughput biological screening of potential drug candidates has led to an ever-increasing demand for novel drug, based on heterocyclic compounds. Noteworthy advantages were observed during the course of our study on “in water” synthesis of heterocyclic compounds. The established advantages of water as a solvent for reactions are: water is the most abundant and available resource on the planet and many biochemical processes occur in aqueous medium. This review is focussed on the use of water in the synthesis of heterocycles. Heterocycles that will be covered include acridine, benzothiazole, benzopyran, isoxazole, pyradazine, pyrazole, pyridine, pyrimidine, pyrrole, quinazoline, quinoline, quinoxaline, thiadiazole, triazole, xanthene, xanthone and others.