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Medicinal and Chemical Perspectives of Nitric Oxide: an Overview

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Medicinal and Chemical Perspectives of Nitric Oxide: an Overview Review Article Published: 15 Jan, 2019 SF Journal of Pharmaceutical and Analytical Chemistry Medicinal and Chemical Perspectives of Nitric Oxide: An Overview Kapoor Y and Kumar K* School of Pharmaceutical Sciences, Apeejay Stya University, Sohna-Palwal Road, Sohna, Gurgaon, Haryana-122103, India Abstract Nitric oxide (NO) is a simple, diatomic molecule possessing unique and fascinating chemistry. Indeed, the utility of NO as a biological effector and/or mediator is due to some of its novel chemical properties along with the properties of other nitrogen oxides derived from it. The multiple effects of NO in biological systems have resulted in intense investigation into the mechanisms of NO-mediated events. The chemistry of NO is the primary determinant of its biological properties. However, not all the reactions of NO that can be performed in the test tube are pertinent in vivo. Due to the discovery of its endogenous biosynthesis and diversity of its biological actions, nitric oxide (NO) is a molecule of extreme physiological, pharmacological, and pathophysiological interest. Much of the biological utility of NO is due to its unique chemical interactions with biological molecules. Thus, an understanding of the basic chemical properties of NO, and derived species, will provide insight into the intimate mechanisms of its biological activity. Keywords: Nitric oxide; In vivo; Biological activity; Chemistry Introduction In the late 1980s and early 1990s it was confirmed by several groups of workers that nitric oxide was a chemical messenger released by the endothelium and other tissues in mammals. It was tentatively identified as being the endothelium-derived relaxing factor (EDRF) discovered earlier in the 1990s and has been linked to a multitude of physiological and pathophysiological states in mammals [1]. It is now known to be involved in the control of blood pressure, neurotransmission and the immune defence system of the body [2,3]. NO in the brain regulates many physiological processes affecting behavior and cognitive function, including synaptic plasticity. In addition, it also OPEN ACCESS controls brain blood flow, promotes angiogenesis, maintains cellular redox state, cell immunity and neuronal survival. Its over-production may lead to neurodegeneration [4]. Excessive production has * Correspondence: been linked to atherosclerosis, hypotension, Huntington’s disease, Alzheimer’s disease and AIDS Kapil Kumar, School of Pharmaceutical dementia whilst underproduction has been related to thrombosis, vasospasm and impotence. This Sciences, Apeejay Stya University, diversity of action has resulted in there being a considerable interest in the relationship of NO to Sohna-Palwal Road, Sohna, Gurgaon, both healthy and diseased states [5]. Haryana-122103, India. E-mail: [email protected] Nitric oxide (NO) released by vascular endothelial cells accounts for the relaxation of strips of Received Date: 17 Nov 2018 vascular tissue [6] and for the inhibition of platelet aggregation [7] and platelet adhesion attributed 0 Accepted Date: 11 Jan 2019 to endothelium-derived relaxing factor [8]. NO is a colourless paramagnetic gas (B.P. -151.7 C), sparingly soluble in water (2-3 mmol dm-3). It is produced in mammals by the enzyme-catalysed Published Date: 15 Jan 2019 interaction of molecular oxygen and arginine. NO is a key molecule involved in a variety of Citation: Kapoor Y, Kumar K. Medicinal biological functions throughout the whole body [9]. However, whether it is NO or a derivative of and Chemical Perspectives of Nitric NO, that is ultimately responsible for the observed physiological response to NO generation is still Oxide: An Overview. SF J Pharm Anal under dispute. Chem. 2019; 2(1): 1015. Structure of Nitric Oxide ISSN 2643-8178 Copyright © 2019 Kumar K. This is an Nitric Oxide (NO) is a linear molecule. NO is colorless gas at room temperature and pressure. open access article distributed under The maximum solubility of ∙NO (at 1atm partial pressure) in water at room temperature and pressure is approximately 2mM which is slightly higher than the solubility of dioxygen (O ) in the Creative Commons Attribution 2 water. Also, like O , ∙NO is somewhat lipophilic and possesses 6- to 8- fold higher solubility in License, which permits unrestricted 2 nonpolar solvents and lipid membranes compared to water. Thus, the rates of ∙NO reactions in use, distribution, and reproduction in hydrophobic environment can be dramatically increased (with O , e.g.) over that found in water due any medium, provided the original work 2 to its increased concentration. It becomes immediately evident from a lewis dot depiction that ∙NO is properly cited. ScienceForecast Publications LLC., | https://scienceforecastoa.com/ 2019 | Volume 2 | Edition 1 | Article 1015 1 Kumar K, et al., SF Journal of Pharmaceutical and Analytical Chemistry orbital terminology, the electronic structure of ∙NO exists as a monomer at room temperature and pressure. The above rationale for N + O N::O the observation that ∙NO exists primarily as a monomeric species at room temperature and pressure is purely qualitative. Based on the Figure 1: Lewis dot structure of ∙NO. argument above, ∙NO is considered a free radical in spite of the fact that it does not have the tendency to dimerise at room temperature and pressure. Nitric oxide will, however, react with other species with ∗ σ z N atomic orbitals O atomic orbitals unpaired electrons. One of the most biologically relevant reactions * π xy of this type is the reaction of ∙NO with O2. As a ground state triplet, O2 has two unpaired electrons [electronic configuration for 2O is KK b 2 * 2 b 4 b 2 * 2 2p 2p (σs ) (σs ) (π xy) (σ z) (π xy) ] and therefore possesses some chemical σb z characteristics of a biradical. As such, O2 is known to react with other radical species. Since O2 has two unpaired electrons, the product of πb xy O2 and ∙NO (and other radicals) still has one unpaired electron which can react further. Thus, the reaction of ∙NO with O2 consumes two * σs equivalents of ∙NO to give two equivalents of nitrogen dioxide (∙NO2) 2s 2s possibly via the reactions (1), (2), and (3). b σs → ∙NO + O2 ∙OONO (1) Figure 2: Molecular orbital diagram for ∙NO. ∙OONO + ∙NO→ONOONO (2) → ONOONO 2∙NO2 (3) 2.5504 Å The simple Molecular orbital (MO) picture for ∙NO shows that O O 97.17o 1.1515Å the unpaired electron is in an antibonding MO (is a type of molecular N N 2.263 Å orbital that if occupied by electrons, weakens the bond between two atoms and helps to raise the energy of the molecule relative to the Figure 3: Structure of ∙NO dimer. separated atoms). Consequently, this electron should be readily lost, resulting in the formation of the nitrosonium ion (NO+). This is in agreement with NO’s low ionisation potential of 9.25 eV compared to A B π−backbond a value of 15.56eV for nitrogen gas. M N M σ−bondNO Chemical Properties of NO O 0 0 120 -140 The chemistry of ∙NO is quite varied. It can be oxidised, form Figure 4: (A) Bent geometry for M-nitrosyl bond. M = metal ion and (B) Linear salts, act as an electrophile, oxidising agent and free radical and geometry (arrows depict bonding interactions). form complexes [10]. It gives rise to a series of salts and complexes with metals. Many of the chemical species identified in laboratory has one unpaired electron and thus is formally a free radical species reactions have also been found in biological systems. It is believed (Figure 1). that many of these species could be formed in biological systems by reactions similar to those found in the in vitro laboratory experiments, The radical nature of ∙NO is evidenced by its ability to react with although at present few of these reactions have been detected in vivo. other species with unpaired electrons such as O , superoxides (O ∙ −) 2 2 In biological systems, ∙NO often appears to be closely associated and other radical species. However, unlike many free radicals, ∙NO with many other simple nitrogen-oxygen containing species such as doesn’t dimerise appreciably in the gas phase at room temperature nitrogen dioxide, nitrogen trioxide, nitrogen tetroxide, nitrate, nitrite and pressure, although it appears to form N O (Figure 3) in the 2 2 and peroxynitrite. Nitrogen dioxide and peroxynitrite are believed to liquid state. This may seem curious since dimerization would lead cause tissue damage [11]. to a structure (ON-NO) whereby all atoms have a full complement of eight valence electrons and, therefore, would satisfy the octet rule. Reaction of ∙NO with metals In order to reconcile this apparent anomaly, the molecular orbital Complexes containing ∙NO ligands have been prepared by the of ∙NO must be considered. Simple combination of the nitrogen direct reaction of ∙NO with the metal ion of complexes that have and oxygen atomic orbital’s gives the following set of molecular unused coordination sites, the displacement of an existing ligand orbital’s (Figure 2). In order of increasing energy, the molecular by ∙NO and the reaction of inorganic or organic nitrites [12]. The b orbital consist of a σ bonding orbital (σs ) and a corresponding σ nitric oxide ligand may be bonded to the metal in three distinct ways * b antibonding orbital (σs ), two degenerate π bonding orbitals (π xy), (Figure 4), namely: b one σ bonding orbital (σ z , only slightly higher in energy than the π bonding orbitals), two degenerate antibonding π orbitals (π* ), and 1. Complexes in which the M-NO bond is linear or almost xy 0 0 * linear, usually lying between 160 and 180 . In these complexes the a high energy σ antibonding orbital (σ z). Since the total number of valence electrons in the ∙NO molecule is 11 (five from nitrogen and ∙NO donates its odd electron to the metal atom.
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