The electron transfer reactions of tetrakis (2, 2′- bipyridine)-μ-oxodiiron(III) complex ion (Fe2O4+) with thiourea (TU), N-methylthiourea (MTU), N–allylthiourea (ATU), N,N’–dimethylthiourea (DMTU), N,N’-dimethylthiourea (DETU), dithionite ion(S2O42–), dithionate ion (S2O62–), semicarbazide (EH2), diphenylcarbazide (DH2) and glutathione (GSH) were studied spectrophotometrically, in aqueous hydrochloric acid medium at T = 27.0 ± 3.0oC, I = 0.3 mol dm-3 (NaCl), [H+] = 1.0 ×10-3 mol dm-3 (HCl) and λmax = 520 nm. The stoichiometry was found to be 1:1(Fe2O4+/reductant) in all other systems but 1:2 in theFe2O4+- GSH system. The reactions of Fe2O4+- thioureas follow identical kinetics, being first order each with respect to [oxidant] and [thiourea] and second order overall. The reaction of Fe2O4+ with dithionite(S2O42-)ion, dithionate(S2O62-) ion, semicarbazide (EH2), diphenylcarbazide (DH2) and glutathione(GSH) follow first order and zero order with respect to oxidant and reductants respectively and first order overall. Changes in hydrogen ion concentration, the dielectric constant and ionic strength of the medium have no considerable influences on the rate in all the systems. The reactions involving thioureas were not affected by addition of cations but there was inhibition of the rate of reaction when anions were added. However, for all other systems the rate of reaction was independent of added cations and anions. Under comparable experimental conditions, the rate of oxidation of the thioureas followed the order k2(TU) < k2(DETU) < k2(ATU) < k2(MTU) < k2(DMTU), sulphur oxyanions kobs(S2O62-) < kobs(S2O42-) and that for the carbazides is kobs(EH2) < kobs(DH2). The observed kinetics in the reactions involving thioureas is in agreement with the rate law;
[Fe2O4+] = k2[Fe2O4+][thioureas]
While that for other reductants can be given as:
[Fe2O4+] = kobs [Fe2O4+]
In all the reactions, Fe2+ was found to be the product of Fe2O4+ reduction. The oxidation products of the reactions of thioureas were urea/urea derivatives and sulphur using both spectroscopic and qualitative methods. Whereas sulphate ion was qualitatively identified in S2O42- and S2O62- reactions, disulphide was identified as the product of the reaction of GSH with Fe2O4+. Spectroscopic evidence and Michaelis-Menten plots did not indicate the formation of intermediate complex prior to electron transfer, there was also absence of free radicals formation in all the systems. Based on the Michaelis- Menten plots, interactions with added ions, all the reactions are proposed to have proceeded through the outer-sphere electron transfer mechanism. Plausible mechanisms have been proposed for all the systems.
TABLE OF CONTENTS
Cover page i
Fly leaf ii
Title page iii
Table of Contents x
List of Figures xv
List of Tables xix
1.0 INTRODUCTION 1
1.2 Electron Transfer Theories 4
1.2.1Franck – Condon principle 4
1.2.2 Marcus theory 5
1.3 Statement of the Research Problem 8
1.4 Justification of the Study 9
1.6 Aim and Objectives of the Study 13
2.0 LITERATURE REVIEW 14
2.1 Binuclear Coordination Compounds of Iron(III) 14
2.2 Electron Transfer Reactions of Iron(III) 16
2.3 Thiourea 20
2.3.1 Electron transfer reactions of thiourea and its derivatives 21
2.3.2 Oxidative degradation of thiourea and its derivatives 27
2.4 Electron Transfer Reactions of Dithionite Ion 28
2.5 Electron Transfer Reactions of Dithionate Ion 34
2.6 Glutathione and its Electron Transfer Reactions 36
2.7 Diphenyl Carbazide and Semicarbazide 41
3.0 EXPERIMENTAL 44
3.1 Materials and Reagents 44
3.1.1 Synthesis and characterisation of
tetrakis(2,2′- bipyridine)-μ-oxo-diiron(III) chloride 44
3.1.2 Thiourea solution 45
3.1.3 N-methylthiourea (CH3HNCSNH2) solution 45
3.1.4 N, N’– dimethylthiourea (CH3HNCSNHCH3) solution 45
3.1.5 N-allylthiourea (CH2=CHCH2HNSNH2) solution 45
3.1.6 N, Nˈ- diethylthiourea (CH2=CHCH2HNSNH2) solution 45
3.1.7 Sodium dithionite(Na2S2O4) solution 46
3.1.8 Sodium dithionate (Na2S2O6) solution 46
3.1.9 Semicarbazide (OC(NH2)(N2H3) HCl) hydrochloride solution 46
3.1.10 Diphenylcarbazide(C13H14N4O) solution 46
3.1.11 Glutathione (C10H17N3O6S) solution 46
3.1.12 Preparation of standard solutions of magnesium chloride,
potassium chloride, sodium sulphate, sodium nitrate, sodium
chloride, sodium ethanoate and sodium formate 47
3.1.13 Preparation of standard solution of hydrochloric acid 47
3.2 Methods 47
3.2.1 Stoichiometric study 47
3.2.2 Products analysis 49
3.2.3 Kinetic measurement 50
3.2.4 Effect of change in hydrogen ion concentration on rate of
3.2.5 Effect of change in ionic strength of reaction medium on rate
of reaction 51
3.3.6 Dielectric constant dependence 52
3.2.7 Effect of addition of ions to reaction medium on rate
of reaction 52
3.2.8 Test for participation of free radicals in the course of reaction 52
3.2.9 Test for formation of intermediate complex prior to
electron transfer 53
4.0 RESULTS 54
4.1 Stoichiometry 54
4.2 Products Analysis 60
4.3 Determination of Pseudo-first Order and Second Order
Rate Constants and Order of Reaction 83
4.3.1 Fe2O4+ reaction with thiourea and thiourea derivatives
(N-methylthiourea, N,N’-dimethylthiourea and N-allylthiourea) 83
4.3.2 Fe2O4+ reaction with S2O42–, S2O62–, EH2, DH2 and GSH 99
4.4 Effect of Changes in Hydrogen Ion Concentration, [H+], on
Rates of Reaction 99
4.4.1Fe2O4+ reaction with thiourea and thiourea derivatives
(N-methyl thiourea, N-allylthiourea, N,N‟-dimethylthioureaandN, 99
Nˈ-diethylthiourea),S2O42–, S2O62–, EH2, DH2 and GSH.
4.5 Effect of Changes of Ionic Strength of Reaction Medium Reaction Rate 110
4.5.1 Fe2O4+ reaction with thiourea and thiourea derivatives
(TU, MTU, ATU DMTU and DETU), S2O42–, S2O62–, EH2,
DH2 and GSH 110
4.6 Effect of Changes in Dielectric Constant of Reaction
Medium on Reaction Rate 110
4.6.1 Fe2O4+ reaction with the thioureas, S2O42–, S2O62–,
EH2, DH2 and GSH 110
4.7 Effect of Added Ions on Reaction Rate 111
4.7.1 Fe2O4+ reaction with thioureas, S2O42–, S2O62–,
EH2, DH2 and GSH . 111
4.8 Test for the Formation of Intermediate Complex 142
4.8.1 Spectroscopic test 142
4.8.2 Kinetic test (Michaelis-Menten plots) 153
4.9 Free Radical Test 153
5.0 DISCUSSION 159
5.1 Fe2 O4+-Reaction with Thiourea and its Derivatives 159
5.2 -Reaction with S2O42- and S2O62- 166
5.3 -Reaction with Carbazides (Semi Carbazide and
5.4 -Reaction with Glutathione (GSH) 178
5.5 Comparison of Fe2O4+ – Thioureas Systems 183
5.6 Comparison of Fe2O4+ – Sulphur Oxyanions 183
5.7 Comparison of Fe2O4+ – Carbazides 184
6.0 SUMMARY, CONCLUSION AND RECOMMENDATIONS 185
6.1 Summary 185
6.2 Conclusion 185
6.3 Recommendations 186
Electron transfer (ET) is one of the unique chemical processes which have received considerable attention due to its role in physical and biochemicalsystems (Marcus and Sutin, 1985).It occurs when an electron moves from an atom or a chemical species (e.g. a molecule) to another atom or chemical species. Since the late 1940s, the field of ET processes has grown enormously. The development of the field, experimentally and theoretically, aswell asits relation to the studyof other kinds of chemical reactions, presents an intriguing history, one inwhich many threads have been brought together(Marcus, 1997). The process is a mechanistic description of the thermodynamic concept of redox, wherein the oxidation states of both reaction partners change. Numerous biological processes involve ET reactions. These processes include oxygen binding,photosynthesis, respiration, and detoxification. Additionally, the process of energy transfer can be formalised as a two-electron exchange (two concurrent ET events in opposite directions) incase of small distances between the transferring molecules. ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic chemistry and other areas as depicted in Figure 1.1(Marcus and Siddarth, 1992; Greenwood and Earnshaw, 1997; Holleman and Wiberg, 2001). Moreover, many reactions in bioinorganic systems involve the electron transfer at one stage or the other and proper understanding of these electron transfer processes would help in the understanding development and eventual effective control of a wide area of science and technology (Iyun, 1982).
Nonradiative and radiative ET are found to be a key elementary step in many important processes involving isolated molecules, ions and excess electrons in solution, condensed phase, surfaces and
Figure 1.1: Examples of topics in the electron transfer field (Marcus and Siddarth, 1992)
interfaces, bioelectrochemical systems and in solar cells, in particular. Marcus (1964) observed that one of the active areas in reaction kinetics during the post-war years has been that of ET reactions. These reactions constitute one type of oxidation-reduction process and include both chemical and electrochemical systems. Many rate constants have now been measured and they have stimulated a variety of theoretical studies. The field has been characterised by a strong interplay of theory and experiment, which now includes the testing of theoretically predicted quantitative correlations.
Chemical kinetics is concerned with the study of rates of chemical reactions and the effect ofphysical conditions such as temperature, light, pressure, ionic strength, and solvent concentration etc. on the reacton rate. The measurement of these rates under different conditions give information about the mechanismof the reaction.Chemical kinetics tries to answer the question of what happens as the reactants are converted to products. Does the reaction occur in one step, or in multiple steps via intermediates? The rates of chemical reactions are of great importance in industrial and biological processesespecially in determining optimum reaction as in organic synthesis and chemical manufacturing (Chigwada, 2005). Scientists use kinetic studies to postulates theories to mimic natural occurrences, calculate how fast the products will be formed and use thermodynamics topredict the equilibrium composition of the reaction mixtures.
The economic viability of many industrial processes is largely affected by the rate at which the reactions occur. Also, every chemical reaction taking place in living organisms occur at a rate carefully controlled by the complex catalysts called enzymes. Life would have been impossible without the rates of countless, complicated chemical processes being controlled with exceptional precision by exquisitely formed enzymes. The above processes in living and chemical industries are essentially redox reactions involving one or more electron transfer between any two chemicalentities (Muhammad, 2003).
A reaction mechanism is a detailed stepwise or step by step sequence of elementary reactions involving molecules, atoms, radicals or ions that occur simultaneously or consecutively andculminate in the overall reaction (Cooke, 1979). It is the theoretical framework accounting forthe fate of bonding electrons and illustrates which bonds are broken and which are formed. Forproper understanding of mechanisms, the experimentally determined rate equation, the exactnature of both reactants and products, the presence of any equilibrium and stoichiometry of the reaction are indispensable (Basolo and Johnson, 1964).
1.2 Electron Transfer Theories
In many instances, theory plays a role in unifying the structural and spectroscopic information, assisting the understanding of the many chemical reactions, including the frequent transfer of electrons between different sites, proton and proton coupled-electron transfers, and various bonds breaking and bondforming reaction steps (Marcus, 2009, 2012).Many theories have been postulated for electron transfer in chemical reactions and each are interrelated to give a
wholesomeand vivid explanationof what happened, how it happened and why it happened. Those relevant to the electron transfer are hereby described.
1.2.1 Franck – Condon principle
The Franck-Condon principle originated in molecular spectroscopy in 1925 when James Franck
proposed (and later Edward Condon provided a theoretical basis for) the idea that, when molecules absorb photons to undergo an electronic transition from the ground state E0 Figure 1.2) to an excited state (E1), the electronic transition occurs so rapidly that heavy nuclei do not have time to rearrange to their new equilibrium positions q01 (Figure 1.2). The electron and nuclear motions being 10-15s-1 and 10-12 s-1 respectively. In effect, this means that the photon-induced electronic transitions are most likely to occur from the ground vibrational level (i.e., ν‟‟ = 0) of the ground electronic state to an excited vibrational level (i.e., ν‟ = 2) of the upper electronic statevertical arrow as illustrated in Figure 1.2.
1.2.2 Marcus theory
Marcus theory is a theory originally developed by Rudolph A. Marcus, starting in 1956, to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species (called the electron donor) to another (called the electron acceptor).Marcus‟ inspiration to develop his theory for what is now referred to as Marcus theory, was a paper of Bill Libbyin which Franck-Condon principle was used to explain some experimental results, namely, why some isotopic exchange reactions which involve electron transfer between pairs of small cations in aqueous solution, such as reaction in equation 1.1, are relatively slow, whereas electron transfers involving larger ions, such as Fe(CN)63- – Fe(CN)64-
and MnO4- – MnO42-, are relatively fast (Marcus, 1997).In contrast with the classical chemical reaction (equation 1.1) in simple electron transfer reactions no chemical bonds are broken or formed, so different picture is needed for the reaction-rate for electron transfer.
AB+C→ A+ BC (1.1)
An example of reaction 1.1is the transfer of an H, such as in
Figure 1.2: A schematic representation of the Franck-Condon principle. The upward arrow indicates the most favored vibronic (i.e., both vibrational and electronic) transition predicted by the Franck – Condon principle
HI + Br-→I- + HBr(1.2)
or the transfer of a CH, group from one aromatic sulphonate to another.Libby (1952) noted that when an electron transfers from one reacting ion or molecule to another, the two new ions or molecules formed are in the wrong environment of the solvent molecules since the nuclei do not have time to move during the fast electron jump. Looking at the electron transfer reaction in equation(1.1), after the electron jump, the Fe2+ ion would be formed in some configuration of the many nearby dipolar solvent molecules that was appropriate to the original Fe3+ ion. This introduces a “solvatation energy barrier” for the process. The “solvatation energy barrier” is not the only mechanism playing an important role in the electron transfer reaction rate: the self-exchange reaction 1.3 involves big ions but the experimental rate for the reaction is really slow, in contrast to the picture of an electron transfer governed by the “solvatation energy barrier”.
[Co*(NH3)6]2+ + [Co(NH3)6]3+→[Co*(NH3)6]3++[Co(NH3)6]2+(1.3)
According to Marcus, the missing ingredient to explain the slow rate of the reaction 1.6 is the factthat there is a dramatic difference in the equilibrium Co-N bond length inthe +3 and +2 ions so that each ions would be formed in a very “foreign “configuration of the vibrational coordinates. It is clear then that electrontransfer implies changing in the chemical structure of the
reactants. To understand how the Franck-Condon principle is used, reference is made to its classical definition in spectroscopy: “an electronic transition is most likelyto occur without changes in thepositions of the nuclei in the molecular entityand its environment”. The resulting state is called a Franck-Condon state,and the transition involved a vertical transition. As electronic transitions,electron transfers are instantaneous compared to the motion of the nuclei ofthe molecules or ions involved in the process and of the orientation of themolecules in the medium (e.g. solvent molecules). The foreign environmentfor the new electronic state after the electronic jump can be seen as anenergetic barrier for the ET process (Marcus, 1997).
1.3Statement of the Research Problem
The kinetic data of electron transfer reaction oftetrakis (2,2′-bipyridine)-μ-oxodiiron(III) complex are scanty, besides there is no kinetic data of reactions between the oxidant and the biochemical species (thioureas, dithionite ion, dithionate ion, semicarbazide, diphenyl carbazide and glutathione) used as reductants in this research. Moreover, there is no established mechanism of reactions between the oxidant and the biomolecules used as reductants. This has constituted a great impediment with respect to proper understanding of some important kinetic information about the oxidant.
In fact, iron deficiency leads to the deficiency of neurotransmitters such as dopamine and serotonin in brain, inducing several mental diseases such as Parkinson‟s disease, depression and schizophrenia(Nishida, 2005). Thus, the ancient Greeks concocted portions of iron filings dissolved in vinegar, hoping that drinking this liquor would empower them with the properties of the element(McCord, 1996).Large amounts of ingested iron can cause excessive levels of iron in the blood. High blood levels of free ferrous iron react with peroxides to produce free radicals,
which are highly reactive and can damage DNA, proteins, lipids, and other cellular components. Thus, iron toxicity occurs when there is free iron in the cell, which generally occurs when iron levels exceed the capacity of transferrin to bind the iron. Damage to the cells of the gastrointestinal tract can also prevent them from regulating iron absorption leading to further increases in blood levels. Iron typically damages cells in the heart, liver and elsewhere, which can cause significant adverse effects, including coma, metabolic acidosis, shock, liver failure, coagulopathy, adult respiratory distress syndrome, long-term organ damage, and even death (Cheney et al., 1995).
As thiourea is toxic and cancer supporting agent, the environmental concerns have promoted studies on the destruction of thiourea.The presence of thiourea in urine was reported to be a non- specific indicator of cancer (Sandhyamayee, 2011). However, it has been tagged as carcinogenic by U.S Department of Health and Human Services (Sandhyamayee, 2011), and so all work with this compound should be performed with greatest care to prevent direct exposure to human.
1.4 Justification of the Study
It is generally recognised that iron, the most abundant transition metal ion in mammalian systems, is a necessary trace element and is required for normal metabolic processes spanning molecular oxygen transport, respiratory electron transfer, DNA synthesis, drug metabolism and numerous biological processes (McCord, 1996;Que and Ho, 1996; Wallar and Lipscomb, 1996; Beutleret al., 2003;Costas et al., 2004;Dlouhy and Outten, 2013;Yee and Talman, 2015).Iron metalloproteins serve as agents for oxygen transport and storage while haemoglobin and myoglobin are essential for electron transfer in the cytochromes. The group of enzymes called nitrogenases are iron complexes containing either iron and molybdenum, iron and vanadium or
only iron. Each of these enzymes contain two metalloproteins and serve as agents for transfering electrons to larger proteins or reservoirs for electrons before they are transferred. They have the ability to transform many substancesin vitro including nitrous oxide, cyanide, isocyanides, unsaturatedhydrocarbon, and so on. Iron enzymes, the hydrogenases also abound and play important physiological role. They convert dihydrogen to protons in vivo and delivered electrons from dihydrogen to the membrane-bound electron transport chain and also trap dihydrogen involvedin the enzyme reactions (Voordouw, 1992; Andrews, 2000).However, the study of the electron transfer reactions of binuclear complex of iron(III) would possibly go a long way to throw more light on some of the observed biological processes involving iron. Oxo-bridged iron complex has beensynthesised and characterised (Khedekar et al., 1967;Schugar et al., 1967, 1969, 1972; Stephen et al., 1967; Reiff et al., 1968; Cohen, 1969;Ménage et al., 1998).
The use of thiourea and its derivatives, dithionite ion, dithionate ion, semicarbazide, diphenyl carbazide and glutathione as choice of reductants in this work is due to their rolesin biochemical andchemical systems.Thiourea is an organosulphur compound, which is of high industrial potential. It is used to tone silver-gelatin photographic prints, as a plant growth stimulator to break bud dormancy and increase crop yield and more recently as a therapeutic agent in the treatment of thyroid dysfunction (Salem, 2010). As a chaotropic agent, it has been used to solubilise membrane and organelle specific proteins for analysis by two-dimentional gel electrophoresis (Rabilloud, 1998). Reactionof thiourea with hydrogen peroxide under certain conditions produces a powerful reductive bleaching agent, which is routinely used in the textile industry (Arifoglu et al., 1992; Cagarra et al., 1988). It is used as corrosion inhibitors (Ayres, 1970;Atwoodand Hale,1971;Klernet al., 1971;Santanu et al., 1997). Solution of thiourea in
dilute hydrochloric acid has found use as acomplexing agent for removing scales from such boilers (Frost, 1993).Thiourea is also used as a spectrophotometric reagent for the determination ofseveralmetals(Snell, 1978). Thiourea has also found use as a vulcanisation accelerator, additive for slurry explosives, viscosity stabiliser for polymer solutions and molality buffer in petroleum and gold extractions (Groenewald, 1977). It is used as reactant in the production of pharmaceuticals (sulfathiazole, thiobarbituric acid, thiouracil, tetramicol and cephalosporin) (Hilsona and Mouhemius, 2006), a class of drugs used in the treatment of hyperthyroidism (Mertschenk and Beck, 1995; Ardiwlaga, 1999). Several thioureaderivatives have various agricultural, biological, pharmaceutical and analytical applications which include applications in rubber industries as accelerators, in photography fixing agents and to remove stains from negatives, and in agriculture as fungicides, herbicides, and rodenticides (Ren, 2000;Yuan et al.,2001; Zhang, et al., 2004; Zhou, et al., 2004; Eweis et al., 2006). Furthermore, in this research it is hopeful that, the carcinogenic substances (thiourea and its derivatives) would be converted to non carcinogenic products that will be of immense benefits to mankind.
Dithionites and dithionates are oxyanions of sulphur which are good reducing agents.The reducing property of the dithionate ion has made it gain wide application in photographic processing as a fixer and also used in gold extraction. Sodium dithionite is used as a reducing agent in dyeing application. It undergoes reduction reaction with water insoluble vat dye and sulphur dye to form water-soluble alkali metal salt of the dye (leuco form) so that they have affinity for the textile fibre. The reductive decomposition of the excessive dye by sodium dithioniteimprove the colour fastness. It is used in bleaching mechanical paper pulp, cotton, wool and kaolin clay. Additional applications include water treatment, leather processing, food
processing, gas purification, cleaning, printing and stripping. In biological sciences sodium dithionite is often used in physiology experiments as a means of lowering solutions’ pH 7 (Mayhew, 1978).Carbazides (semicarbazide and diphenyl carbazide) are used in preparing pharmaceuticals including nitrofuran antibacterials (furazolidone, nitrofurazone,and nitrofurantoin) and related compounds.Semicarbazide products (semicarbazones and thiosemicarbazones) are known to have an activity of antiviral, antiinfective, and antineoplastic through binding to copper or iron in cells.
Glutathione (GSH) is awater soluble tripeptide thiol compound of the amino acids glutamine, cysteine, and glycine. The thiol group is a potent reducing agent, rendering GSH the most abundant intracellular small molecule thiol, reaching millimolar concentrations in some tissues. As an important antioxidant, GSH plays a role in the detoxification of a variety of electrophilic compounds and peroxides via catalysis by glutathione S-transferases and glutathione peroxidases. The importance of GSH is evident by the widespread utility in plants, mammals, fungi and some prokaryotic organisms (Anderson, 1998). In addition to detoxification, GSH plays a role in other cellular reactions, including, the glyoxalase system,reduction of ribonucleotides to deoxyribonucleotides, regulation of protein and gene expression via thiol:disulphide exchange reactions (Pastore et al., 2003).
In view of the roles played by these reductants, as enumerated above, there is need for kinetic data of the electron transfer of these importants reductants with tetrakis(2,2ˈ- bipyridine)-μ- oxodiiron(III) complex.The kinetic data generated from the electron transfer reaction between these biochemical compounds and Fe2O4+ will complement the much needed kinetic information
and will bring to the limelighttheir electron transfer properties with the aim of explaining and subsequently improving their physico-chemical properties. Also, the reactions will biomimic some biochemical processes in vitro.
1.5 Aim and Objectives of the Study
The aim of this research is to carry out kinetic studies and propose mechanismsof electron transfer reactions of tetrakis (2, 2′- bipyridine) – μ – oxodiiron(III) ions and some reductants. The objectives to this study are to:
i. synthesise and characterise the tetrakis(2,2′-bipyridine)-μ-oxodiiron(III) complex ions,
ii. determine the stoichiometries of the reactions.
iii. determine the pseudo-first order and second order rate constants of the reactions,
iv. determine the order of reactions with respect to each of the reductants,
v. determine the effects of changing the hydrogen ion concentration on the rates of the reactions that took place in acid medium,
vi. determine the effects of changes of ionic strength and dielectric constant of reaction medium on the rates of the reactions,
vii. determine the effect of added ions to the reaction medium on the rates of reactions,
viii. determine the participation of free radicals in the various reactions and
ix. determine the formation of intermediate complexes in the course of the reactions.
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