Chlorophyll is any of several related green pigments found in cyanobacteria and the chloroplasts of algae and plants. Its name is derived from the Greek words chloros and φύλλον, phyllon. Chlorophyll is essential in photosynthesis, allowing plants to absorb energy from light. Chlorophylls absorb light most in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum, which it reflects, producing the green color of chlorophyll-containing tissues. Two types of chlorophyll exist in the photosystems of green plants: chlorophyll a and b. Chlorophyll was first isolated and named by Joseph Bienaimé Caventou and Pierre Joseph Pelletier in 1817; the presence of magnesium in chlorophyll was discovered in 1906, was the first time that magnesium had been detected in living tissue. After initial work done by German chemist Richard Willstätter spanning from 1905 to 1915, the general structure of chlorophyll a was elucidated by Hans Fischer in 1940.
By 1960, when most of the stereochemistry of chlorophyll a was known, Robert Burns Woodward published a total synthesis of the molecule. In 1967, the last remaining stereochemical elucidation was completed by Ian Fleming, in 1990 Woodward and co-authors published an updated synthesis. Chlorophyll f was announced to be present in cyanobacteria and other oxygenic microorganisms that form stromatolites in 2010. Chlorophyll is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll molecules are arranged in and around photosystems that are embedded in the thylakoid membranes of chloroplasts. In these complexes, chlorophyll serves three functions; the function of the vast majority of chlorophyll is to absorb light. Having done so, these same centers execute their second function: the transfer of that light energy by resonance energy transfer to a specific chlorophyll pair in the reaction center of the photosystems; this pair effects the final function of charge separation, leading to biosynthesis.
The two accepted photosystem units are photosystem II and photosystem I, which have their own distinct reaction centres, named P680 and P700, respectively. These centres are named after the wavelength of their red-peak absorption maximum; the identity and spectral properties of the types of chlorophyll in each photosystem are distinct and determined by each other and the protein structure surrounding them. Once extracted from the protein into a solvent, these chlorophyll pigments can be separated into chlorophyll a and chlorophyll b; the function of the reaction center of chlorophyll is to absorb light energy and transfer it to other parts of the photosystem. The absorbed energy of the photon is transferred to an electron in a process called charge separation; the removal of the electron from the chlorophyll is an oxidation reaction. The chlorophyll donates the high energy electron to a series of molecular intermediates called an electron transport chain; the charged reaction center of chlorophyll is reduced back to its ground state by accepting an electron stripped from water.
The electron that reduces P680+ comes from the oxidation of water into O2 and H+ through several intermediates. This reaction is how photosynthetic organisms such as plants produce O2 gas, is the source for all the O2 in Earth's atmosphere. Photosystem I works in series with Photosystem II. Electron transfer reactions in the thylakoid membranes are complex and the source of electrons used to reduce P700+ can vary; the electron flow produced by the reaction center chlorophyll pigments is used to pump H+ ions across the thylakoid membrane, setting up a chemiosmotic potential used in the production of ATP or to reduce NADP+ to NADPH. NADPH is a universal agent used to reduce CO2 into sugars as well as other biosynthetic reactions. Reaction center chlorophyll–protein complexes are capable of directly absorbing light and performing charge separation events without the assistance of other chlorophyll pigments, but the probability of that happening under a given light intensity is small. Thus, the other chlorophylls in the photosystem and antenna pigment proteins all cooperatively absorb and funnel light energy to the reaction center.
Besides chlorophyll a, there are other pigments, called accessory pigments, which occur in these pigment–protein antenna complexes. Chlorophylls are numerous in types, but all are defined by the presence of a fifth ring beyond the four pyrrole-like rings. Most chlorophylls are classified as chlorins, they share a common biosynthetic pathway as porphyrins, including the precursor uroporphyrinogen III. Unlike hemes, which feature iron at the center of the tetrapyrrole ring, chlorophylls bind magnesium. For the structures depicted in this article, some of the ligands attached to the Mg2+ center are omitted for clarity; the chlorin ring can have various side chains including a long phytol chain. The most distributed form in terrestrial plants is chlorophyll a; the structures of chlorophylls are summarized below: When leaves degreen in the process of plant senescence, chlorophyll is converted to a group of colourless tetrapyrroles known as nonfluorescent chlorophyll cataboli
Cytochromes are proteins containing heme as a cofactor. They are classified according to its mode of binding. Four varieties are recognized by the IUBMB, cytochromes a, cytochromes b, cytochromes c and cytochrome d. Cytochrome function is linked to the reversible redox change from ferrous to the ferric oxidation state of the iron found in the heme core. In addition to the classification by the IUBMB into four cytochrome classes, several additional classifications such as cytochrome o and cytochrome P450 can be found in biochemical literature. Cytochromes were described in 1884 by MacMunn as respiratory pigments. In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”, he classified these heme proteins on the basis of the position of their lowest energy absorption band in their reduced state, as cytochromes a, b, c. The ultra-violet to visible spectroscopic signatures of hemes are still used to identify heme type from the reduced bis-pyridine-ligated state, i.e. the pyridine hemochrome method.
Within each class, cytochrome a, b, or c, early cytochromes are numbered consecutively, e.g. cyt c, cyt c1, cyt c2, with more recent examples designated by their reduced state R-band maximum, e.g. cyt c559. The heme group is a conjugated ring system surrounding an iron ion; the iron in cytochromes exists in a ferrous and a ferric state with a ferroxo state found in catalytic intermediates. Cytochromes are, capable of performing electron transfer reactions and catalysis by reduction or oxidation of their heme iron; the cellular location of cytochromes depends on their function. They can be found as membrane proteins. In the process of oxidative phosphorylation, a globular cytochrome c protein is involved in the electron transfer from the membrane-bound complex III to complex IV. Complex III itself is composed of several subunits, one of, a b-type cytochrome while another one is a c-type cytochrome. Both domains are involved in electron transfer within the complex. Complex IV contains a cytochrome a/a3-domain that transfers electrons and catalyzes the reaction of oxygen to water.
Photosystem II, the first protein complex in the light-dependent reactions of oxygenic photosynthesis, contains a cytochrome b subunit. Cyclooxygenase 2, an enzyme involved in inflammation, is a cytochrome b protein. In the early 1960s, a linear evolution of cytochromes was suggested E. Margoliash which led to the molecular clock hypothesis; the constant evolution rate of cytochromes can be a helpful tool in trying to determine when various organisms may have diverged from a common ancestor. Several kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, reduction potential. Four types of cytochromes are distinguished by their prosthetic groups: There is no "cytochrome e," but cytochrome f, found in the cytochrome b6f complex of plants is a c-type cytochrome. In mitochondria and chloroplasts, these cytochromes are combined in electron transport and related metabolic pathways: A distinct family of cytochromes is the cytochrome P450 family, so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced and complexed to carbon monoxide.
These enzymes are involved in steroidogenesis and detoxification. Scripps Database of Metalloproteins Cytochromes at the US National Library of Medicine Medical Subject Headings
A photosensitizer is a molecule that produces a chemical change in another molecule in a photochemical process. Photosensitizers are used in polymer chemistry in reactions such as photopolymerization, photocrosslinking, photodegradation. Photosensitizers are used to generate triplet excited states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Photosensitizers act by absorbing ultraviolet or visible region of electromagnetic radiation and transferring it to adjacent molecules. Photosensitizers have large de-localized π systems, which lower the energy of HOMO orbitals and its absorption of light might be able to ionize the molecule. There are examples of using semiconductor quantum dots as photosensitizers. Chlorophyll acts as a photosensitizer during the photosynthesis of carbohydrates in plants: 6CO2 + 6H2O → C6H12O6 + 6O2 Photosensitisers are a key part of Photodynamic therapy, used to treat some cancers, they help to produce singlet oxygen to damage tumours.
They can be divided into porphyrins and dyes - See Photodynamic_therapy#Photosensitizers In February 2019, medical scientists announced that iridium attached to albumin, creating a photosensitized molecule, can penetrate cancer cells and, after being irradiated with light, destroy the cancer cells. Artificial photosynthesis Photodynamic therapy Photosensitivity Phototoxicity
Hemoglobin or haemoglobin, abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates as well as the tissues of some invertebrates. Haemoglobin in the blood carries oxygen from the gills to the rest of the body. There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 16 grams of haemoglobin in every 100 ml of blood. In mammals, the protein makes up about 96% of the red blood cells' dry content, around 35% of the total content. Haemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind up to four oxygen molecules. Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide as carbaminohemoglobin, in which CO2 is bound to the heme protein.
The molecule carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen. Haemoglobin is found outside red blood cells and their progenitor lines. Other cells that contain haemoglobin include the A9 dopaminergic neurons in the substantia nigra, alveolar cells, retinal pigment epithelium, mesangial cells in the kidney, endometrial cells, cervical cells and vaginal epithelial cells. In these tissues, haemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism. Haemoglobin and haemoglobin-like molecules are found in many invertebrates and plants. In these organisms, haemoglobins may carry oxygen, or they may act to transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghaemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen-fixing nodules of leguminous plants, before the oxygen can poison the system.
In 1825 J. F. Engelhard discovered that the ratio of iron to protein is identical in the hemoglobins of several species. From the known atomic mass of iron he calculated the molecular mass of hemoglobin to n × 16000, the first determination of a protein's molecular mass; this "hasty conclusion" drew a lot of ridicule at the time from scientists who could not believe that any molecule could be that big. Gilbert Smithson Adair confirmed Engelhard's results in 1925 by measuring the osmotic pressure of hemoglobin solutions; the oxygen-carrying property of hemoglobin was discovered by Hünefeld in 1840. In 1851, German physiologist Otto Funke published a series of articles in which he described growing hemoglobin crystals by successively diluting red blood cells with a solvent such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the resulting protein solution. Hemoglobin's reversible oxygenation was described a few years by Felix Hoppe-Seyler. In 1959, Max Perutz determined the molecular structure of hemoglobin by X-ray crystallography.
This work resulted in his sharing with John Kendrew the 1962 Nobel Prize in Chemistry for their studies of the structures of globular proteins. The role of hemoglobin in the blood was elucidated by French physiologist Claude Bernard; the name hemoglobin is derived from the words heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme group. Each heme group contains one iron atom, that can bind one oxygen molecule through ion-induced dipole forces; the most common type of hemoglobin in mammals contains four such subunits. Hemoglobin consists of protein subunits, these proteins, in turn, are folded chains of a large number of different amino acids called polypeptides; the amino acid sequence of any polypeptide created by a cell is in turn determined by the stretches of DNA called genes. In all proteins, it is the amino acid sequence that determines the protein's chemical properties and function. There is more than one hemoglobin gene: in humans, hemoglobin A is coded for by the genes, HBA1, HBA2, HBB.
The amino acid sequences of the globin proteins in hemoglobins differ between species. These differences grow with evolutionary distance between species. For example, the most common hemoglobin sequences in humans and chimpanzees are nearly identical, differing by only one amino acid in both the alpha and the beta globin protein chains; these differences grow larger between less related species. Within a species, different variants of hemoglobin always exist, although one sequence is a "most common" one in each species. Mutations in the genes for the hemoglobin protein in a species result in hemoglobin variants. Many of these mutant forms of hemoglobin cause no disease; some of these mutant forms of hemoglobin, cause a group of hereditary diseases termed the hemoglobinopathies. The best known hemoglobinopathy is sickle-cell disease, the first human disease whose mechanism was understood at the molecular level. A separate set of diseases called thalassemias involves underproduction of normal and sometimes abnormal hemoglobins, through problems and mutations in globin gene regulation.
All these diseases produce anemia. Variations in hemoglobin amino acid sequences, as with other proteins, may be adaptive. For example, hemoglobin has been found to adapt in different ways to
Molecular scale electronics
Molecular scale electronics called single-molecule electronics, is a branch of nanotechnology that uses single molecules, or nanoscale collections of single molecules, as electronic components. Because single molecules constitute the smallest stable structures imaginable, this miniaturization is the ultimate goal for shrinking electrical circuits; the field is termed as "molecular electronics", but this term is used to refer to the distantly related field of conductive polymers and organic electronics, which uses the properties of molecules to affect the bulk properties of a material. A nomenclature distinction has been suggested so that molecular materials for electronics refers to this latter field of bulk applications, while molecular scale electronics refers to the nanoscale single-molecule applications treated here. Conventional electronics have traditionally been made from bulk materials. Since their invention in 1958, the performance and complexity of integrated circuits has undergone exponential growth, a trend named Moore’s law, as feature sizes of the embedded components have shrunk accordingly.
As the structures shrink, the sensitivity to deviations increases. In a few technology generations, when the minimum feature sizes reaches 13 nm, the composition of the devices must be controlled to a precision of a few atoms for the devices to work. With bulk methods growing demanding and costly as they near inherent limits, the idea was born that the components could instead be built up atom by atom in a chemistry lab versus carving them out of bulk material; this is the idea behind molecular electronics, with the ultimate miniaturization being components contained in single molecules. In single-molecule electronics, the bulk material is replaced by single molecules. Instead of forming structures by removing or applying material after a pattern scaffold, the atoms are put together in a chemistry lab. In this way, billions of billions of copies are made while the composition of molecules are controlled down to the last atom; the molecules used have properties that resemble traditional electronic components such as a wire, transistor or rectifier.
Single-molecule electronics is an emerging field, entire electronic circuits consisting of molecular sized compounds are still far from being realized. However, the unceasing demand for more computing power, along with the inherent limits of lithographic methods as of 2016, make the transition seem unavoidable; the focus is on discovering molecules with interesting properties and on finding ways to obtain reliable and reproducible contacts between the molecular components and the bulk material of the electrodes. Molecular electronics operates in the quantum realm of distances less than 100 nanometers; the miniaturization down to single molecules brings the scale down to a regime where quantum mechanics effects are important. In conventional electronic components, electrons can be filled in or drawn out more or less like a continuous flow of electric charge. In contrast, in molecular electronics the transfer of one electron alters the system significantly. For example, when an electron has been transferred from a source electrode to a molecule, the molecule gets charged up, which makes it far harder for the next electron to transfer.
The significant amount of energy due to charging must be accounted for when making calculations about the electronic properties of the setup, is sensitive to distances to conducting surfaces nearby. The theory of single-molecule devices is interesting since the system under consideration is an open quantum system in nonequilibrium. In the low bias voltage regime, the nonequilibrium nature of the molecular junction can be ignored, the current-voltage traits of the device can be calculated using the equilibrium electronic structure of the system. However, in stronger bias regimes a more sophisticated treatment is required, as there is no longer a variational principle. In the elastic tunneling case, the formalism of Rolf Landauer can be used to calculate the transmission through the system as a function of bias voltage, hence the current. In inelastic tunneling, an elegant formalism based on the non-equilibrium Green's functions of Leo Kadanoff and Gordon Baym, independently by Leonid Keldysh was advanced by Ned Wingreen and Yigal Meir.
This Meir-Wingreen formulation has been used to great success in the molecular electronics community to examine the more difficult and interesting cases where the transient electron exchanges energy with the molecular system. Further, connecting single molecules reliably to a larger scale circuit has proven a great challenge, constitutes a significant hindrance to commercialization. Common for molecules used in molecular electronics is that the structures contain many alternating double and single bonds; this is done because such patterns delocalize the molecular orbitals, making it possible for electrons to move over the conjugated area. The sole purpose of molecular wires is to electrically connect different parts of a molecular electrical circuit; as the assembly of these and their connection to a macroscopic circuit is still not mastered, the focus of research in single-molecule electronics is on the functionalized molecules: molecular wires are characterized by containing no functional groups and are hence composed of plain repetitions of a conjugated building block.
Among these are the carbon nanotubes that are quite large compared to
The density, or more the volumetric mass density, of a substance is its mass per unit volume. The symbol most used for density is ρ, although the Latin letter D can be used. Mathematically, density is defined as mass divided by volume: ρ = m V where ρ is the density, m is the mass, V is the volume. In some cases, density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials have different densities, density may be relevant to buoyancy and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. To simplify comparisons of density across different systems of units, it is sometimes replaced by the dimensionless quantity "relative density" or "specific gravity", i.e. the ratio of the density of the material to that of a standard material water.
Thus a relative density less than one means. The density of a material varies with pressure; this variation is small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object and thus increases its density. Increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a fluid results in convection of the heat from the bottom to the top, due to the decrease in the density of the heated fluid; this causes it to rise relative to more dense unheated material. The reciprocal of the density of a substance is called its specific volume, a term sometimes used in thermodynamics. Density is an intensive property in that increasing the amount of a substance does not increase its density. In a well-known but apocryphal tale, Archimedes was given the task of determining whether King Hiero's goldsmith was embezzling gold during the manufacture of a golden wreath dedicated to the gods and replacing it with another, cheaper alloy.
Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated and compared with the mass. Baffled, Archimedes is said to have taken an immersion bath and observed from the rise of the water upon entering that he could calculate the volume of the gold wreath through the displacement of the water. Upon this discovery, he leapt from his bath and ran naked through the streets shouting, "Eureka! Eureka!". As a result, the term "eureka" entered common parlance and is used today to indicate a moment of enlightenment; the story first appeared in written form in Vitruvius' books of architecture, two centuries after it took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time. From the equation for density, mass density has units of mass divided by volume; as there are many units of mass and volume covering many different magnitudes there are a large number of units for mass density in use.
The SI unit of kilogram per cubic metre and the cgs unit of gram per cubic centimetre are the most used units for density. One g/cm3 is equal to one thousand kg/m3. One cubic centimetre is equal to one millilitre. In industry, other larger or smaller units of mass and or volume are more practical and US customary units may be used. See below for a list of some of the most common units of density. A number of techniques as well as standards exist for the measurement of density of materials; such techniques include the use of a hydrometer, Hydrostatic balance, immersed body method, air comparison pycnometer, oscillating densitometer, as well as pour and tap. However, each individual method or technique measures different types of density, therefore it is necessary to have an understanding of the type of density being measured as well as the type of material in question; the density at all points of a homogeneous object equals its total mass divided by its total volume. The mass is measured with a scale or balance.
To determine the density of a liquid or a gas, a hydrometer, a dasymeter or a Coriolis flow meter may be used, respectively. Hydrostatic weighing uses the displacement of water due to a submerged object to determine the density of the object. If the body is not homogeneous its density varies between different regions of the object. In that case the density around any given location is determined by calculating the density of a small volume around that location. In the limit of an infinitesimal volume the density of an inhomogeneous object at a point becomes: ρ = d m / d V, where d V is an elementary volume at position r; the mass of the body t
European Chemicals Agency
The European Chemicals Agency is an agency of the European Union which manages the technical and administrative aspects of the implementation of the European Union regulation called Registration, Evaluation and Restriction of Chemicals. ECHA is the driving force among regulatory authorities in implementing the EU's chemicals legislation. ECHA helps companies to comply with the legislation, advances the safe use of chemicals, provides information on chemicals and addresses chemicals of concern, it is located in Finland. The agency headed by Executive Director Bjorn Hansen, started working on 1 June 2007; the REACH Regulation requires companies to provide information on the hazards and safe use of chemical substances that they manufacture or import. Companies register this information with ECHA and it is freely available on their website. So far, thousands of the most hazardous and the most used substances have been registered; the information is technical but gives detail on the impact of each chemical on people and the environment.
This gives European consumers the right to ask retailers whether the goods they buy contain dangerous substances. The Classification and Packaging Regulation introduces a globally harmonised system for classifying and labelling chemicals into the EU; this worldwide system makes it easier for workers and consumers to know the effects of chemicals and how to use products safely because the labels on products are now the same throughout the world. Companies need to notify ECHA of the labelling of their chemicals. So far, ECHA has received over 5 million notifications for more than 100 000 substances; the information is available on their website. Consumers can check chemicals in the products. Biocidal products include, for example, insect disinfectants used in hospitals; the Biocidal Products Regulation ensures that there is enough information about these products so that consumers can use them safely. ECHA is responsible for implementing the regulation; the law on Prior Informed Consent sets guidelines for the import of hazardous chemicals.
Through this mechanism, countries due to receive hazardous chemicals are informed in advance and have the possibility of rejecting their import. Substances that may have serious effects on human health and the environment are identified as Substances of Very High Concern 1; these are substances which cause cancer, mutation or are toxic to reproduction as well as substances which persist in the body or the environment and do not break down. Other substances considered. Companies manufacturing or importing articles containing these substances in a concentration above 0,1% weight of the article, have legal obligations, they are required to inform users about the presence of the substance and therefore how to use it safely. Consumers have the right to ask the retailer whether these substances are present in the products they buy. Once a substance has been identified in the EU as being of high concern, it will be added to a list; this list is available on ECHA's website and shows consumers and industry which chemicals are identified as SVHCs.
Substances placed on the Candidate List can move to another list. This means that, after a given date, companies will not be allowed to place the substance on the market or to use it, unless they have been given prior authorisation to do so by ECHA. One of the main aims of this listing process is to phase out SVHCs where possible. In its 2018 substance evaluation progress report, ECHA said chemical companies failed to provide “important safety information” in nearly three quarters of cases checked that year. "The numbers show a similar picture to previous years" the report said. The agency noted that member states need to develop risk management measures to control unsafe commercial use of chemicals in 71% of the substances checked. Executive Director Bjorn Hansen called non-compliance with REACH a "worry". Industry group CEFIC acknowledged the problem; the European Environmental Bureau called for faster enforcement to minimise chemical exposure. European Chemicals Bureau Official website