Heterofunctional carboxylic acids.
1. Halogenoacids
Halogenoacids are the derivatives of carboxyl acids that contain halogen radical (1 or more).
β α
α-bromopropanoic acid
2-bromopropanoic acid
Methods of extraction:
1. Halogenation of saturated carboxylic acids:
2. Hydrohalogenation of unsaturated carboxylic acids:
3. Halogenation of aromatic carboxylic acids:
Chemical properties: in the molecule of halogenoacids either carboxyl group or halogen radical can react.
Carboxyl group can react forming:
a) salts
b) complex ethers:
c) amides:
Halogen radical can react with:
1. ammonium:
2. NaOH (water solution)
a) for α-halogenoacids
b) for β-halogenoacids
c) for γ,σ-halogenoacids
Representatives:
1. Monochloroacetic acid
2. Dichloroacetic acid
3. Trichloroacetic acid
2. Hydroxyacids
Hydroxyacids are the derivatives of carboxyl acids that contain –OH group (1 or more).
β α
2-hydroxypropionic acid
α-hydroxypropionic acid
Methods of extraction:
1. Hydrolysis of α-halogenoacids
2. Oxidations of diols and hydroyaldehydes
3. Hydration of α,β-unsaturated carboxylic acids
Chemical properties: in the molecule of hydroxyacids either –OH group or carboxyl group can react.
Carboxyl group can react forming:
a) salts
b) complex ethers:
c) amides:
–OH group can react with:
1. hydrohalogens (HCl, HBr, HI, HF)
2. can oxidize
Representatives:
1. Milk acid . Milk acid is a trivial name because at first it was extracted from milk. It is present in kefir, yogurt, sour milk and other milk products. It can form in muscles during hard and prolonged work. That is why peoples can feel ache in their muscles after physical training. Salts of milk acid are used in medicine.
2. Apple acid . It is present in green apples and some berries. It takes part in biological processes in human organisms and organisms of other alive creatures. In industry it is used for manufacturing of wine, fruit waters and sweets. It is used in medicine for synthesis of some medical preparations.
3. Tartaric acid . It is present in grape. It is used in medicine for synthesis of some medical preparations.
4. Citric acid . It is present in orange, lemon and other citric fruits. It takes part in biological processes in human organism.
3. Oxyacids
Oxyacids – are the derivatives of carboxylic acids that contain carbonyl group.
2-oxyethanoic acid
a-oxyethanoic acid
Methods of extraction:
1. Oxidation of hydroxyacids:
Derivates of carbonate acid. Sulfoacids. Tasks of synthesis and analysis of organic compounds.
Carbonic acid (ancient name acid of air or aerial acid) has the formula H2CO3. It is also a name sometimes given to solutions of carbon dioxide in water, which contain small amounts of H2CO3. The salts of carbonic acids are called bicarbonates (or hydrogen carbonates) and carbonates. It is a weak acid. Carbonic acid should never be confused with carbolic acid, an antiquated name for phenol.
Carbon dioxide dissolved in water is in equilibrium with carbonic acid:
CO2 + H2O ⇌ H2CO3
The hydration equilibrium constant at 25°C is Kh= 1.70×10−3: hence, the majority of the carbon dioxide is not converted into carbonic acid and stays as CO2molecules. In the absence of a catalyst, the equilibrium is reached quite slowly. The rate constants are 0.039 s−1 for the forward reaction (CO2 + H2O → H2CO3) and 23 s−1 for the reverse reaction (H2CO3 → CO2 + H2O). Carbonic acid is used in the making of soft drinks, inexpensive and artificially carbonated sparkling wines, and other bubbly drinks.
Role of carbonic acid in blood
Carbonic acid is an intermediate step in the transport of CO2 out of the body via respiratory gas exchange. The hydration reaction of CO2 is generally very slow in the absence of a catalyst, but red blood cells contain carbonic anhydrase which both increases the reaction rate and disassociates a hydrogen ion (H+) from the resulting carbonic acid, leaving bicarbonate (HCO3–) dissolved in the blood plasma. This catalysed reaction is reversed in the lungs, where it converts the bicarbonate back into CO2 and allows it to be expelled. Carbonic acid also plays a very important role as a buffer in mammalian blood. The equilibrium between carbon dioxide and carbonic acid is very important for controlling the acidity of body fluids, and the carbonic anhydrase increases the reaction rate by a factor of nearly a billion to keep the fluids at a stable pH.
Role of carbonic acid in anthropogenic climate change
The oceans of the world have absorbed almost half of the CO2 emitted by humans from the burning of fossil fuels. The extra dissolved carbon dioxide has caused the ocean’s average surface pH to shift by about 0.1 unit from pre-industrial levels. This process is known as ocean acidification. Depending on the rate and magnitude of future emissions, the ocean’s pH could drop by as much as 0.35 units by the mid-21st century.
Amino acids, peptides
Next to water, proteins are the most abundant substances in most cells – from 10% to 20% of the cell’s mass. All proteins contain the elements carbon, hydrogen, oxygen, and nitrogen; most also contain sulfur. The presence of nitrogen in proteins sets them apart from carbohydrates and lipids, which generally do not contaiitrogen. The average nitrogen content of proteins is 15.4% by mass. Other elements, such as phosphorus and iron, are essential constituents of certain specialized proteins.
Casein, the main protein of milk, contains phosphorus, an element very important in the diet of infants and children. Hemoglobin, the oxygen-transporting protein of blood, contains iron.
Amino acids – the building blocks for proteins. The word protein comes from the Greek proteios, which means “of first importance.” This derivation alludes to the key role that proteins play in life processes.
А protein is in polymer in which the monomer units are amino acids. Thus the starting point for а discussion of proteins is an understanding of the structures and chemical properties of amino acids.
An amino acid is an organic compound that contains both an amino (–NН3) group and a carboxyl (-СООН) group. The amino acids found in proteins are always α-amino acids – that is, amino acids in which the amino group is attached to the α-carbon atom of the carboxylic acid carbon chain. The general structural formula for an α-amino acid is:
The R group present in an α-amino acid is called the amino acid side chain. The nature of this side chain distinguishes а-amino acids from each other. Side chains vary in size, shape, charge, acidity, functional groups present, hydrogen-bonding ability, and chemical reactivity.
Over 700 different naturally occurring amino acids are known, but only 20 of them, called standard amino acids, are normally present in proteins. А standard amino acid is one of the 20 α-amino acids normally found in proteins. Amino acids are grouped according to side-chain polarity. In this system there are four categories: (1) nonpolar amino acids, (2) polar neutral amino acids, (3) polar acidic amino acids, and (4) polar basic amino acids. This classification system gives insights into how various types of amino acid side chains help determine the properties of proteins.
Nonpolar amino acids contain one amino group, one carboxyl group, and a nonpolar side chain. When incorporated into а protein, such amino acids are hydrophobic (“water fearing”); that is, they are not attracted to water molecules. They are generally found in the interior of proteins, where there is limited contact with water. There are eight nonpolar amino acids.
The three types of polar amino acids have varying degrees of affinity for water. Within а protein, such amino acids are said to be hydrophilic (“water-loving”). Hydrophilic amino acids are often found on the surfaces of proteins.
Polar neutral amino acids contain one amino group, one carboxyl group, and а side chain that is polar but neutral. The side chain is neutral in that it is neither acidic nor basic in solution at physiological pH. There are seven polar neutral amino acids.
Polar acidic amino acids contain one amino group and two carboxyl groups, the second carboxyl group being part of the side chain. In solution at physiological рН, the side chain of а polar acidic amino acid bears а negative charge; the side-chain carboxyl group has lost its acidic hydrogen atom. There are two polar acidic amino acids: aspartic acid and glutamic acid.
Polar basic amino acids contain one amino groups and one carboxyl group, the second amino group being part of the side chain. In solution at physiological рН, the side chain of а polar basic amino acid bears а positive charge; the nitrogen atom of the amino group has accepted а proton. There are three polar basic amino acids: lysine, arginine, and histidine.
Chirality and amino acids. Four different groups are attached to the α-carbon atom in all of the standard amino acids except glycine, where the R group is а hydrogen atom.
This means that the structures of 19 of the 20 standard amino acids possess а chiral center at this location, so enantiomeric forms (left- and right-handed forms) exist for each of these amino acids.
With few exceptions (in some bacteria), the amino acids found iature and in proteins are isomers. Thus, as is the case with monosaccharides, nature favors one mirror-image form over the other. Interestingly, for amino acids the L isomer is the preferred form, whereas for monosaccharides theisomer is preferred.
The rules for drawing Fischer projections for amino acid structures follow.
1. The – СООН group is put at the top of the projection, the R group at the bottom. This positions the carbon chain vertically.
2. The – NН2 group is in а horizontal position. Positioning it on the left denotes the L isomer, and positioning it on the right denotes the D isomer.
Acid – base properties of amino acids. In pure form, amino acids are white crystalline solids with relatively high decomposition points. (Most amino acids decompose before they melt.) Also most amino acids are not very soluble in water because of strong intermolecular forces within their crystal structures. Such properties are those often exhibited by compounds in which charged species are present. Studies of amino acids confirm that they are charged species both in the solid state and in solution.
Both an acidic group (-СООН) and а basic group (-NН2) are present on the same carbon in an α-amino acid.
We learned that ieutral solution, carboxyl groups have а tendency to lose protons (Н+), producing а negatively charged species:
–СООН = – СОО– + Н+
We learned that ieutral solution, amino groups have а tendency to accept protons (Н+), producing а positively charged species:
–NH2 + H+ == –NH3+
As is consistent with the behavior of these groups, ieutral solution, the –СООН group of an amino acid donates а proton to the –NH2 of the same amino acid. We can characterize this behavior as an internal acid — base reaction. The net result is that ieutral solution, amino acid molecules have the structure
Such а molecule is known as а zwitterion, from the German term meaning “double ion”. А zwitterion is а molecule that has а positive charge on one atom and а negative charge on another atom. Note that the net charge on а zwitterion is zero even though parts of the molecule carry charges. In solution and also in the solid state, α-amino acids are zwitterions.
Zwitterion structure changes when the pH of а solution containing an amino acid is changed from neutral either to acidic (low pH) by adding an acid such as НС1 or to basic (high pH) by adding а base such as NaOH. In an acidic solution, the zwitterion accepts а proton (Н+) to form а positively charged ion.
In basic solution, the –NH3+ of the zwitterion loses а proton, and а negatively charged species is formed.
Thus, in solution, three different amino acid forms can exist (zwitterion, negative ion, and positive ion). The three species are actually in equilibrium with each other, and the equilibrium shifts with pH change. The overall equilibrium process can be represented as follows:
In acidic solution, the positively charged species on the left predominates; nearly neutral solutions have the middle species (the zwitterion) as the dominant species; in basic solution, the negatively charged species on the right predominates.
The previous discussion assumed that the side chain (R group) of an amino acid remains unchanged in solution as the pH is varied. This is the case for neutral amino acids but not for acidic or basic ones. For these latter compounds, the side chain can also acquire а charge, because it contains an amino or а carboxyl group that can, respectively, gain or lose а proton.
Because of the extra site that can be protonated or deprotonated, acidic and basic amino acids have four charged forms in solution.
The existence of two low-pH forms for aspartic acid results from the two carboxyl groups being deprotonated at different pH values. For basic amino acids, two high-pH forms exist because deprotonation of the amino groups does not occur simultaneously. The side-chain amino group deprotonates before the α-amino group.
The isoelectric point for an amino acid is the pH at which the total charge on the amino acid is zero. Every amino acid has а different isoelectric point. Fifteen of the 20 amino acids, those with nonpolar or polar neutral side chains, have isoelectric points in the range of 4.8 – 6.3. The three basic amino acids have higher isoelectric points (His = 7.59, Lys = 9.74, Arg = 10.76), and the two acidic amino acids have lower ones (Asp = 2.77, Glu = 3.22).
Reaction of amino acids.
i) Reaction with alcohols – esters formation:
ii) Reaction with ammonia – amides formation. The amides of aspartic and glutamic acid acids, asparagine and glutamine, play important role in the transport of ammonia in the body.
iii) Decarboxylation. Amino acids may be decarboxylated by heat, acids, bases or specific enzymes to the primary amines:
Some of the decarboxylation reaction are of great importance in the body, decarboxylation of histidine to histamine:
In the presence of foreign protein introduced into the body, very large quantities of histamine are produced in the body and allergic reactions become evident. In extreme cases shock may result. The physiological effects of histamine may be neutralized or minimized by the use of chemical compounds known as antihistamines.
iv) Salts are formed. All amino acids can react with some inorganic acids and bases and form two kind sold:
v) Deamination:
1. oxidation deamination – important pathway for the biodegradation of α-amino acids:
2. hydrolitic deamination – reaction with nitrous acid. Amino acids react with nitrous acid to give hydroxy acid along with the evolution of nitrogen.
The nitrogen can be collected and measured. Thus this reaction constitutes one of the methods for the estimation of amino acids.
3. intramolecular deamination – unsaturated acids are formed
4. redaction deamination – saturated carboxylic acid formation:
vi) Peptide formation. Two amino acids can react in а similar way – the carboxyl group of one amino acid reacts with the amino group of the other amino acid. The products are а molecule of water and а molecule containing the two amino acids linked by an amide bond.
Removal of the elements of water from the reacting carboxyl and amino groups and the ensuing formation of the amide bond are better visualized when expanded structural formulas for the reacting groups are used.
In amino acid chemistry, amide bonds that link amino acids together are given the specific name of peptide bond. А peptide bond is а bond between the carboxyl group of one amino acid and the amino group of another amino acid.
Under proper conditions, many amino acids can bond together to give chains of amino acids containing numerous peptide bonds. For example, four peptide bonds are present in а chain of five amino acids.
Short to medium-sized chains of amino acids are known as peptides. А peptide is а sequence of amino acids, of up to 50 units, in which the amino acids are joined together through amide (peptide) bonds. А compound containing two amino acids joined by а peptide bond is specifically called а dipeptide; three amino acids in а chain constitute а tripeptide; and so on. The name oligopeptide is loosely used to refer to peptides with 10 to 20 amino acid residues and polypeptide to larger peptides.
In all peptides, the amino acid at one end of the amino acid sequence has а free H3N+ group, and the amino acid at the other end of the sequence has а free СОО–group. The end with the free H3N+ group is called the N-terminal end, and the end with the free СОО– group is called the С-terminal end. By convention, the sequence of amino acids in а peptide is written with the N-trminal end amino acid at the left. The individual amino acids within а peptide chain are called amino acid residues.
Proteins are polypeptides that contain more than 50 amino acid units. The dividing line between а polypeptide and а protein is arbitrary. The important point is that proteins are polymers containing а large number of amino acid units linked by peptide bonds. Polypeptides are shorter chains of amino acids. Some proteins have molecular masses in the millions. Some proteins also contain more than one polypeptide chain.
To aid us in describing protein structure, we will consider four levels of substructure: primary, secondary, tertiary, and quaternary. Even though we consider these structure levels one by one, remember that it is the combination of all four levels of structure that controls protein function.
The primary structure of а protein is the sequence of amino acids present in its peptide chain or chains. Knowledge of primary structure tells us which amino acids are present, the number of each, their sequence, and the length and number of polypeptide chains.
The first protein whose primary structure was determined was insulin, the hormone that regulates blood-glucose level; а deficiency of insulin leads to diabetes. The sequencing of insulin, which took over 8 years, was completed in 1953. Today, thousands of proteins have been sequenced; that is, researchers have determined the order of amino acids within the polypeptide chain or chains.
The secondary structure of а protein is the arrangement in space of the atoms in the backbone of the protein. Three major types of protein secondary structure are known; the alpha helix, the beta pleated sheet, and the triple helix. The major force responsible for all three types of secondary structure is hydrogen bonding between а carbonyl oxygen atom of а peptide linkage and the hydrogen atom of an amino group (-NH) of another peptide linkage farther along the backbone. This hydrogen-bonding interaction may be diagrammed as follows:
The Alpha Helix The alpha helix (α-helix) structure resembles а coiled helical spring, with the coil configuration maintained by hydrogen bonds between N – Н and С= О groups of every fourth amino acid, as is shown diagrammatically in Figure.
The beta pleated sheet (β-pleated sheet) secondary structure involves amino acid chains that are almost completely extended. Hydrogen bonds form between two different side-by-side protein chains (interchain bonds) as shown in Figure.
The tertiary structure of а protein is the overall three-dimensional shape that results from the attractive forces between amino acid side chains (R groups) that are widely separated from each other within the chain.
А good analogy for the relationships among the primary, secondary, and tertiary structures of а protein is that of а telephone cord. The primary structure is the long, straight cord. The coiling of the cord into а helical arrangement gives the secondary structure. The supercoiling arrangement the cord adopts after you hang up the receiver is the tertiary structure.
Interactions responsible for tertiary structure. Four types of attractive interactions contribute to the tertiary structure of а protein:
(1) covalent disulfide bonds,
(2) electrostatic attractions (salt bridges),
(3) hydrogen bonds,
(4) hydrophobic attractions.
Quaternary structure is the highest level of protein organization. It is found only in proteins that have structures involving two or more polypeptide chains that are independent of each other — that is, are not covalently bonded to each other. These multichain proteins are often called oligomeric proteins. The quaternary structure of а protein involves the associations among the separate chains in an oligomeric protein.
Simple and Conjugated Proteins
Proteins are classified as either simple proteins or conjugated proteins. А simple protein is made up entirely of amino acid residues. More than one polypeptide chain may be present, but all chains contain only amino acids. А conjugated protein has other chemical components in addition to amino acids. These additional components, which may be organic or inorganic, are called prosthetic groups. А prosthetic group is а non-amino acid unit permanently associated with а protein.
Conjugated proteins may be further classified according to the nature of the prosthetic group. For example, proteins containing lipids, those containing carbohydrates, and those containing metal ions are called lipoproteins, glycoproteins, and metalloproteins, respectively.
