Name
: Tiara
Student
Number : RSA1C110003
1. Explain the triterpenoid biosynthetic pathway, identify important factors that determine the quantities produced of many triterpenoids.
There are at least 4000
known triterpenes. They are derived from mevalonic acid. Triterpenes are
precursors to steroids in both plants and animals. Many triterpenes occur free,
but others occur as glycosides (saponins) or in special combined forms.
Triterpenes
are derived from a triterpene squalene that is formed by condensation of two
units of farnesyl pyrophosphate. Squalene is then converted to 2,3-squalene
epoxide and cyclized by two different pathways to make most triterpenes.
squalene
----> 2,3-squalene epoxide ----> cyclization
Terpenoids
are built up from C5 units, isopentenyl diphosphate (IPP). IPP is supplied from
the cytosolic mevalonic acid (MVA) pathway and the plastidal methylerythritol
phosphate (MEP) pathway. Triterpenoids and sesquiterpenoids are biosynthesized
via the MVA pathway. The first diversifying step in triterpenoid biosynthesis
is the cyclization of 2,3-oxidosqualene catalyzed by oxidosqualene cyclase
(OSC). After the cyclization of 2,3-oxidosqualene catalyzed by OSC, a
triterpenoid undergoes various modifications including P450-catalyzed oxidation
and UGT-catalyzed glycosylation. Blue arrows, OSC-catalyzed steps; red arrows,
P450-catalyzed steps; green arrows, additional modifications including
UGT-catalyzed steps.
some important
factors are:
·
squalene
synthetase (enzyme, , carries out all the steps from FPP to squalene and also
there are two catalytic sites involved.
·
oxidosqualene cyclase as a catalyse in cyclization
The two most important types of
triterpenes are derived by orientation of the precursor squalene epoxide in two
distinct arrangements followed by condensation. The other main type leads to
pentacyclic triterpenes. Pentacyclic triterpenes are produced by arrangement of
squalene epoxide in a chair-chair-chair-boat arrangement followed by
condensation. These compounds are also extremely common and are found in most
plants. Common representatives of this group are α-amyrin, β-amyrin, friedelin,
oleanolic acid and taraxerane. (see the picture above)
The other image
for more understand
2. Describe the structure determination of flavonoids, specificity and intensity
of absorption signal by using
IR and NMR
spectra. Give the
example, at least two different structures.
Determining
the Structure of an Organic Compound
- The analysis of the outcome of a reaction requires that we know the full structure of the products as well as the reactants
- In the 19th and early 20th centuries, structures were determined by synthesis and chemical degradation that related compounds to each other
- Physical methods now permit structures to be determined directly. We will examine:
- mass spectrometry (MS)
- infrared (IR)
spectroscopy
- nuclear magnetic
resonance spectroscopy (NMR)
- ultraviolet-visible
spectroscopy (VIS)
IR Spectroscopy
An important
tool of the organic chemist is Infrared Spectroscopy, or "IR".
IR spectra are acquired on a special instrument, called an IR spectrometer. IR
is used to gather information about compound's structure, assess its purity,
and sometimes to identify it.
Infrared
radiation is that part of the electromagnetic spectrum between the visible and
microwave regions. Infrared radiation is absorbed by organic molecules and
converted into energy of molecular vibration, either stretching or bending.
Different types of bonds, and thus different functional groups, absorb infrared
radiation of different wavelengths. A IR spectrum is a plot of wavenumber
(X-axis) vs percent transmittance (Y-axis). (Note: wavelength can be used
instead of wavenumber and absorbance instead of percent transmittance)
- IR region lower energy than visible light (< red – produces heating as with a heat lamp)
- 2.5 ´ 10-6 m to 2.5 ´ 10-5 m region used by organic chemists for structural analysis
- IR energy in a spectrum is usually measured as wavenumber (cm-1), the inverse of wavelength and proportional to frequency
- Specific IR absorbed by organic molecule related to its structure
- IR energy absorption corresponds to atomic movements, such as vibrations and rotations from bending and stretching of bonds between groups of atoms
- Energy is characteristic of the bonding of atoms in a functional group
- Bond stretching dominates higher energy modes
- Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H
Infrared Spectra: Functional Groups
Characteristic higher energy IR absorptions used to
confirm the existence of the presence of functional groups
NMR Spectroscopy
Nuclear magnetic resonance spectroscopy,
most commonly known as NMR spectroscopy,
is a research technique that exploits the magnetic properties of certain atomic
nuclei to determine physical and chemical properties of atoms or the molecules
in which they are contained. It relies on the phenomenon of nuclear magnetic
resonance and can provide detailed information about the structure, dynamics,
reaction state, and chemical environment of molecules.
Chemical shift : The location of different nmr
resonance signals is dependent on both the external magnetic field strength and
the rf frequency. Since no two magnets will have exactly the same field,
resonance frequencies will vary accordingly and an alternative method for
characterizing and specifying the location of nmr signals is needed.
Signal Strength :
The
magnitude or intensity of nmr resonance signals is displayed along the vertical
axis of a spectrum, and is proportional to the molar concentration of the
sample. Thus, a small or dilute sample will give a weak signal, and doubling or
tripling the sample concentration increases the signal strength proportionally.
π-Electron Functions : Pi-electrons are more polarizable than
are sigma-bond electrons, as addition reactions of electrophilic reagents to
alkenes testify. Therefore, we should not be surprised to find that field induced
pi-electron movement produces strong secondary fields that perturb nearby
nuclei.
Solvent Effects : Chloroform-d
(CDCl3) is the most common solvent for nmr measurements
Catechin
IR Spectra
The IR spectra of (+)-catechin has a
broad band around 3400-2600 cm-1 region corresponding to aliphatic and aromatic
C-H, phenolic and alcoholic O-H stretching. A band at 1620 cm-1 observed may be
due to aromatic C=C stretching. Other stretchings were comparable with IR
spectra of authentic (+)-catechin.
NMR spectra
Catechin molecule contains 15 carbons.
In the 13C-NMR spectra, signals appeared at δ
27.7, 66.3, 80.9, 93.9, 95.1, 114.5, 115.1, 118.4 due to C-4, C-3, C-2, C-8,
C-2́, C-5́, C-6́
carbons respectively and others aromatic carbons showed peaks at δ 99.1, 130.6, 144.6, 144.8, 155.3, 156.1 and 156.4.
Quercetin
IR spectra
IR spectrum of the
isolated compounds provide information regarding the absorption peak wavenumber
hydroxyl group at 3369 cm-1. Hydroxyl group is an OH stretch-bonded (bonded to hydrogen),
OH bound to look at the wave numbers 3450-3200 cm-1 which form a broad band with
a strong intensity. The existence of the hydroxyl group is also strengthened by
the emergence of a wave-CO-at 2956 cm. showed a stretch - CH aliphatic and reinforced
by the emergence of absorption at 1498-1359 cm, indicating a stalling - CH. The
existence of strain-C = O carbonyl indicated by absorption at wave numbers 1658
cm-1. Wavenumber absorption band at 1606 cm-1 indicate the presence of C = C stretch
Absorption band at wavenumber 1574 cm. Regional absorption at wave number 821 cm-1
indicate the presence of two neighboring H in the aromatic ring.
NMR spectra
3. In isolation of
alkaloids, in initial stages required
acid or base conditions. Explain
the basis of the
use that reagents, and give
examples of at least three kinds of alkaloids.
The use of thet reagent is to liberate
the salt bond into Free Alkaloidal. It has been observed that the alkaloids invariably occur in the plant sources as the salt of
acids, such as: oxalates, tannates etc. Therefore, when the plant substance is
exposed to an alkaline medium, the alkaloidal salts are readily converted to
the corresponding alkaloid bases.
Choice of Alkali Indeed, the choice of a suitable mineral base (alkali)
for the ease of liberation of the alkaloid from the salts is not only very
vital but also equally significant and largely depend on the following factors,
namely:
(a) Natural state of the alkaloids: It has been observed that the salt
of a strongly basic alkaloid with a mineral acid usually tends to
undergo cleavage under the influences of a stronger base. Likewise, the
corresponding salt of a weakly basic alkaloid and a relatively weak
organic acid shall require a rather weaker base for its cleavage.
(b) Chemical characteristics of the alkaloidal base: The usage of
strong alkali e.g., NaOH or KOH should be avoided as far as possible by
virtue of the fact that certain alkaloids undergo hydrolysis on prolonged
contact with a strong base.
Example :
1.
Nicotine
Alkaloid
extracted from the leaves of plants, flowers, fruit, bark, and roots
are dried and then crushed. Alkaloid extracted
with solvents, eg ethanol,
and then evaporated. Then, Extracts were obtained
inorganic acids to produce a quaternary ammonium salt
and then extracted again. Quaternary ammonium
salt obtained was
treated with sodium carbonate
to produce these alkaloids were then extracted with
a solvent-free such as ether
and chloroform. A
mixture of alkaloids obtained
finally separated in various ways,
such as chromatographic methods.
2. Caffeine
Caffeine extraction from tea powder, the
solubility of caffeine in water is 22mg/ml at 25°C, 180mg/ml at 80°C, and 670mg/ml
at 100°C. Here the organic solvent Dichloromethane is used to extract caffeine
from aqueous extract of tea powder because caffeine is more soluble in
dichloromethane (140mg/ml) than it is in water (22mg/ml).The dichloromethane -
caffeine mixture can then be separated on the basis of the different densities
of dichloromethane and water because dichloromethane is much denser than water
and insoluble in it. Residual water is separated from dichloromethane by drain
out the dichloromethane through separating funnel, thus dichloromethane passed
through the funnel while polar solvents such as water is still remains in the
funnel. Water and dichloromethane is slightly soluble in each other. So, after
separating the solvents, residual water will remain the organic layer. Mainly
anhydrous sodium sulfite is used for the removal of water from organic layer.
Anhydrous sodium sulfite is an insoluble inorganic solid which will absorb
water, thus drying it.
3. Morphine
Extract morphine from poppy straw. In the
opium poppy the alkaloids are bound to meconic acid. The method is to extract
from the crushed plant with diluted sulfuric acid, which is a stronger acid
than meconic acid, but not so strong to react with alkaloid molecules. The
extraction is performed in many steps (one amount of crushed plant is at least
six to ten times extracted, so practically every alkaloid goes into the
solution). From the solution obtained at the last extraction step, the
alkaloids are precipitated by either ammonium hydroxide or sodium carbonate.
The last step is purifying and separating morphine from other opium alkaloids.
Opium poppy contains at least 40 different alkaloids, but most of them are of
very low concentration. Morphine is the principal alkaloid in raw opium and
constitutes ~8-19% of opium by dry weight (depending on growing conditions)
4. Describe the relationship
between biosynthesis, methods of isolation and
structural determination of natural
product compounds. Give an example.
The relationship
between biosynthesis, isolation and structural
determination were biosynthesis conducted for the formation of natural products compound
with some specific molecules using basic reaction, after knowing the result of the natural compound
than isolate it to test the component of natural product in
the sample to be tested. Isolation is done by separating some compounds present
in the sample using a solvent to produce a single compound structure to be
determined in several ways. Identification cab be use IR or NMR spectra.
Example :
Nicotine
Nicotine
(C10H14N2) is a naturally occurring liquid
alkaloid. An alkaloid is an organic compound made out of carbon,
hydrogen, nitrogen and sometimes oxygen. These chemicals have potent effects on
the human body.
Nicotine biosynthesis
The biosynthetic pathway of nicotine involves a
coupling reaction between the two cyclic structures that compose nicotine.
Metabolic studies show that the pyridine ring of nicotine is derived from
nicotinic acid while the pyrrolidone is derived from N-methyl-Δ1-pyrrollidium cation. Biosynthesis of the two component
structures proceeds via two independent syntheses, the NAD pathway for
nicotinic acid and the tropane pathway for N-methyl-Δ1-pyrrollidium cation.
The NAD pathway in the genus nicotiana begins
with the oxidation of aspartic acid into α-imino succinate by
aspartate oxidase (AO) (an enzyme that catalyzes the chemical reaction). This
is followed by a condensation with glyceraldehyde-3-phosphate and a cyclization
catalyzed by quinolinate synthase (QS) to give quinolinic acid. Quinolinic acid
then reacts with phosphoriboxyl pyrophosphate catalyzed
by quinolinic acid phosphoribosyl transferase (QPT) to form nicotinic acid
mononucleotide (NaMN). The reaction now proceeds via the NAD salvage cycle to
produce nicotinic acid via the conversion of nicotinamide by the enzyme
nicotinamidase.
The N-methyl-Δ1-pyrrollidium cation used in the synthesis of nicotine is an intermediate
in the synthesis of tropane-derived alkaloids. Biosynthesis begins with decarboxylation
of ornithine by ornithine decarboxylase (ODC) to produce putrescine. Putrescine
is then converted into N-methyl putrescine via methylation by SAM catalyzed by
putrescine N-methyltransferase (PMT). N-methylputrescine then undergoes
deamination into 4-methylaminobutanal by the N-methylputrescine oxidase (MPO)
enzyme, 4-methylaminobutanal then spontaneously cyclize into N-methyl-Δ1-pyrrollidium cation.
The final step
in the synthesis of nicotine is the coupling between N-methyl-Δ1-pyrrollidium cation and nicotinic acid. Although studies conclude some
form of coupling between the two component structures, the definite process and
mechanism remains undetermined. The current agreed theory involves the
conversion of nicotinic acid into 2,5-dihydropyridine through
3,6-dihydronicotinic acid. The 2,5-dihydropyridine intermediate would then
react with N-methyl-Δ1-pyrrollidium cation to
form enantiomerically pure (–)-nicotine
Isolation
Alkaloid
extracted from the leaves of plants, flowers, fruit, bark, and roots
are dried and then crushed. Alkaloid extracted
with solvents, eg ethanol,
and then evaporated. Then, Extracts were obtained
inorganic acids to produce a quaternary ammonium salt
and then extracted again. Quaternary ammonium
salt obtained was
treated with sodium carbonate
to produce these alkaloids were then extracted with
a solvent-free such as ether
and chloroform. A
mixture of alkaloids obtained
finally separated in various ways,
such as chromatographic methods.
Structural
Determination using IR spectra
On this
spectrum, we can notice several peaks, which characterise the different
chemical functions of nicotine:
·
Around 3400 cm-1, we can see the large peak of water (it deals with
a liquid film).
·
Between 2970 and 2780 cm-1 : C-H stretching.
·
The peak at 1677 cm-1 : aromatic C=N double bond stretching.
·
The peak at 1691 cm-1 : aromatic C=C double bond stretching.
· The peaks at 717 cm-1
and 904 cm-1 correspond to the out of plane bending of the C-H bond
of the monosubstituted pyridinic cycle.
