What are Lipids?

Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

The membrane that surrounds a cell is made up of proteins and lipids. Depending on the membrane’s location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins.Cholesterol, which is not found in plant cells, is a type of lipid that helps stiffen the membrane. Image Credit: National Institute of General Medical Sciences

Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or "building blocks": ketoacyl and isoprene groups. Using this approach, lipids may be divided into eight categories: fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids and polyketides (derived from condensation of ketoacyl subunits); and sterol lipids and prenol lipids (derived from condensation of isoprene subunits).

Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids calledtriglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol. Although humans and other mammals use various biosynthetic pathways to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

Further Reading

• Types of Lipids
• Lipid Biological Functions
• Lipid Metabolism
• Lipid Health and Nutrition

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What is a Mutation?

MUTATION

As we learned earlier, the sequence of deoxyribonucleotide bases in the genes (def) that make up a organism's DNA determines the order of amino acids in the proteins and polypeptides made by that organism. This order of DNA bases constitutes an organism's genotype (def). A particular organism may possess alternate forms of some genes. Such alternate forms of genesare referred to as alleles (def). The physical characteristics an organism possesses, based on its genotype and the interaction with its environment, make up its phenotype .

Mutation (def) is an error during DNA replication that results in a change in the sequence of deoxyribonucleotide bases (def) in the DNA. Spontaneous mutation (def) occurs naturally (a normal mistake rate) about one in every million to one in every billion divisions and is probably due to low level natural mutagens normally present in the environment. Induced mutation (def) is caused by mutagens, substances that cause a much higher rate of mutation.

A. Spontaneous Mutation (def)

1. Mechanisms of mutation

a. Substitution of a nucleotide (point mutations (def)): substitution of one deoxyribonucleotide for another during DNA replication (see Fig. 17). This is the most common mechanism of mutation. Substitution of one nucleotide for another is a result of tautomeric shift, a rare process by which the hydrogen atoms of a deoxyribonucleotide base move in a way that changes the properties of its hydrogen bonding. For example, a shift in the hydrogen atom of adenine enables it to form hydrogen bonds with cytosine rather than thymine. Likewise, a shift in the hydrogen atom in thymine allows it to bind with guanine rather than adenine.

b. Deletion or addition of a nucleotide (frameshift mutations (def)): deletion or addition of a deoxyribonucleotide during DNA replication (see Fig. 18 andFig. 19).

2. Results of mutation

One of four things can happen as a result of these mechanisms of mutation and the resulting change in the deoxyribonucleotide base sequence mentioned above:

a. A missense mutation (def) occurs. This is usually seen with a single substitution mutation and results in one wrong codon (def) and one wrong amino acid (see Fig. 20).

b. A nonsense mutation occurs (def). If the change in the deoxyribonucleotide base sequence results in transcription (def) of a stop or nonsense codon(def), the protein would be terminated at that point in the message (see Fig. 21).

c. A sense mutation (def) occurs. This is sometimes seen with a single substitution mutation when the change in the DNA base sequence results in a new codon still coding for the same amino acid (see Fig. 22). (With the exception of methionine, all amino acids are coded for by more than one codon.)

d. A frameshift mutation occurs (def). This is seen when a number of DNA nucleotides not divisible by three is added or deleted. Remember, the genetic code is a triplet code where three consecutive nucleotides code for a specific amino acid. This causes a reading frame shift and all of the codons and all of the amino acids after that mutation are usually wrong (see Fig. 23); frequently one of the wrong codons turns out to be a stop or nonsense codon and the protein is terminated at that point.

B. Induced Mutation (def) is caused by mutagens, substances that cause a much higher rate of mutation.

Chemical mutagens generally work in one of three ways.

·         Some chemical mutagens, such as nitrous acid and nitrosoguanidine, work by causing chemical modifications of purine and pyrimidine bases that alter their hydrogen-bonding properties. For example, nitrous acid converts cytosine to uracil which then forms hydrogen bonds with adenine rather than guanine.

·         Other chemical mutagens function as base analogs. They are compounds that chemically resemble a nucleotide base closely enough that during DNA replication, they can be incorporated into the DNA in place of the natural base. Examples include 2-amino purine, a compound that resembles adenine, and 5-bromouracil, a compound that resembles thymine. The base analogs, however, do not have the hydrogen-bonding properties of the natural base.

·         Still other chemical mutagens function as intercalating agents. Intercalating agents are planar three-ringed molecules that are about the same size as a nucleotide base pair. During DNA replication, these compounds can insert or intercalate between adjacent base pairs thus pushing the nucleotides far enough apart that an extra nucleotide is often added to the growing chain during DNA replication. An example is ethidium bromide.

Certain types of radiation can also function as mutagens.

·         Ultraviolet Radiation. The ultraviolet portion of the light spectrum includes all radiations with wavelengths from 100 nm to 400 nm. It has low wave length and low energy. The microbicidal activity of ultraviolet (UV) light depends on the length of exposure: the longer the exposure the greater the cidal activity. It also depends on the wavelength of UV used. The most cidal wavelengths of UV light lie in the 260 nm - 270 nm range where it is absorbed by nucleic acid.

In terms of its mode of action, UV light is absorbed by DNA and causes adjacent thymine bases on the same DNA strand to covalently bond together, forming what are calledthymine-thymine dimers (see Fig. 24). As the DNA replicates, nucleotides do not complementary base pair with the thymine dimers and this terminates the replication of that DNA strand. In the case of bacteria, however, most of the damage from UV radiation actually comes from the cell trying to repair the damage to the DNA by a process called SOS repair. In very heavily damaged DNA containing large numbers of thymine dimers, a process called SOS repair is activated as kind of a last ditch effort to repair the DNA. In this process, a gene product of the SOS system binds to DNA polymerase allowing it to synthesize new DNA across the damaged DNA. However, this altered DNA polymerase loses its proofreading ability resulting in the synthesis of DNA that itself now contains many misincorporated bases. (Most of the chemical mutagens mentioned above also activate SOS repair.)

·         Ionizing Radiation. Ionizing radiation, such as X-rays and gamma rays, has much more energy and penetrating power than ultraviolet radiation. It ionizes water and other molecules to form radicals (molecular fragments with unpaired electrons) that can break DNA strands and alter purine and pyrimidine bases.

A point mutation is the simplest kind of genetic mutation. See the dramatic effects that can arise from such a small substitution in the genome.

Simple Substitution

A genetic mutation is any change in the DNA code (genome) of a cell. Of all the ways the genome can be changed, the simplest type of mutation is the point mutation. In a point mutation, a single base is substituted for another, changing the meaning of a single codon, leaving the rest of the gene unchanged. Geneticists recognize four types of point mutations, based on the effect on the genome.

Synonymous Mutations (Silent Mutations)

With 61 codons for 20 amino acids, many of the codons are "synonyms," coding for the same amino acid. Each amino acid can be indicated by up to six different codons (in the case of leucine); only two (methionine and tryptophan) have only one codon. In most cases, the synonyms differ by only one base, so it is possible for a point mutation to result in a codon for the same amino acid.

For example, the DNA codons CAA, CAG, CAT, and CAC all code for the amino acid valine. If a strand of DNA undergoes a point mutation in the codon CAA that changes it to CAG, it would still code for valine. This type of substitution is called a synonymous or sense mutation; it is also known as a silent mutation because there is no change in the amino acid sequence. (Find out how "silent" mutations can still have an effect on the gene and its resulting protein.)

Missense Mutations

A missense mutation is a point mutation that causes a codon to code for a different amino acid. The most notorious missense mutation is the one that causes sickle cell anemia. In this disease, one of the codons in an important hemoglobin gene has changed from CTC to CAC, resulting in the amino acid valine instead of glutamic acid. Chemically, these two amino acids are very dissimilar, so this simple change has a significant effect on the structure of hemoglobin protein, causing the disease symptoms.

A missense mutation might be less significant if the change is between two similar amino acids. For example, a change from CTC (glutamic acid) to CTG (aspartic acid) may not have a dramatic effect on the resulting protein because these two amino acids are similar (both are acidic).

Nonsense Mutations and Stop-Codon Mutations

Three codons in the genetic code tell the cell to stop adding amino acids to a protein because the end of the gene has been reached. In a nonsense mutation, a codon that stands for an amino acid mutates to one of these three stop codons. (The term "nonsense mutation" is used because the stop codon has "no sense" for an amino acid—as opposed to a "missense mutation," in which the resulting codon has the "wrong sense" for an amino acid.) Nonsense mutations cause the protein to be cut off early and therefore incomplete, which usually renders it non-functional. Cystic fibrosis is a disease caused by a nonsense mutation.

A stop-codon mutation is the opposite of a nonsense mutation: it changes a stop codon into a codon for an amino acid, causing the protein to become too large. The added section may consist of part of another protein from the genome—or it may be complete "gibberish," if the addition comes from a non-coding region ofDNA. This lesser-known type of mutation, like a nonsense mutation, generally renders its protein non-functional, and may even result in a harmful protein. A rare disease called familial British dementia is caused by a stop-codon mutation that causes mutated amyloid protein to "clog" the brain (see Nature vol. 399 (1999), pp. 776-781).

LIPIDS

All Lipids are hydrophobic: that’s the one property they have in common. This group of molecules includes fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.

		Fats and oils are made from two kinds of molecules: glycerol (a type of alcohol with a hydroxyl group on each of its three carbons) and three fatty acids joined by dehydration synthesis. Since there are three fatty acids attached, these are known as triglycerides. “Bread” and pastries from a “bread factory” often contain mono- and diglycerides as “dough conditioners.” Can you figure out what these molecules would look like? The main distinction between fats and oils is whether they’re solid or liquid at room temperature, and this, as we’ll soon see, is based on differences in the structures of the fatty acids they contain.

Structure of Fatty Acids

The “tail” of a fatty acid is a long hydrocarbon chain, making it hydrophobic. The “head” of the molecule is a carboxyl group which is hydrophilic. Fatty acids are the main component of soap, where their tails are soluble in oily dirt and their heads are soluble in water to emulsify and wash away the oily dirt. However, when the head end is attached to glycerol to form a fat, that whole molecule is hydrophobic.

The terms saturated, mono-unsaturated, and poly-unsaturated refer to the number of hydrogens attached to the hydrocarbon tails of the fatty acids as compared to the number of double bonds between carbon atoms in the tail. Fats, which are mostly from animal sources, have all single bonds between the carbons in their fatty acid tails, thus all the carbons are also bonded to the maximum number of hydrogens possible. Since the fatty acids in these triglycerides contain the maximum possible amouunt of hydrogens, these would be called saturated fats. The hydrocarbon chains in these fatty acids are, thus, fairly straight and can pack closely together, making these fats solid at room temperature. Oils, mostly from plant sources, have some double bonds between some of the carbons in the hydrocarbon tail, causing bends or “kinks” in the shape of the molecules. Because some of the carbons share double bonds, they’re not bonded to as many hydrogens as they could if they weren’t double bonded to each other. Therefore these oils are called unsaturatedfats. Because of the kinks in the hydrocarbon tails, unsaturated fats can’t pack as closely together, making them liquid at room temperature. Many people have heard that the unsaturated fats are “healthier” than the saturated ones. Hydrogenated vegetable oil (as in shortening and commercial peanut butters where a solid consistency is sought) started out as “good” unsaturated oil. However, this commercial product has had all the double bonds artificially broken and hydrogens artificially added (in a chemistry lab-type setting) to turn it into saturated fat that bears no resemblance to the original oil from which it came (so it will be solid at room temperature).

In unsaturated fatty acids, there are two ways the pieces of the hydrocarbon tail can be arranged around a C=C double bond. In cis bonds, the two pieces of the carbon chain on either side of the double bond are either both “up” or both “down,” such that both are on the same side of the molecule. Intrans bonds, the two pieces of the molecule are on opposite sides of the double bond, that is, one “up” and one “down” across from each other. Naturally-occurring unsaturated vegetable oils have almost all cis bonds, but using oil for frying causes some of the cis bonds to convert to trans bonds. If oil is used only once like when you fry an egg, only a few of the bonds do this so it’s not too bad. However, if oil is constantly reused, like in fast food French fry machines, more and more of the cis bonds are changed to trans until significant numbers of fatty acids with trans bonds build up. The reason this is of concern is that fatty acids with trans bonds are carcinogenic, or cancer-causing. The levels of trans fatty acids in highly-processed, lipid-containing products such as margarine are quite high, and I have heard that the government is considering requiring that the amounts of trans fatty acids in such products be listed on the labels.

We need fats in our bodies and in our diet. Animals in general use fat for energy storage because fat stores 9 KCal/g of energy. Plants, which don’t move around, can afford to store food for energy in a less compact but more easily accessible form, so they use starch (a carbohydrate, NOT A LIPID) for energy storage. Carbohydrates and proteins store only 4 KCal/g of energy, so fat stores over twice as much energy/gram as carbohydrates or proteins. By the way, this is also related to the idea behind some of the high-carbohydrate weight loss diets. The human body burns carbohydrates and fats for fuel in a given proportion to each other. The theory behind these diets is that if they supply carbohydrates but not fats, then it is hoped that the fat needed to balance with the sugar will be taken from the dieter’s body stores. Fat is also is used in our bodies to a) cushion vital organs like the kidneys and b) serve as insulation, especially just beneath the skin.

Phospholipids

Phospholipids are made from glycerol, two fatty acids, and (in place of the third fatty acid) a phosphate group with some other molecule attached to its other end. The hydrocarbon tails of the fatty acids are still hydrophobic, but the phosphate group end of the molecule is hydrophilic because of the oxygens with all of their pairs of unshared electrons. This means that phospholipids are soluble in both water and oil.

An emulsifying agent is a substance which is soluble in both oil and water, thus enabling the two to mix. A “famous” phospholipid is lecithinwhich is found in egg yolk and soybeans. Egg yolk is mostly water but has a lot of lipids, especially cholesterol, which are needed by the developing chick. Lecithin is used to emulsify the lipids and hold them in the water as an emulsion. Lecithin is the basis of the classic emulsion known as mayonnaise.

Our cell membranes are made mostly of phospholipids arranged in a double layer with the tails from both layers “inside” (facing toward each other) and the heads facing “out” (toward the watery environment) on both surfaces.

Steroids

The general structure of cholesterol consists of two six-membered rings side-by-side and sharing one side in common, a third six-membered ring off the top corner of the right ring, and a five-membered ring attached to the right side of that. The central core of this molecule, consisting of four fused rings, is shared by all steroids, including estrogen (estradiol), progesterone, corticosteroids such as cortisol (cortisone), aldosterone, testosterone, and Vitamin D. In the various types of steroids, various other groups/molecules are attached around the edges. Know how to draw the four rings that make up the central structure.

Cholesterol is not a “bad guy!” Our bodies make about 2 g of cholesterol per day, and that makes up about 85% of blood cholesterol, while only about 15% comes from dietary sources. Cholesterol is the precursor to our sex hormones and Vitamin D. Vitamin D is formed by the action of UV light in sunlight on cholesterol molecules that have “risen” to near the surface of the skin. At least one source I read suggested that people not shower immediately after being in the sun, but wait at least ½ hour for the new Vitamin D to be absorbed deeper into the skin. Our cell membranes contain a lot of cholesterol (in between the phospholipids) to help keep them “fluid” even when our cells are exposed to cooler temperatures.

Many people have hear the claims that egg yolk contains too much cholesterol, thus should not be eaten. An interesting study was done at Purdue University a number of years ago to test this. Men in one group each ate an egg a day, while men in another group were not allowed to eat eggs. Each of these groups was further subdivided such that half the men got “lots” of exercise while the other half were “couch potatoes.” The results of this experiment showed no significant difference in blood cholesterol levels between egg-eaters and non-egg-eaters while there was a very significant difference between the men who got exercise and those who didn’t.

Lipoproteins are clusters of proteins and lipids all tangled up together. These act as a means of carrying lipids, including cholesterol, around in our blood. There are two main categories of lipoproteins distinguished by how compact/dense they are. LDL or low density lipoprotein is the “bad guy,” being associated with deposition of “cholesterol” on the walls of someone’s arteries. HDL or high density lipoprotein is the “good guy,” being associated with carrying “cholesterol” out of the blood system, and is more dense/more compact than LDL.

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References:

• Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.
• Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5^th Ed.   Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
• Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3^rd Ed.   Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
• Lappé, Francis Moore. 1982. Diet for a Small Planet, 10th Anniversary Ed. Ballantine Books. New York.
• Lappé, Francis Moore. 1991. Diet for a Small Planet, 20th Anniversary Ed. Ballantine Books. New York.
• Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.
• Sienko, Michell J. and Robert A. Plane. 1966. Chemistry: Principles and Properties. McGraw-Hill Book Co., NY. (and other chemistry texts and handbooks)

MACAM DESTILASI KIMIA

Macam-Macam Destilasi

Distilasi Sederhana

Pada distilasi sederhana, dasar pemisahannya adalah perbedaan titik didih yang jauh atau dengan salah satu komponen bersifat volatil. Jika campuran dipanaskan maka komponen yang titik didihnya lebih rendah akan menguap lebih dulu. Selain perbedaan titik didih, juga perbedaan kevolatilan, yaitu kecenderungan sebuah substansi untuk menjadi gas. Distilasi ini dilakukan pada tekanan atmosfer. Aplikasi distilasi sederhana digunakan untuk memisahkan campuran air dan alkohol.

Distilasi Fraksionisasi

Fungsi distilasi fraksionasi adalah memisahkan komponen-komponen cair, dua atau lebih, dari suatu larutan berdasarkan perbedaan titik didihnya. Distilasi ini juga dapat digunakan untuk campuran dengan perbedaan titik didih kurang dari 20 °C dan bekerja pada tekanan atmosfer atau dengan tekanan rendah. Aplikasi dari distilasi jenis ini digunakan pada industri minyak mentah, untuk memisahkan komponen-komponen dalam minyak mentah

Perbedaan distilasi fraksionasi dan distilasi sederhana adalah adanya kolom fraksionasi. Di kolom ini terjadi pemanasan secara bertahap dengan suhu yang berbeda-beda pada setiap platnya. Pemanasan yang berbeda-beda ini bertujuan untuk pemurnian distilat yang lebih dari plat-plat di bawahnya. Semakin ke atas, semakin tidak volatil cairannya.

Minyak mentah (crude oil) sebagian besar tersusun dari senyawa-senyawa hidrokarbon jenuh (alkana). Adapun hidrokarbon tak jenuh (alkena, alkuna dan alkadiena) sangat sedikit dkandung oleh minyak bumi, sebab mudah mengalami adisi menjadi alkana.

Oleh karena minyak bumi berasl dari fosil organisme, mak minyak bumi mengandung senyawa-senyawa belerang (0,1 sampai 7%), nitrogen (0,01 sampai 0,9%), oksigen (0,6-0,4%) dan senyawa logam dalam jumlah yang sanagt kecil. Minyak mentah dipisahkan menjadi sejumlah fraksi-fraksi melalui proses destilasi (penyulingan).

Pemisahan minyak mentah ke dalam komponen-komponen murni (senyawa tunggal) tidak mungkin dilakukan dan juga tidak prakstis sebab terlalu banyak senyawa yang ada dalam minyak tersebut dan senyawa hidrokarbon memiliki isomer-isomer dengan titik didih yang berdekatan. Fraksi-fraksi yang diperoleh dari destilasi minyak bumi adalah campuran hidrokarbon yang mendidih pada trayek suhu tertentu. Misalnya fraksi minyak tanah (kerosin) tersusun dari campuran senyawa-senyawa yang mendidih antar 180⁰C-250⁰C. Proses destilasi dikerjakan dengan menggunakan kolom atau menara destilasi (Gambar 19.5).

Proses pertama dalam pemrosesan minyak bumi adalah fraksionasi dari minyak mentah dengan menggunakan proses destilasi bertingkat, adapun hasil yang diperoleh adalah sebagai berikut:

Sisa :

1.      Minyak bisa menguap : minyak-minyak pelumas, lilin, parafin, dan vaselin.

2.      Bahan yang tidak bisa menguap : aspal dan arang minyak bumi

Distilasi bertingkat

Dalam proses distilasi bertingkat, minyak mentah tidak dipisahkan menjadi komponen-komponen murni, melainkan ke dalam fraksi-fraksi, yakni kelompok-kelompok yang mempunyai kisaran titik didih tertentu. Hal ini dikarenakan jenis komponen hidrokarbon begitu banyak dan isomer-isomer hidrokarbon mempunyai titik didih yang berdekatan. Proses distilasi bertingkat ini dapat dijelaskan sebagai berikut:

• Minyak mentah dipanaskan dalam boiler menggunakan uap air bertekanan tinggi sampai suhu ~600oC. Uap minyak mentah yang dihasilkan kemudian dialirkan ke bagian bawah menara/tanur distilasi.

• Dalam menara distilasi, uap minyak mentah bergerak ke atas melewati pelat-pelat (tray). Setiap pelat memiliki banyak lubang yang dilengkapi dengan tutup gelembung (bubble cap) yang memungkinkan uap lewat.

• Dalam pergerakannya, uap minyak mentah akan menjadi dingin. Sebagian uap akan mencapai ketinggian di mana uap tersebut akan terkondensasi membentuk zat cair. Zat cair yang diperoleh dalam suatu kisaran suhu tertentu ini disebut fraksi.

• Fraksi yang mengandung senyawa-senyawa dengan titik didih tinggi akan terkondensasi di bagian bawah menara distilasi. Sedangkan fraksi senyawa-senyawa dengan titik didih rendah akan terkondensasi di bagian atas menara.

Sebagian fraksi dari menara distilasi selanjutnya dialirkan ke bagian kilang minyak lainnya untuk proses konversi.

Gambar 2. Menara destilasi

Distilasi Uap

Distilasi uap digunakan pada campuran senyawa-senyawa yang memiliki titik didih mencapai 200 °C atau lebih. Distilasi uap dapat menguapkan senyawa-senyawa ini dengan suhu mendekati 100 °C dalam tekanan atmosfer dengan menggunakan uap atau air mendidih. Sifat yang fundamental dari distilasi uap adalah dapat mendistilasi campuran senyawa di bawah titik didih dari masing-masing senyawa campurannya. Selain itu distilasi uap dapat digunakan untuk campuran yang tidak larut dalam air di semua temperatur, tapi dapat didistilasi dengan air. Aplikasi dari distilasi uap adalah untuk mengekstrak beberapa produk alam seperti minyak eucalyptus dari eucalyptus, minyak sitrus dari lemon atau jeruk, dan untuk ekstraksi minyak parfum dari tumbuhan.

Campuran dipanaskan melalui uap air yang dialirkan ke dalam campuran dan mungkin ditambah juga dengan pemanasan. Uap dari campuran akan naik ke atas menuju ke kondensor dan akhirnya masuk ke labu distilat.

Distilasi Vakum

Distilasi vakum biasanya digunakan jika senyawa yang ingin didistilasi tidak stabil, dengan pengertian dapat terdekomposisi sebelum atau mendekati titik didihnya atau campuran yang memiliki titik didih di atas 150 °C. Metode distilasi ini tidak dapat digunakan pada pelarut dengan titik didih yang rendah jika kondensornya menggunakan air dingin, karena komponen yang menguap tidak dapat dikondensasi oleh air. Untuk mengurangi tekanan digunakan pompa vakum atau aspirator. Aspirator berfungsi sebagai penurun tekanan pada sistem distilasi ini.