WEEK-1 Amino Acids PROF. SBW 2

BLOG: www.simonbw.lecture.ub.ac.id Page/HALAMAN: KULIAH S-1 dibawah: KIMIA PANGAN BARU SEM 2012/2013 PASSWORD: cucu1

BAGI MHSW BARU SEM 2 TERLAMBAT > 5 MENIT, JANGAN MASUK, KARENA SDR AKAN DIUSIR DARI RUANGAN. KERJAKAN LATIHAN DIBLOG, AGAR MUDAH LULUSNYA. QUIS 15-20 MENIT SETELAH KULIAH KE-3

Today’s Lecture Topics Covered: Amino Acids - Classification & Physical Properties Stereochemistry of a-Amino Acids Common a-Amino Acids: Review of Structure Effect of pH on the Structure of a-Amino Acids Isoelectric Point of Amino Acids 6

27.1 Classification of Amino Acids

Classification of Amino Acids amino acids are classified as , , , etc. to indicate where the nitrogen atom is relative to the carboxylic acid: 3

a-Amino Acids are Most Ubiquitous in Nature 3

Revision: Amines are Brønsted Bases Amines are electron-rich and have a reactive lone pair, which can form a covalent bond to a proton to form an ammonium ion. 3

Amino Acids Exist as Zwitterions While their name implies that amino acids are compounds that contain an amine (—NH2) and a carboxylic acid (-CO2H), these groups are actually present as their conjugate acid (—NH3+) and conjugate base (—CO2–), respectively. 3

Amino Acids Exist as Zwitterions Resonance-Stabilized Zwitterion 4

Zwitterions Defined IUPAC Compendium of Chemical Terminology Zwitterionic Compounds/Zwitterions neutral compounds having formal unit electrical charges of opposite sign. Some chemists restrict the term to compounds with the charges on non-adjacent atoms. Sometimes referred to as inner salts, dipolar ions (a misnomer), e.g. +H3N-CH2CO2- ammonioacetate (glycine). IUPAC Compendium of Chemical Terminology

Amino Acids Exist as Zwitterions What evidence do we have for this behavior……… 4

Properties of Glycine Reflect its Zwitterionic Structure High Water Solubility glycine is soluble in water but not in non-polar solvents. High Melting Point when heated to 233°C it decomposes before it melts. 4

Properties of Glycine Reflect its Zwitterionic Structure The physical properties of glycine are consistent with this structure 4

27.2 Stereochemistry of a-Amino Acids 4

Chirality, Chiral & Stereogenic Centers the term describing an object that is not superimposable on its mirror image Chiral Center an atom that has four nonequivalent atoms or groups attached to it. At various times, chiral centers have been and are called asymmetric centers or stereogenic centers

Fisher Projections and the D/L Stereochemical Convention

With the Exception of Glycine, a-Amino Acids are Chiral Molecules has no stereogenic center and is therefore achiral All Other A.A.s have a stereogenic center and are therefore chiral 0857 Non- superimposable isomers (enantiomers)

Configuration of a-Amino Acids in Nature Most of the a-amino acids in proteins have the L-configuration at their carbon

Configuration of a-Amino Acids in Nature

Occurrence of a-Amino Acids in Nature More than 700 amino acids occur naturally, but 20 of them are especially important. These 20 amino acids are the building blocks of proteins. All are -amino acids. They differ in respect to the group attached to the  carbon. These 20 are listed in Table 27.1. tolong cari sendiri.

The Nature of a-Side Chain Determines the Physical Properties of Amino Acids The major differences among the side chains concern: Size and shape Electronic characteristics

Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acids Non-polar side chains Polar but non-ionized side chains Brønsted acidic side chains Brønsted basic side chains

Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acids Non-polar side chains Polar but non-ionized side chains Brønsted acidic side chains Brønsted basic side chains

Non-Polar Side Chains: Glycine (Gly, G) Glycine is the simplest amino acid and is achiral

Non-Polar Side Chains: Alanine (Ala, A) Alanine, valine, leucine, and isoleucine have alkyl groups as side chains, which are non-polar and hydrophobic

Non-Polar Side Chains: Valine (Val, V)

Non-Polar Side Chains: Leucine (Leu, L)

Non-Polar Side Chains: Isoleucine (Ile, I)

Non-Polar Side Chains: Methionine (Met, M) the side chain in methionine is non-polar, but the presence of sulfur makes it somewhat polarizable.

Non-Polar Side Chains: Proline (Pro, P) Among the 20 essential amino acids, proline is the only amino acid that contains a secondary amine function. Its side chain is non-polar and cyclic.

Non-Polar Side Chains: Phenylalanine (Phe, F) The side chain in phenylalanine (a non-polar amino acid) is a benzyl group.

Non-Polar Side Chains: Tryptophan (Trp, W) The side chain in tryptophan (a non-polar amino acid) is larger and more polarizable than the benzyl group of phenylalanine.

Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acids Non-polar side chains Polar but non-ionized side chains Brønsted acidic side chains Brønsted basic side chains

Polar, Non-Ionized Side Chains: Serine (Ser, S) The hydroxymethyl (-CH2OH) side chain in serine can be involved in hydrogen bonding and can behave as a nucleophile.

Polar, Non-Ionized Side Chains: Threonine (Thr, T) The side chain in threonine can be involved in hydrogen bonding, but is somewhat more crowded than in serine

Polar, Non-Ionized Side Chains: Cysteine (Cys, C) Cysteine is a thiol. In proteins, the side chains of two remote cysteine residues can be joined, via oxidation, to form a S-S bond, or disulfide bridge.

Polar, Non-Ionized Side Chains: Tyrosine (Tyr, Y) The side chain of tyrosine is similar to that of phenylalanine but can participate in hydrogen bonding

Polar, Non-Ionized Side Chains: Asparagine (Asn, N) The side chains of asparagine and glutamine (next slide) terminate in amide functions that are polar and can engage in hydrogen bonding.

Polar, Non-Ionized Side Chains: Glutamine (Gln, Q)

Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acids Non-polar side chains Polar but non-ionized side chains Brønsted acidic side chains Brønsted basic side chains

Brønsted Acidic Side Chains: Aspartic Acid Aspartic acid and glutamic acid (next slide) exist as their conjugate bases at biological pH. They are negatively charged and can form ionic bonds with positively charged species.

Brønsted Acidic Side Chains: Glutamic Acid (Glu, U)

Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acids Non-polar side chains Polar but non-ionized side chains Brønsted acidic side chains Brønsted basic side chains

Brønsted Basic Side Chains: Lysine (Lys, K) Lysine and arginine (next slide) exist as their conjugate acids at biological pH. They are positively charged and can form electrostatic interactions with negatively charged species

Brønsted Basic Side Chains: Arginine (Arg, G) Arginine is a derivative of lysine: it contains a basic guanidine group.

New Functional Group: Guanidine Guanidinium Ion Guanidine is a significantly stronger base thantriethylamine Triethylamine

Brønsted Basic Side Chains: Histidine (His, H) Histidine is a basic amino acid, but less basic than lysine and arginine. Histidine can interact with metal ions and can help move protons from one site to another.

Protonation of Imidazole q. Which nitrogen atom is protonated in the imidazole ring of histidine? a. This one  11

Protonation of Histidine 11

27.3 Acid-Base Behavior of Amino Acids 4

Effects of pH on Structure of Glycine At low pH, that is in strongly acidic solution, glycine is protonated and exists as a monocation.

pH of Solution Increases? What Happens when pH of Solution Increases? As the pH of solution increases, the concentration of hydroxide ions also increases. At a give point, the most acidic group in the ion shown below will be deprotonated. Typical ammonium ion: pKa ~9 Typical carboxylic acid: pKa ~5

Glycine is a Stronger Acid than Acetic Acid As the pH of solution increases the carboxylic acid is deprotonated to form a neutral zwitterion. The pKa of glycine is 2.34. This is lower than acetic acid (4.76) because of the presence of the electron withdrawing ammonium substituent.

Isoelectric Point Isoelectric point The pH value at which the net electric charge of an elementary entity is zero. pI is a commonly used symbol for this kind-of-quantity. It should be replaced by pH(I) because it is a pH determined under that particular condition.

Zwitterionic structure is neutral and its value of pH is called isoelectric point.

Nilai pKa dan pI dari gugus asam amino bebas dan protein @ 25 0C

Kurva titrasi asam amino dengan basa (NaOH) dan Asam (HCL)

REAKSI GUGUS ASAM DAN BASA DARI ASAM AMINO BILA DITITRASI ZWITTER ION ATAU SALING MENETRALKAN

Calculation of Isoelectric Point The pH(I) is the numerical average of the pKa values for the carboxylic acid and the ammonium group. 9.60+2.37/2 = 5.97 The pH(I) of glycine is 5.97

pH of Solution Rises Above pH(I)? What Happens when pH of Solution Rises Above pH(I)? As the pH of solution increases above the isoelectric point, a proton is removed from the ammonium group (pKa = 9.3) of the zwitterion to generate the anionic form of the amino acid.

The Overall Charge of Amino Acids Changes as a Function of pH Increasing pH 4

Isoelectric Points of Essential a-Amino Acids General categories of a-amino acids Amino acids with neutral side chains Amino acids with ionizable side chains

Neutral Side Chains: Glycine (Gly, G) pKa1 = 2.34 pKa2 = 9.60 pH(I) = 5.97

Neutral Side Chains: Alanine (Ala, A) pKa1 = 2.34 pKa2 = 9.69 pH(I) = 6.00

Neutral Side Chains: Valine (Val, V) pKa1 = 2.32 pKa2 = 9.62 pH(I) = 5.96

Neutral Side Chains: Leucine (Leu, L) pKa1 = 2.36 pKa2 = 9.60 pH(I) = 5.98

Neutral Side Chains: Isoleucine (Ile, I) pKa1 = 2.36 pKa2 = 9.60 pH(I) = 5.98

Neutral Side Chains: Methionine (Met, M) pKa1 = 2.28 pKa2 = 9.21 pH(I) = 5.74

Neutral Side Chains: Proline (Pro, P) pKa1 = 1.99 pKa2 = 10.60 pH(I) = 6.30

Neutral Side Chains: Phenylalanine (Phe, F) pKa1 = 1.83 pKa2 = 9.13 pH(I) = 5.48

Neutral Side Chains: Tryptophan (Trp, W) pKa1 = 2.83 pKa2 = 9.39 pH(I) = 5.89

Neutral Side Chains: Serine (Ser, S) pKa1 = 2.02 pKa2 = 8.80 pH(I) = 5.41

Neutral Side Chains: Threonine (Thr, T) pKa1 = 2.09 pKa2 = 9.10 pH(I) = 5.60

Neutral Side Chains: Cysteine (Cys, C) pKa1 = 1.96 pKa2 = 8.18 pH(I) = 5.07

Neutral Side Chains: Tyrosine (Tyr, Y) pKa1 = 2.20 pKa2 = 9.11 pH(I) = 5.66

Neutral Side Chains: Asparagine (Asn, N) pKa1 = 2.02 pKa2 = 8.80 pH(I) = 5.41

Neutral Side Chains: Glutamine (Gln, Q) pKa1 = 2.17 pKa2 = 9.13 pH(I) = 5.65

Isoelectric Points of Essential a-Amino Acids General categories of a-amino acids Amino acids with neutral side chains Amino acids with ionizable side chains

Amino Acids with Acidic Side Chains have Three pKa Values to Consider pH(I) is between 1.88 and 3.65

Calculation of pH(I) for Amino Acids with Acidic Side Chains For amino acids with acidic side chains, pH(I) is the average of pKa1 and pKa2 1.88+3.65/2 = 2.77 The pH(I) of aspartic acid is 2.77

Ionizable Side Chains: Aspartic Acid pKa1 = 1.88 pKa2 = 3.65 pKa3 = 9.60 pH(I) = 2.77

Ionizable Side Chains: Glutamic Acid (Glu, U) pKa1 = 2.19 pKa2 = 4.25 pKa3 = 9.67 pH(I) = 3.22

Amino Acids with Ionizable Side Chains have Three pKa Values to Consider pH(I) is between 8.95 and 10.53

Calculation of pH(I) for Amino Acids with Basic Side Chains For amino acids with basic side chains, pH(I) is the average of pKa2 and pKa3 8.95+10.53/2 = 9.74 The pH(I) of lysine is 9.74

Ionizable Side Chains: Lysine (Lys, K) pKa1 = 2.18 pKa2 = 8.95 pKa3 = 10.53 pH(I) = 9.74 For amino acids with basic side chains, pH(I) is the average of pKa2 and pKa3.

Ionizable Side Chains: Arginine (Arg, G) pKa1 = 2.17 pKa2 = 9.04 pKa3 = 12.48 pH(I) = 10.76

Ionizable Side Chains: Histidine (His, H) pKa1 = 1.82 pKa2 = 6.00 pKa3 = 9.17 pH(I) = 7.59

Properties of Amino-Acids Because they are zwitterions at neutral pH, amino acids have many of the physical properties we associate with salts: can form crystals have high melting points are soluble in water not soluble in hydrocarbon solvents

REAKSI GUGUS ASAM AMINO REAKSI DENGAN REAGEN NINHYDRIN Reaksi ini digunakan untuk analisa/mendeteksi adanya asam amino bebas dari suatu bahan: Diukur dengan spektro @ 570 nm, warna ungu, krn adanya seny. Hydrindantin + CO2+H2O+ aldehide

REAKSI DENGAN REAGEN FLUORESCAMINE DIGUNAKAN UNTUK MENDETEKSI ASAM-2 AMINO, PEPTIDA DAN PROTEIN yang mengandung gugus amine primer, diukur dengan spektro @ 470 nm, warnanya fluorescence tajam.

REAKSI KIMIA GUGUS ASAM AMINO SECARA KIMIA LIHAT E-BOOK: FOOD CHEMISTRY OLEH FENNEMA di BLOG SBW JUDUL: FOOD CHEMISTRY, 3rd EDITION, FENNEMA, 1997. MARCEL DEKKER PUB.

SIFAT FISIKO-KIMIA ASAM AMINO YANG LAIN SIFAT HIDROFOBISITAS: REAKSI KELARUTAN ASAM AMINO DALAM AIR DAN ETANOL (LIHAT DETAIL DI E-BOOK) ARTINYA MANA-2 JENIS ASAM AMINO YG LARUT DALAM AIR DAN LARUT DALAM ETANOL. ARGININE, VALINE, LYSINE, ALANINE LARUT DALAM AIR. SIFAT OPTIK: TRP, TYR DAN Phe Mengobsorpsi sinar pada ג 250-300 nm dan Tyr dan Trp berfluorescene pada lingkungan polaritas medium larutan.

Latihan AA APA YANG DIMAKSUD DGN pI, kapan terjadinya Jelaskan klasifikasi AA dan beri contoh Bagaimana proses protonasi dan deprotonasi asam amino dapat terjadi Apakah beda produk pangan nabati dan hewani ditinjau dari jenis dan jumlah asam amino yang ada dalam ke dua produk pangan tsb.