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4 Today’s Lecture Topics Covered: Amino Acids - Classification & Physical PropertiesStereochemistry of a-Amino AcidsCommon a-Amino Acids: Review of StructureEffect of pH on the Structure of a-Amino AcidsIsoelectric Point of Amino Acids6
8 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
9 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
10 Amino Acids Exist as Zwitterions Resonance-Stabilized Zwitterion4
11 Zwitterions Defined IUPAC Compendium of Chemical Terminology Zwitterionic Compounds/Zwitterionsneutral 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
12 Amino Acids Exist as Zwitterions What evidence do we have for this behavior………4
13 Properties of Glycine Reflect its Zwitterionic Structure High Water Solubilityglycine is soluble in water but not in non-polar solvents.High Melting Pointwhen heated to 233°C it decomposes before it melts.4
14 Properties of Glycine Reflect its Zwitterionic Structure The physical properties of glycine are consistent with this structure4
16 Chirality, Chiral & Stereogenic Centers the term describing an object that is not superimposable on its mirror imageChiral Centeran 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
17 Fisher Projections and the D/L Stereochemical Convention
18 With the Exception of Glycine, a-Amino Acids are Chiral Molecules has no stereogenic center and is therefore achiralAll Other A.A.shave a stereogenic center and are therefore chiral0857Non- superimposable isomers(enantiomers)
19 Configuration of a-Amino Acids in Nature Most of the a-amino acids in proteins have the L-configuration at their carbon
21 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 tolong cari sendiri.
22 The Nature of a-Side Chain Determines the Physical Properties of Amino Acids The major differences among the side chains concern:Size and shapeElectronic characteristics
23 Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acidsNon-polar side chainsPolar but non-ionized side chainsBrønsted acidic side chainsBrønsted basic side chains
24 Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acidsNon-polar side chainsPolar but non-ionized side chainsBrønsted acidic side chainsBrønsted basic side chains
25 Non-Polar Side Chains: Glycine (Gly, G)Glycine is the simplest amino acid and is achiral
26 Non-Polar Side Chains: Alanine (Ala, A)Alanine, valine, leucine, and isoleucine have alkyl groups as side chains, which are non-polar and hydrophobic
30 Non-Polar Side Chains: Methionine (Met, M)the side chain in methionine is non-polar, but the presence of sulfur makes it somewhat polarizable.
31 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.
32 Non-Polar Side Chains: Phenylalanine (Phe, F)The side chain in phenylalanine (a non-polar amino acid) is a benzyl group.
33 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.
34 Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acidsNon-polar side chainsPolar but non-ionized side chainsBrønsted acidic side chainsBrønsted basic side chains
35 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.
36 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
37 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.
38 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
39 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.
40 Polar, Non-Ionized Side Chains: Glutamine (Gln, Q)
41 Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acidsNon-polar side chainsPolar but non-ionized side chainsBrønsted acidic side chainsBrønsted basic side chains
42 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.
43 Brønsted Acidic Side Chains: Glutamic Acid (Glu, U)
44 Essential a-Amino Acids can be Subdivided According to the Nature of Side Chain General categories of a-amino acidsNon-polar side chainsPolar but non-ionized side chainsBrønsted acidic side chainsBrønsted basic side chains
45 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
46 Brønsted Basic Side Chains: Arginine (Arg, G)Arginine is a derivative of lysine: it contains a basic guanidine group.
47 New Functional Group: Guanidine Guanidinium IonGuanidine is a significantly stronger base thantriethylamineTriethylamine
48 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.
49 Protonation of Imidazole q. Which nitrogen atom is protonated in the imidazole ring of histidine?a. This one 11
52 Effects of pH on Structure of Glycine At low pH, that is in strongly acidic solution, glycine is protonated and exists as a monocation.
53 pH of Solution Increases? What Happens whenpH 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.Typicalammoniumion: pKa ~9Typicalcarboxylicacid: pKa ~5
54 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 This is lower than acetic acid (4.76) because of the presence of the electron withdrawing ammonium substituent.
55 Isoelectric PointIsoelectric pointThe 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.
56 Zwitterionic structure is neutral and its value of pH is called isoelectric point.
57 Nilai pKa dan pI dari gugus asam amino bebas dan protein @ 25 0C
58 Kurva titrasi asam amino dengan basa (NaOH) dan Asam (HCL)
59 REAKSI GUGUS ASAM DAN BASA DARI ASAM AMINO BILA DITITRASI ZWITTER ION ATAU SALING MENETRALKAN
60 Calculation of Isoelectric Point The pH(I) is the numerical average of the pKa values for the carboxylic acid and the ammonium group./2 = 5.97The pH(I) of glycine is 5.97
61 pH of Solution Rises Above pH(I)? What Happens whenpH 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.
62 The Overall Charge of Amino Acids Changes as a Function of pH Increasing pH4
63 Isoelectric Points of Essential a-Amino Acids General categories of a-amino acidsAmino acids with neutral side chainsAmino acids with ionizable side chains
89 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 crystalshave high melting pointsare soluble in waternot soluble in hydrocarbon solvents
90 REAKSI GUGUS ASAM AMINO REAKSI DENGAN REAGEN NINHYDRINReaksi ini digunakan untuk analisa/mendeteksi adanya asam amino bebas dari suatu bahan:Diukur dengan 570 nm, warna ungu, krn adanya seny. Hydrindantin + CO2+H2O+ aldehide
91 REAKSI DENGAN REAGEN FLUORESCAMINE DIGUNAKAN UNTUK MENDETEKSI ASAM-2 AMINO, PEPTIDA DAN PROTEIN yang mengandung gugus amine primer, diukur dengan 470 nm, warnanya fluorescence tajam.
92 REAKSI KIMIA GUGUS ASAM AMINO SECARA KIMIA LIHAT E-BOOK: FOOD CHEMISTRY OLEH FENNEMA di BLOG SBWJUDUL: FOOD CHEMISTRY, 3rd EDITION, FENNEMA, MARCEL DEKKER PUB.
93 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 nm dan Tyr dan Trp berfluorescene pada lingkungan polaritas medium larutan.
94 Latihan AAAPA YANG DIMAKSUD DGN pI, kapan terjadinyaJelaskan klasifikasi AA dan beri contohBagaimana proses protonasi dan deprotonasi asam amino dapat terjadiApakah beda produk pangan nabati dan hewani ditinjau dari jenis dan jumlah asam amino yang ada dalam ke dua produk pangan tsb.