Drug Discovery & Biotechnological Drugs PDF

Drug Discovery & Biotechnological Drugs Master’s degree in Medical and Pharmaceutical Biotechnology Giacomo Rossino, PhD [email protected] Do not distribute, photocopy or use this material for other purposes than individual...

Drug Discovery & Biotechnological Drugs Master’s degree in Medical and Pharmaceutical Biotechnology Giacomo Rossino, PhD [email protected] Do not distribute, photocopy or use this material for other purposes than individual study. 1  Outline 1. Amino acids 3. Carbohydrates a. Nomenclature, structure, a. Monosaccharides: structures, Fisher stereochemistry projections, cyclic hemiacetals b. acid-base properties b. Glycosides 2. Peptides and proteins c. Oligosaccharides and polysaccharides a. Primary structure d. Aminosugars b. Secondary structure 4. Peptide and peptidomimetic drugs c. Tertiary and quaternary structure 5. Chemical protein modifications d. Protein binding sites, weak interactions Do not distribute, photocopy or use this material for other purposes than individual study. 2  Aminoacids: definition and structure Amino acids: organic compounds that contain both an amino group (-NH2 ) and a carboxylic acid group (–COOH). L-alanine γ-aminobutyric acid (GABA) tyrosine p-aminobenzoic acid (PABA) Natural α-amino acids are the constituents of proteins, where thy are bond together through amide bonds (a.k.a. peptide bonds) to form long peptide chains. Peptide bonds aminoacids tripeptide Do not distribute, photocopy or use this material for other purposes than individual study. 3 Functional groups Do not distribute, photocopy or use this material for other purposes than individual study. 4 Functional groups Do not distribute, photocopy or use this material for other purposes than individual study. 5 Functional groups Indicate with a circle and name the functional groups of the following drugs. Do not distribute, photocopy or use this material for other purposes than individual study. 6 Functional groups Indicate with a circle and name the functional groups of the following drugs. Do not distribute, photocopy or use this material for other purposes than individual study. 7 Functional groups Indicate with a circle and name the functional groups of the following drugs. Do not distribute, photocopy or use this material for other purposes than individual study. 8 Functional groups Indicate with a circle and name the functional groups of the following drugs. Do not distribute, photocopy or use this material for other purposes than individual study. 9  Amino acids: definition and structure Amino acids can be classified as α-, β-, γ-, etc., according to the relative position of the functional groups. About 700 natural amino acids are known, although by this name we generally refer to the 20 α-amino acids that make up peptides and proteins. Zwitterion α-amino acid β-amino acid γ –amino acid Neutral form L-alanine γ-aminobutyric acid (GABA) ε-aminocaproic acid One of the 20 α-amino acids Neurotransmitter Constituent of Nylon-6 that constitute proteins Do not distribute, photocopy or use this material for other purposes than individual study. 10  Amino acids: definition and structure Some amino acids are named substitutively as carboxylic acids with amino substituents. The 20 α-amino acids that form proteins are known by widely accepted traditional names. All of these α-amino acids, with the exception of proline, have the same general structure, differing only in the identity of the side chain R. Proline is the only naturally occurring amino acid with a secondary amino group. Do not distribute, photocopy or use this material for other purposes than individual study. 11  Amino acids: definition and structure According to the nature of the R side chain, the 20 natural α-amino acids can be classified as follows: 1. Apolar amino acids 2. Polar amino acids 3. Acidic amino acids 4. Basic amino acids The variety of these substituents determines the structural differences of proteins and their high functional diversity. Do not distribute, photocopy or use this material for other purposes than individual study. 12  Amino acids: definition and structure Apolar amino acids (with aliphatic side chain). Glycine Leucine* Alanine Isoleucine* Valine* * Essential amino acids: they cannot be synthesized by the organism and must therefore come from the diet. Do not distribute, photocopy or use this material for other purposes than individual study. 13  Amino acids: definition and structure Apolar amino acids. Sulfur-containing side chain Phenylalanine* Methionine* Aromatic groups Tryptophan* Trp pyrrolidine indole Heterocycles * Essential amino acids: they cannot be Proline synthesized by the organism and must + H2 therefore come from the diet. Do not distribute, photocopy or use this material for other purposes than individual study. 14  Amino acids: definition and structure Polar amino acids. Hydroxyl-containing side chain Amide-containing side chain Serine Asparagine Threonine* Glutamine Sulfur-containing side chain Tyrosine Cysteine * Essential amino acids Do not distribute, photocopy or use this material for other purposes than individual study. 15  Amino acids: definition and structure Acidic amino acids. Basic amino acids. Aspartate Lysine* (aspartic acid) Guanidine group Glutamate Glu Arginine* (glutamic acid) imidazole (aromatic heterocycle) Histidine* His * Essential amino acids Do not distribute, photocopy or use this material for other purposes than individual study. 16  Aromatic compounds Aromatic compounds are cyclic unsaturated molecules (i.e. containing C=C double bonds). These double bonds are conjugated (i.e. the double bonds are separated by a single bond). Conjugation increases the stability of the molecule. different chemical reactivity and spectral properties Do not distribute, photocopy or use this material for other purposes than individual study. 17  Aromatic compounds Aromatic compounds are cyclic unsaturated molecules (i.e. containing C=C double bonds). These double bonds are conjugated (i.e. the double bonds are separated by a single bond). Conjugation increases the stability of the molecule. However, aromatic compounds have characteristic reactivity and unusual stability (more than what could be accounted for by simple conjugation). The special stability afforded by this planar cyclic array is called aromaticity. Hückel’s rule: a planar cyclic conjugated system is aromatic if the number of π electrons is 4n + 2, where n is an integer (0, 1, 2, 3, etc). Benzene, with six π electrons, is aromatic (n = 1, 4n + 2 = 6). Cyclooctatetraene, with eight π electrons, is not. Do not distribute, photocopy or use this material for other purposes than individual study. 18  Aromatic compounds Aromatic heterocycles: unsaturated cyclic compounds containing atoms other than carbon (e.g. nitrogen, nitrogen provides oxygen or sulfur) that are aromatic. two of the six π electrons Aromatic compounds can also feature fused rings. 10 π electron 14 π electron 10 π electron system system system Heteroaromatic rings found in amino acids. indole imidazole 10 π electron system 6 π electron system Do not distribute, photocopy or use this material for other purposes than individual study. 19  Amino acids: definition and structure Stereochemistry of α-amino acids. The α carbon of all natural amino acids, except glycine, is an asymmetric carbon → 19 of the 20 amino acids previously mentioned can exist as pairs of enantiomers. Most amino acids found in nature belong to the L-stereoisomeric series according to Fischer's definition. Chiral carbon (asymmetric carbon) Fischer projection R/S notation * * Alanine (S)-Alanine (R)-Alanine (in glycine R = H, therefore it is achiral) Do not distribute, photocopy or use this material for other purposes than individual study. 20  Amino acids: definition and structure Stereochemistry of α-amino acids: the D,L-system. An L amino acid has the amino group on the left and the hydrogen on the right when the carboxylic acid group is up and the side chain is down in a Fischer projection of the α-carbon. L-Threonine Projection of a tetrahedral molecule onto a planar surface. Do not distribute, photocopy or use this material for other purposes than individual study. 21  Amino acids: definition and structure Fischer projection: the main chain should be drawn vertically, with the group in the highest oxidation state at the top. The two substituents on the chiral carbons should be placed on horizontal bonds to the right or left of the chain and, by convention, they project toward the viewer. Glyceraldehyde General amino acid Threonine Glucose Projection of a tetrahedral molecule onto a planar surface. Do not distribute, photocopy or use this material for other purposes than individual study. 22  Amino acids: definition and structure The stereoscriptors D- (from Latin dexter, right) and L- (Latin laevus, left) are used to describe the configuration of α-amino acids and sugars. * * The simplest polyhydroxylated aldehyde is 2,3- dihydroxypropanal (glyceraldehyde), which has a single chiral center and exists as two enantiomers. D-(+)-glyceraldehyde L-(−)-glyceraldehyde (R)-(+)-glyceraldehyde (S)-(−)- glyceraldehyde All carbohydrates whose chiral carbon farthest from In amino acids, the D- or L- configuration is the carbonyl group has the same configuration as D- assigned based on the position of the amino group (+)-glyceraldehyde belong to the D-series. on the first chiral carbon (α-carbon) * The –OH group on the The –NH2 group on the last chiral carbon is on * first chiral carbon is on the right. The sugar the left. The amino acid has configuration D. has configuration L. D-threose L-threonine Most natural carbohydrates belongs to the D-series. Most natural amino acids belongs to the L-series. Do not distribute, photocopy or use this material for other purposes than individual study. 25  Amino acids: definition and structure Absolute configuration is attributed according to the Cahn, Ingold and Prelog rules. (R)/(S) descriptors (S)-Alanine (R)-Alanine Fischer projection D/L descriptors L-Alanine D-Alanine Do not distribute, photocopy or use this material for other purposes than individual study. 26  Absolute configuration The Cahn, Ingold, and Prelog protocol works as follows: Assigning priorities in complex situations. What do we do in situations where a chiral center has two or more identical atoms attached? How do we break ties? What if the #4 substituent is not pointing away from the viewer? What if it’s in the “front” (attached to a “wedged” bond) or in the plane of the page? Do not distribute, photocopy or use this material for other purposes than individual study. 27  Absolute configuration Assigning priorities in complex situations. Do not distribute, photocopy or use this material for other purposes than individual study. 28  Absolute configuration Assigning priorities in complex situations. Do not distribute, photocopy or use this material for other purposes than individual study. 29  Absolute configuration Assigning priorities in complex situations: multiple bonds. Do not distribute, photocopy or use this material for other purposes than individual study. 30  Absolute configuration What if the #4 substituent is not pointing away from the viewer? Case A: the #4 substituent is in the front. There are two ways to determine the (R)/(S) in this case. 1. Rotation: “simply” rotate the molecule in your head so that the #4 is pointing away. Do not distribute, photocopy or use this material for other purposes than individual study. 31  Absolute configuration What if the #4 substituent is not pointing away from the viewer? Case A: the #4 substituent is in the front. There are two ways to determine the (R)/(S) in this case. 2. Use the “opposite rule”: when the #4 priority is on a wedge you can just reverse the rules → Clockwise = S; Counterclockwise = R Do not distribute, photocopy or use this material for other purposes than individual study. 32  Absolute configuration What if the #4 substituent is not pointing away from the viewer? Case B: the #4 substituent is in the plane. Also in this case we can rotate the molecule, or use another approach. This last is based on the single swap concept: swapping any two groups on a chiral centre will flip the configuration of the chiral centre from R to S (and vice versa). Do not distribute, photocopy or use this material for other purposes than individual study. 33  Absolute configuration What if the #4 substituent is not pointing away from the viewer? Case B: the #4 substituent is in the plane. Also in this case we can rotate the molecule, or use another approach. This last is based on the single swap concept: swapping any two groups on a chiral centre will flip the configuration of the chiral centre from R to S (and vice versa). Do not distribute, photocopy or use this material for other purposes than individual study. 34  Amino acids: definition and structure Absolute configuration is attributed according to the Cahn, Ingold and Prelog rules. (S) (S) (S) (S) (S) (R) (R) The (+) and (−) symbols refer to the direction of the rotation of the polarized light, dextrorotatory and laevorotatory respectively. The letters (R) and (S) refer to the absolute configuration of the chiral center. The letters D and L refer to the steric series according to Fischer’s definition for carbohydrates and amino acids. Do not distribute, photocopy or use this material for other purposes than individual study. 35 Chirality Indicate the chiral center(s) with a star. * * * * * * * * Do not distribute, photocopy or use this material for other purposes than individual study. 36 Chirality Indicate the chiral center(s) with a star. Do not distribute, photocopy or use this material for other purposes than individual study. 37  Amino acids: acid-base properties Amino acids contain (at least) one weakly acidic group (i.e., the carboxylic acid) and one weakly basic group (i.e., the amine), and, depending on the nature of the side chain R, they may contain more. In the Brønsted–Lowry definition: an acid is a substance that will donate a proton (H+); a base is a substance that will accept a proton (H+). Do not distribute, photocopy or use this material for other purposes than individual study. 38  Amino acids: acid-base properties Amino acids contain (at least) one weakly acidic group (i.e., the carboxylic acid) and one weakly basic group (i.e., the amine), and, depending on the nature of the side chain R, they may contain more. Both the amine and carboxyl groups of an amino acid are more acidic than they would be if they were alone in the molecule, due to the electron-withdrawing inductive effect they exert on each other. Acetic acid Chloroacetic acid Dichloroacetic acid Amino acid pKa 4.7 pKa 2.9 pKa 1.3 Ethylamine Amino acid Do not distribute, photocopy or use this material for other purposes than individual study. 39  Amino acids: acid-base properties For the ionization of the acid HA in water, the equilibrium constant K is given by the formula: Since the concentration of water is essentially constant in aqueous solution, we can define a new constant Ka (acidity constant): The Ka indicates the strength of an acid: strong acids produce high concentration of H3O+ and therefore have high Ka values. Since these values range from very small (e.g. CH3CO2H Ka = 1.76 × 10−5) to very high (e.g. HCl Ka = 107) the Ka is usually expressed in the logarithmic form pKa: a difference of n pKa units indicates a 10n-fold difference in acidity! Do not distribute, photocopy or use this material for other purposes than individual study. 40 Decreasing acidity Amino acids: acid-base properties Decreasing acidity Do not distribute, photocopy or use this material for other purposes than individual study. 41 Amino acids: acid-base properties pKa values can be used to predict: nucleophilicity whether a reagent is a good leaving Decreasing acidity group, the percentage of ionization of a compound under particular conditions and, therefore, its solubility, absorption, excretion, etc. Do not distribute, photocopy or use this material for other purposes than individual study. 42  Amino acids: acid-base properties The functional groups of an amino acid can be in neutral or charged (ionized) form depending on the pH of the medium. Henderson-Hasselbalch equation: it is used to calculate the amount of ionized form of an acid or base present at a given pH, provided we know the pKa. If we take negative logarithms of both sides, we get Henderson–Hasselbalch equation Do not distribute, photocopy or use this material for other purposes than individual study. 43 Amino acids: acid-base properties ▪ the pKa of an acid is the pH at which HA it is exactly half dissociated, since A− log 1 = 0 ▪ As we increase the pH, the acid becomes more ionized; as we lower the pH, the acid becomes less ionized. ▪ A shift in pH by one unit to either side of the pKa value changes the ratio of ionized to non-ionized forms by a factor of 10. Do not distribute, photocopy or use this material for other purposes than individual study. 44  Amino acids: acid-base properties The ionization of amino acids at pH 7 The carboxylic acid groups of amino acids have pKa values in a range from about 1.8 to 2.6. Consider a typical carboxylic acid group with pKa 2.0. The carboxylic acid group of an amino acid can be considered as completely ionized in solution at pH 7.0. Consider the amino group in α-amino acids: the pKa values of the conjugate acids range from about 8.8 to 10.8. We shall consider a typical group with pKa 10.0. Do not distribute, photocopy or use this material for other purposes than individual study. 45  Amino acids: acid-base properties Some amino acids have additional ionizable groups in their side-chains. These may be (potentially) acidic (aspartic acid, glutamic acid, tyrosine, cysteine), or basic (lysine, arginine, histidine). Calculations for the basic side-chain groups of arginine (pKa 12.48) and lysine (pKa 10.52), and the acidic side- chains of aspartic acid (pKa 3.65) and glutamic acid (pKa 4.25) show essentially complete ionization at pH 7.0. However, for cysteine (pKa of the thiol group 10.29) and for tyrosine (pKa of the phenol group 10.06) there will be negligible ionization at pH 7.0. For cysteine The heterocyclic side-chain of histidine is partially ionized at pH 7.0 0.1 Do not distribute, photocopy or use this material for other purposes than individual study. 46 Amino acids: acid-base properties The ionic states at pH 7.0 of amino acids with ionizable side-chains Do not distribute, photocopy or use this material for other purposes than individual study. 47  Amino acids: acid-base properties The functional groups of an amino acid can be in neutral or charged (ionized) form depending on the pH of the medium. For example, alanine can exist in three different forms depending on the pH of the solution. The two pKa values of alanine are 2.34 and 9.69. zwitterion Alanine with charge +1 Alanine with charge 0 Alanine with charge −1 pH 0 pH 6.01 pH 14 Around pH 2.34 the Around pH 9.69 the ammonium carboxylic acid loses its H+ group loses its H+ Titration of alanine by addition of a strong base, starting from a pH 0 medium. Do not distribute, photocopy or use this material for other purposes than individual study. 48  Amino acids: acid-base properties Isoelectric point (pI): the pH at which the concentration of the zwitterion is a maximum (the total charge is zero). Under these conditions, the amino acid has its lowest solubility in water and it does not migrate if placed in an electric field (electrophoresis). For all the amino acids that do not contain an ionizable group in the side chain, the pI corresponds to the mean of the two pKa values. Titration of alanine by addition of a strong base, starting from a pH 0 medium. Do not distribute, photocopy or use this material for other purposes than individual study. 49  Amino acids: acid-base properties Amino acids bearing an acidic or basic group on the side chain can exist in four different forms. For example, lysine is a basic amino acid with three ionizable groups: the carboxylic acid (pKa1 = 2.18), the amine group in α position (pKa2 = 8.95), the amine group in ε (pKa3 = 10.79). L-lysine Do not distribute, photocopy or use this material for other purposes than individual study. 50  Amino acids: acid-base properties Lysine with charge +2 Lysine with charge +1 Lysine with charge 0 Lysine with charge −1 pH 0 pH 5.57 pH 9.87 pH 14 Around pH 2.18 the Around pH 8.95 the α Around pH 10.79 the ε carboxylic acid loses its H+ ammonium group loses its H+ ammonium group loses its H+ Also in this case, the pI is the pH at which the molecule has total charge zero. For amino acids with a basic side chain, the pI corresponds to the mean of pKa2 and pKa3 values. Do not distribute, photocopy or use this material for other purposes than individual study. 51  Amino acids: acid-base properties pKa1 2.09 Aspartic acid is an acidic amino acid with three ionizable groups: the pKa2 3.86 carboxylic acid (pKa1 = 2.09), the carboxyl on the side chain (pKa2 = 3.86) and the amino group in α (pKa3 = 9.82),. pKa3 9.82 pKa2 3.86 pKa3 9.82 Aspartic acid Aspartic acid Aspartic acid Aspartic acid with charge +1 with charge 0 with charge -1 with charge -2 pH 0 pH 2.97 pH 6.84 pH 14 At pH 2.09 the α carboxyl At pH 3.86 the second At pH 9.82 the ammonium loses its H+ carboxyl loses its H+ group loses its H+ For the amino acids with an acidic side chain, the pI corresponds to the mean of 2,09 + 3,86 5,95 pI = = = 2.98 the pKa1 and pKa2 values. 2 2 Do not distribute, photocopy or use this material for other purposes than individual study. 52 Amino acids: acid-base properties Do not distribute, photocopy or use this material for other purposes than individual study. 53 Amino acids: acid-base properties Do not distribute, photocopy or use this material for other purposes than individual study. 54 Amino acids: acid-base properties Do not distribute, photocopy or use this material for other purposes than individual study. 55 Amino acids: acid-base properties Do not distribute, photocopy or use this material for other purposes than individual study. 56 Amino acids: acylation and esterification Amino acids undergo many of the reactions characteristic of both amines and carboxylic acids. Acylation is an example of reaction involving the amine group. Amino acids, like ordinary carboxylic acids, are easily esterified by heating with an alcohol and a strong acid catalyst (acid-catalyzed esterification). Do not distribute, photocopy or use this material for other purposes than individual study. 57  Peptides Amino acids can bond with each other to form an amide between the amino group of one amino acid and the carboxyl group of another. This bond is called a peptide bond. In this way, long chains called proteins can be formed, containing sequences of hundreds or even thousands of amino acids. Shorter chains of amino acids are called peptides. Peptide bonds N-terminus C-terminus Amino acids tripeptide Oligopeptides: 2 – 7 amino acids Insulin (a protein hormone made up of 51 amino acids that Polypeptides: 8 – 49 amino acids prompts cells to absorb glucose from the blood) is often Proteins: > 50 amino acids referred to as the smallest protein. Do not distribute, photocopy or use this material for other purposes than individual study. 58  Peptides By convention, polypeptide chains are written horizontally with the N-terminus (the amino acid with the free α-amine) on the left and the C-terminus (the amino acid with the free α-carboxyl group) on the right. Met-enkephalin is a natural pentapeptide that modulates the transmission of pain signals and an endogenous ligand of the δ-opioid receptor. The aminoacidic sequence is tyrosine-glycine-phenylalanine-methionine. The numbering starts from the left (N-terminus) and the sequence can be written more concisely using the three- letters abbreviations (Tyr-Gly-Gly-Phe-Met) or the one-letter abbreviation (YGGFM). Do not distribute, photocopy or use this material for other purposes than individual study. 59  Peptides Which of these two structures can be represented as Ser-Phe-Ala? Ala-Phe-Ser Ser-Phe-Ala H-Ala-Phe-Ser-OH H-Ser-Phe-Ala-OH Ala→Phe→Ser Ser→Phe→Ala AFS SFA Sometimes, the termini identities are emphasized by showing H– for the amino group and –OH for the carboxyl group. Some peptides are cyclic, and this convention can have no significance, so arrows are incorporated into the sequence to indicate peptide bonds in the direction CO→NH. Do not distribute, photocopy or use this material for other purposes than individual study. 60  Peptides Glutathione is a tripeptide and a natural antioxidant found in plants, animals, fungi, and some bacteria. It has an amide linkage that involves the γ-carboxyl of glutamic acid rather than a normal amide bond utilizing the C-1 carboxyl group. β α γ γ-Glu−Cys−Gly Do not distribute, photocopy or use this material for other purposes than individual study. 61  Peptides Peptides and proteins may be hydrolyzed to their constituent amino acids by either acid or base hydrolysis. However, the amide bond is quite resistant to hydrolytic conditions, an important feature for natural proteins. Hydrolysis of peptides and proteins, therefore, requires heating with quite concentrated strong acid or strong base. This can lead to degradation of some of the constituent amino acids that are sensitive to these reagents. Do not distribute, photocopy or use this material for other purposes than individual study. 62  Peptides A much milder and more selective alternative for peptide hydrolysis is to use specific enzymes, called proteases, peptidases, or proteolytic enzymes. One of the most widely used proteases is the enzyme trypsin, a digestive enzyme which biological role is to catalyze the hydrolytic breakdown of dietary proteins in the intestinal tract. Trypsin catalyzes the hydrolysis of peptides or proteins at the carbonyl group of arginine or lysine residues, provided the following conditions: 1. these residues are not at the amino end of the protein 2. they are not followed by a proline residue Because trypsin catalyzes the hydrolysis of peptides at internal rather than terminal residues, it is called an endopeptidase. Do not distribute, photocopy or use this material for other purposes than individual study. 63  Peptides The peptide bond is planar: resonance of the nitrogen with the carbonyl gives 40% of double bond character to the C-N bond, hampering free rotation about this bond (it would break the alignment of p orbitals of the π system). The α carbons, due to steric hinderance, have trans configuration respect to the C=N double bond. Proline is an exception, as it can assume both cis and trans configuration. α carbons are trans-configured respect to the C=N bond The protein chain is composed of a succession of planar ‘’blocks’’ that can only bend at the α carbons. Do not distribute, photocopy or use this material for other purposes than individual study. 64  Resonance Resonance is a way of describing delocalized electrons within certain molecules where the bonding cannot be expressed by a single Lewis formula. A molecule with such delocalized electrons is represented by several resonance structures. The nuclear skeleton of the Lewis Structure of these resonance structures remains the same, only the electron locations differ. The combination of possible resonance structures is defined as a resonance hybrid. Molecules with multiple resonance structures are more stable than one with fewer, and some resonance structures contribute more to the stability of the molecule than others. Do not distribute, photocopy or use this material for other purposes than individual study. 65  Resonance Delocalization and resonance structures rules: 1. Resonance structures should have the same number of electrons. 2. Each resonance structures follows the rules of writing Lewis Structures. 3. The hybridization of the structure must stay the same. 4. The skeleton of the structure can not be changed (only the electrons move). 5. Resonance structures must also have the same number of lone pairs. Resonance of the arginine’s guanidyl group (protonated form) Resonance of the imidazole group of histidine (non-protonated form) Do not distribute, photocopy or use this material for other purposes than individual study. 66  Resonance Curly arrows are useful for predicting the outcome of chemical reactions, following bond making and bond breaking processes, and to highlight movement of electrons in resonance forms. The curly arrow represents the movement of two electrons. The tail of the arrow indicates where the electrons are coming from, and the arrowhead where they are going to. Curly arrows must start from an electron-rich species. This can be a negative charge, a lone pair, or a bond. Arrowheads must be directed towards an electron- deficient species. This can be a positive charge, the positive end of a polarized bond, or a suitable atom capable of accepting electrons, i.e. an electronegative atom. α carbons are trans-configured respect to the C=N bond Do not distribute, photocopy or use this material for other purposes than individual study. 67