Chemical and Physical Foundations (PDF)

Configurations of Geometric Isomers geometric isomers, or cis-trans isomers = differ in the arrangement of substituent groups with respect to the double bond  Chiral and Achiral Molecules chiral centers = asymmetric carbons a molecule can have 2n stereoisomers, where n is the n...

Configurations of Geometric Isomers geometric isomers, or cis-trans isomers = differ in the arrangement of substituent groups with respect to the double bond  Chiral and Achiral Molecules chiral centers = asymmetric carbons a molecule can have 2n stereoisomers, where n is the number of chiral carbons  Enantiomers and Diastereomers enantiomers = stereoisomers that are mirror images of each other diastereomers = stereoisomers that are not mirror images of each other  Question Molecules that differ in configuration cannot be: A. stereoisomers. B. cis-trans isomers. C. chiral centers that are mirror images of each other. D. chiral centers that can be interchanged by rotation of a single bond.  Question , Response Molecules that differ in configuration cannot be: D. chiral centers that can be interchanged by rotation of a single bond. Configuration describes the fixed spatial arrangement of atoms, conferred by the presence of either (1) double bonds, around which there is little or no freedom of rotation, or (2) chiral centers, around which substituent groups are arranged in a specific orientation. Molecules that differ in configuration cannot be chiral centers that can be interchanged by rotation of a single bond.  Optical Activity of Enantiomers enantiomers have nearly identical chemical reactivities, but differ in optical activity a racemic mixture (equimolar solution of two enantiomers) shows no optical rotation Plane-polarized light is created by passing ordinary light through a polarizing device  Question Enantiomers: A. are only associated with amino acids. B. can be specific types of diastereomers. C. are always designated either D or L. D. can exist for molecules with more than one chiral carbon.  Question , Response Enantiomers: D. can exist for molecules with more than one chiral carbon. A carbon atom with four different substituents is said to be asymmetric, and asymmetric carbons are called chiral centers. A molecule can have 2n stereoisomers, where n is the number of chiral carbons. Stereoisomers that are mirror images of each other are called enantiomers. Enantiomers can exist for molecules that have more than one chiral carbon.  Molecular Conformation conformation = the spatial arrangement of substituent groups that are free to assume different positions in space  Principle Cells use a relatively small set of carbon- based metabolites to create polymeric machines, supramolecular structures, and information repositories. The chemical structure of these components defines their cellular function. The collection of molecules carries out a program, the end result of which is reproduction of the program and self-perpetuation of that collection of molecules—in short, life.  Biological Systems Can Distinguish Stereoisomers stereospecificity = the ability to distinguish between stereoisomers  Interactions between Biomolecules Are Stereospecific FIG. Complementary fit between a macromolecule and a small molecule. A glucose molecule fits into a pocket on the surface of the enzyme hexokinase (PDB ID 3B8A),and is held in this orientation by several noncovalent interactions between the protein and the sugar. This representation of the hexokinase molecule is produced with software that can calculate the shape of the outer surface of a macromolecule, defined either by the van der Waals radii of all the atoms in the molecule or by the “solvent exclusion volume,” the volume a water molecule cannot penetrate. hexokinase = an enz. that phosphorylates hexoses (six-carbon glucose ), forming hexose phosphate. Ie ) G-6-P.  Question The antidepressant medication Celexa is a racemic mixture of two stereoisomers, but only (S)-citalopram has the therapeutic effect. A stereochemically pure preparation of (S)- citalopram is sold under the trade name Lexapro. Which is true about the medications? A. A solution of Lexapro will show no optical activity. B. Biological systems are not able to distinguish between the two stereoisomers. C. The effective dose of Lexapro is one-half the effective dose of Celexa. D. Both stereoisomers are biologically active.  Question , Response The antidepressant medication Celexa is a racemic mixture of two stereoisomers, but only (S)-citalopram has the therapeutic effect. A stereochemically pure preparation of (S)- citalopram is sold under the trade name Lexapro. Which is true about the medications? C. The effective dose of Lexapro is one-half the effective dose of Celexa. Celexa is an equimolar solution of two enantiomers, whereas Lexapro contains only (S)-citalopram. Since only (S)- citalopram is therapeutically active, the effective dose of Lexapro is one-half the effective dose of Celexa. 1.3 Physical Foundations