《食品化学 Food Chemistry》课程教学课件（讲稿，英文版）Carbohydrates.pdf_大学文库

Carbohydrates z Polyhydroxy compounds (poly-alcohols) that contain a carbonyl (C=O) group z Elemental composition C x(H 2O) y z About 80% of human caloric intake z >90% dry matter of plants z Functional properties – Sweetness – Chemical reactivity – Polymer functionality

Types of Carbohydrates

Monosaccharides z Monosaccharides are categorized by the number of carbons (typically 3-8) and whether an aldehyde or ketone z Most abundant monosaccharides are hexoses (6 carbons) z Most monosaccharides are aldehydes, i.e. aldoses C C H O H OH C C O H H OH aldehyde ketone

Fisher projections H OH H OH H OH HO H C C C C C C H 2OH H O H OH H OH O HO H C C C C C H OH 2 C H OH 2 1 2 3 4 5 6 D-glucose (an aldohexose) D-fructose (an ketohexose) 1 2 3 4 5 6

Six carbons =0 H H H H 0 H H- OH HO-C-H H-C-OH HO-C-H H-C-OH HO-C-H H-C -OH HO-C-H H-C-OH H-C-OH HO- -C-H HO-C-H H-C-OH H-C-OH HO- C-H HO-C-H H-C-OH H-C-OH H-C-OH H-C-OH HO-C-H HO-C-H HO-C-H HO-C-H H-C-OH H-C-OH H-C-OH H-C-OH H-C一OH H-C-OH H-C -OH H-C一OH CH2OH CH2OH CH2OH CH2OH CH2OH CH2OH CHOH CH2OH D-Allose D-Altrose D-Glucose D-Mannose D-Gulose D-Idose D-Galactose D-Talose D-Aldoses (a)

Six carbons CH2OH CH2OH CH2OH CH2OH C=0 C=0 C=0 C=0 H-C-OH HO-C-H H-C-OH HO-C-H H-C-OH H-C-OH HO-C-H HO-C-H H-C-OH H-C-OH H-C-OH H-C- OH CH2OH CH2OH CH2OH CH2OH D-Psicose D-Fructose D-Sorbose D-Tagatose D-Ketoses (b)

Cyclic Forms z Lowest energy state H C C C O C C OH HO OH HO H H H CH2OH H 1 2 3 4 5 6 β-D-glucopyranose (glucose) —an aldose a hexose an aldohexose —C1 chair conformation β-D-fructopyranose (fructose) —a ketose a hexulose a ketohexose — 1C chair conformation C C O C C H H C OH H H OH OH OH CH2OH 1 2 3 4 5 6 H

Ring Nomenclature z pyranose is a six-membered ring (a very stable form due to optimal bond angles) z furanose is a five-membered ring

Chirality z Geometric property of a rigid object (or spatial arrangement of atoms) of being non-super-imposable on its mirror image H OH H OH H OH HO H C C C C C C its mirror image is an "optical H2OH isomer" H O 4 chiral centerse.g. at C2 carbon: This structure has a nonsuperimposable mirror image CHO (CHOH)3CH2OH H OH C2

Isomers z Isomers are molecules that have the same chemical formula but different structures z Stereoisomer differs in the 3-D orientation of atoms z Diastereomers are isomers with > 1 chiral center. – Pairs of isomers that have opposite configurations at one or more of the chiral centers but that are not mirror images of each other. z Epimers are a special type of diastereomer. – Stereoisomers with more than one chiral center which differ in chirality at only one chiral center. – A chemical reaction which causes a change in chirality at one one of many chiral center is called an epimerisation