Lecture 25 Carbohydrates.docx

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Michael Baker

BCH210H © Lisa | Page 12 Lecture 25: Carbohydrates  CHO provides NRG  fuel, ATP  monosaccharides consist of one polyhydroxy aldehyde (ketone)  sugaes = -ose Monosaccharides  each C has an –OH  except for 1 which is either an aldehyde (-CHO) or a ketone (C=O)  chiral—can exist in stereoisomeric forms  white crystalline solids, freely water soluble, sweet taste (variable) Stereoisomers  have same # fxal groups  only orientation/distribution of groups around asymmetric C are diff  can’t be superimposed  all monosaccharides have at least 1 or more chiral centres except dihydroxyacetone  solns of chiral molecules will rotate the polarized light  (sunglasses only allow light vibrating at one plane to come into eyes, resulting in reduction of glare)  glare results in vibration of light in multiple planes  if shine polarized light vibrating in one plane through a soln of a stereoisomer, it will turn plane to the right or left Aldohexoses  6Cs  aldehyde group at C-1  D-sugars are defined by having the –OH of the last chiral centre on the right  they are the biologically active sugars  D-glucose is the biologically active form of glucose  D-glucose and D-mannose are epimers  they are stereoisomers that only differ at C-2 (at 1 chiral C)  D-glucose and D-galactose are epimers at C-4  enantiomers are mirror images, epimers differ at one  D-ribose and D-fructose have 3 chiral centres, 8 stereoisomers  4Ds and 4Ls  sugars are relatively reactive molecules  aldehydes are reactive groups  straight chain glucose  aldehyde can interact w –OH or –NH 3  C-1 can interact w C-5 to form a cyclic structure (can interact 6on both sides—2 possibilities, 2 stereoisomeric forms) BCH210H © Lisa| Page 212  C-1 aldehyde reacts with C-5 alcohol to form a hemiacetal  hemiacetal is chiral at C-1 o thus, formation of new chiral centre  hemiacetal defines a pyranose ring  alcohol attacks C-1 aldehyde  results in 2 posibilities  pyran ring—ring w 5C and 1O  anomeric carbon—the new chiral centre that is formed  anomers—plane orientation differs at new chiral centre  β— -OH is above the plane of the ring  α— -OH is below the plane of the ring  α & β infront of the name indicates an anomer, a cyclic structure  α-D-glucose and α-D-glucopyranose are the same thing  α-D-glucose has a rotation of polarized light of 112°  cyclic structures can open up—hemiacetal can go back into straight chain form and then form the other anomer
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