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Lecture 25 Carbohydrates.docx

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University of Toronto St. George
Michael Baker

BCH210H © Lisa| Page 112 L E C T U R E 2 5 : C A R B O H Y D R A T E S  role of CHO:  NRG source (fuel; simple sugars, ATP)  storage form of chemical NRG (high molecular weight sugar polymers)  source of C in synthesis of other molecules (sugar → fat)  structural elements in cells and tissues (ex. cellulose in plants)  components of nucleotides, nucleic acids, glycoproteins, glycolipids MONOSACCHARIDES  simple sugars that consist of 1 polyhydroxy aldehyde or ketone  general formula: (CH O2 , n = 3—7  considered hydrates of C, H:O ratio = 2:1  ex. glucose, C6H 12 6  each C has a –OH group  except for one which is either an aldehyde (-CHO) or a ketone (C=O)  straight chain forms: aldose, ketose  most are white crystalline solids, freely water soluble, has a sweet taste  glucose is the most abundant monosaccharide, the most imp fuel for many organisms  they are chiral—can exist in stereoisomeric forms  stereoisomers: have same numbers of functional groups, but position of these around an asymmetric C is diff  can’t be superimposed  all monosaccharides have at least 1 or more chiral centres (except dihydroxyacetone)  asymmetric (chiral) Cs have 4 diff groups about it  glyceraldehyde (aldotriose) has 1 chiral C (2 stereoisomers, D- & L-)  D- & L- are mirror images  enantiomers are stereoisomers that are mirror images  solns of chiral molecules will rotate 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 vibration in one plant through a soln of a stereoisomer, it will turn plane to the right or left (hence, D- & L-)  aldohexoses are ssugars w 6Cs and 4 chiral centres 4  each chiral C can have 2 configurations—2 = 16 possible stereoisomers  D-sugars are defined by having the –OH of the last chiral C on the right  they are the biologically active sugars  the most common aldohexoses are D-glucose, D-mannose, D-galactose  D-glucose is the biologically active form of glucose  D-glucose and D-mannose are epimers (stereoisomers that differ at one C) o they are epimers at C2  D-glucose and D-galactose are epimers at C4  a common aldopentose is D-ribose  D-ribose has 3 chiral Cs w 8 steroisomers (4D- & 4L-)  a common ketohexose is D-fructose  D-fructose has 3 chiral Cs w 8 steroisomers (4D- & 4L-)  it has a C=O at C2 BCH210H © Lisa | Page 22 RING STRUCTURES  while pentoses, hexoses, and heptoses can be shown as straight chains, it is more common for them to exist as rings  the aldehyde at C1 of a straight chain D-glucose can interact w the –OH at C5 in a cyclization rxn  creates a new chiral centre at C1  it can interact on both sides—2 possibilities, 2 stereoisomers  the new ring structure is a hemiacetal (pyranose ring)—hemiacetal is chiral at C1  if a ketose is involved , the product is a hemiketal (furanose ring)  pyran ring: ring w 5C and 1O  anomeric carbon: the new chiral centre that is formed  in the ring structure for D-glucopyranose, C1 is the anomeric C  anomers are stereoisomers w a plane orientation differing at the new chiral centre
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