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Lecture 4

lecture 4.odt

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University of Toronto St. George
Michael Baker

Lecture 4 – we will see how glucose is a good energy source – we will talk about: – Monosacchrides – cyclization reactions of common forms of sugars – sugar derivatives – and ultimately higher forms that have two or more monosacchrides linked in chain – any glucose that you eat that your body doesn't eat will change into fat Roles of carbohydrates 1. Energy source – fuels (simple sugars like glucose) - too little sugar in blood= hypoglycemia 2. Storage form of chemical energy (high molecular weight sugar polymers 3. source of C in synthesis of other molecules (sugar into fat) 4. structural elements in cells and tissues- paper; cellulose, cotton 5. components of nucleotides, nucleic acids, glycoproteins and glycolipids Classes of carbohydrates Monosacchrides – simple sugar with one polyhydroxy aldehyde or ketone – most sugars end in ose – lactose, sucrose, amylose, galactose, glucose (most sugar endings were ase) – general formula monosacchrise: (CH2O)n where n could be 3 -7 – minimum 3 carbons and maximum 7 carbons in the monosacchride sugar – Carbohydrate – for each carbon there is a hydrate (H2O)- for most cases – C6H12O6 – glucose – above 2 mm to be in consciousness – each carbon on a straight chain has a hydroxy group (OH) except for one which is either an aldehyde (first carbon) or a ketone group (second carbon) – aldehyde group – aldose sugar – ketone group- ketose sugar – mono sacchride – white crystalline solids – freely water soluble with a sweet tast – sugars can be defined by a number of carbons it has – triose= 3 carbon; pentose - 5; hexose=6 – there could be aldotriose and ketotriose (dihydroxyacetone) – two forms of aldo triose ( glyceraldehyde) – L and D form- the OH group on the middle carbon – if left side = L – if right side= D – they can vary with respect to sweetness- fructose much sweeter so used by manufacturers – if too much sugar in the blood – proteins and hemoglobin reacts with it- interaction of high sugar levels with tissues = diabetes Stereoisomers – same number of functional groups – cannot be super imposed – ALL monosacchrides have one or more chiral centres except for dihydroxyacetone ( has no single carbon with 4 carbons around it) – tetrahedral geometry for stereocentres (fisher projection) Rotation of plane polarized light – chirality produces interesting optical projections – chiral centres rotate plane polarized light – if you wear sunglasses, you realize that it only allows light vibrating in one plane to come in your eye hence reducing glare – light usually come in a number of different planes – if you take a polarized light in one plane and shine it down in a solution containing stereoisomer- it cranks the plane to the right or left – if you have d-glucose- cranks the light to the right Aldohexoses – 6 carbons with aldehyde group 6 carbon 1 – 4 chiral carbons in a straight chain – 4 chiral centres -each exist in 2 – L or D – 2*2*2*2 = 16 – memorize three of them – 8 D stereoisomers come from carbon 5 (furthest chiral centre away from the aldehyde) – last asymetric centre – the hydroxyl on the right – D sugars are the ones biologically active – 3 most common D-glucose; D- mannose; D-galactose – D-glucose = biologically active form of glucose – D-glucose and D-mannose are epimers at C-2 – epimers = stereoisomer that differ only at one chiral carbon – glucose has the OH on the right side; mannose has it on the left side – D glucose and D galactose – are epimers at carbon 4 – glucose has the OH on the right side; galactose on the left side Enantiomers – complete mirror images – D and L form are mirror images of each other Riboses- aldo pentose – of the 5 carbons,3 are chiral – 8 stereoisomers (2^3) – 4 Ds and 4Ls – Rememember Ketohexose – 3 chiral carbons again – 8 stereoisomers Aldehyde – very reactive – Too much blood sugar- hyperglycemia (prolonged blood sugar) aldehyde react with amino groups- problem – aldehyde interacts with hydroxyl groups (alcohol) as well – after the aldehyde group interacts it alcohol it forms a prochiral carbon 1- then C 1 interacts with carbon 5 to form a cyclic structure – depending on what side you go at you can form an alpha or a beta structure- this is only possible because carbon will now become chiral Hemiacetal – you form a new chiral carbon as a result of this cyclization reaction – it forms a pentose (6 is a stretch) – hemicacetal comes and defines a pyranose ring – pyrane ring – 5 carbons and an oxygen – new chiral centre – C1 = anomeric isomers Recall: Enantiomers – 2 mirror images of each other – L and D form Epimers – differ only at one carbon D-glucose and D- galactose Anomers – carbon 1 could be axial (below the plane) or equitorial (up)- alpha and beta form respectively Hayworth projections – rings are drawn flat- 2D structure – alpha D glucose (glucopyronose) and beta D glucose (glucopyronose) = hemiacetals Potential experiment – if you have alpha D glucose – dissolve in water – you do have alpha D glucose – very common form used anobalically? to give you energy – alpha D glucose has a rotation of polarized light of 112 degrees – Do optical rotation to confirm – put in a polarimeter- which shines the polarized light by grading one plane – it cranks it to the right direction (+) 112 degrees – you come back in a couple of hours – repeat the experiment – it's 88 degrees now- getting lower – It's because the cyclic structure can open up – and go to the other side – it passes in between through a straigh chain form – equilibrium between alpha and beta form if dissolved in water – this is called mutarotation – Beta is more stable than alpha–2/3 will appear to be in beta form in the solution and 1/3 in alpha Hemiketals – fructose = ketohexose – ketone group at C2 – C2 and C5 come together – makes C2 a chiral carbon as a result of cyclization – which is attacted by carbon 5 hydroxy group – in this case both C1 and C 6 are outside the ring (only C6 is outside the ring for hemiacetal) – again two different ways of cyclization- – furane ring- 4 carbons and 2 oxygens – looks like an organic molecule called furane – the smell coming from corn boiling = furane – again 5
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