Redox Reactions and Reducing Sugars PDF

Redox Reactions and Reducing Sugars Aldose Ketose http://commons.wikimedia.org/wiki/File:D-Glucose_vs._D-Fructose_Structural_Formulae_V.1.svg Aldose s Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. Ketoses Fig. Mathews, van Holde, and Ahern...

Redox Reactions and Reducing Sugars Aldose Ketose http://commons.wikimedia.org/wiki/File:D-Glucose_vs._D-Fructose_Structural_Formulae_V.1.svg Aldose s Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. Ketoses Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. http://www.rpi.edu /dept/chem-eng/Bi otech-Environ/FUN DAMNT/hemiacet. htm & Mathews, van Holde, and Ahern Biochemistry 3rd edition. The two possible anomers are referred to as  and .  and  forms can interconvert when the ring opensMathews, van Holde, and Ahern Biochemistry 3rd In formation of glycosidic linkages: The hydrogen of the –OH group of the anomeric carbon is replaced with another group is said to form a glycosidic bond, and the molecules formed are collectively known as glycosides. (The hemiacetal or hemiketal of the sugar is converted to an acetal or ketal in this type of substitution.) 1.Monosaccharides (“Simple sugars”). 2.Disaccharides – Two linked monosaccharides. 3.Oligosaccharides – Three to “several” linked monosaccharides. 4.Polysaccharides – Large numbers of linked monosaccharides (>10-20). Because starch is formed from the  anomer, both bonds are on the same side of the ring and so a spiral structure results. Figs. Mathews, van Holde, and Ahern Biochemistry 3rd edition.  Fig. 5-6 Chloroplast Starch Mitochondria Glycogen granules 0.5 µm 1 µm Amylose Glycogen Amylopectin (a) Starch: a plant polysaccharide (b) Glycogen: an animal polysaccharide Cellulose is poly--(1-4)-glucose. Because of this, the two linkages are on opposite sides of the ring, so that a ribbon structure results. http:// www.fibersource.com/f- tutor/cellulose.htm Figs. Mathews, van Holde, and Ahern Biochemistry 3rd edition. M.C. McCann, B. Wells & K. Roberts (1990) Journal of Cell Science 96: 33-334 Cellulose is a structural carbohydrate, utilised by nearly all plants and some fungi. It is the most common macromolecule on Earth. Hydrogen bonding between adjacent ribbons leads to formation of ordered bundles of parallel cellulose molecules called microfibrils. Individual microfibrils have been estimated to have a Young’s modulus of 140 GPa. Steel has a modulus of 200 Reducing sugars http://jchemed.chem.wisc.edu/jcesoft/CCA/CCA5/MAIN/1ORGANIC/ORG18/TRAM18/B/0595723/ THUMBS.HTM Hydroxyl and aldehyde oxidation A hydroxyl group (C-OH) can be oxidised to an aldehyde or ketone (C=O). An aldehyde can be oxidised to a carboxylic acid (COOH or COO-). In many cases redox reactions involve increase of number of oxygen atoms (oxidation) or addition of hydrogen atoms (reduction). However, the fundamental and essential element of redox reactions is transfer of electrons from the reducing agent to the oxidising agent. OIL RIG Oxidation Is Loss of electrons Reduction Is Gain of electrons For example: Fe3+ + e- ⇌ Fe2+ Note that this equation is not correct. The Iron (III) is the oxidised form and has been reduced to Iron (II). The compound from which the electron has come has been oxidised by loss of the electron. Consider the following reaction: Zn + Cu2+ ⇌ Zn2+ + Cu This can be treated as two “half-cell reactions” i.e. Zn ⇌ Zn2+ + 2e- Cu2+ + 2e- ⇌ Cu Note: The notation here is not exactly correct. Zn ⇌ Zn2+ + 2e- involves OXIDATION (i.e. Loss of Electrons) Cu2+ + 2e- ⇌ Cu involves REDUCTION (i.e. Gain of Electrons)  OIL RIG Oxidation Is Loss of electrons Reduction Is Gain of electrons Electrons cannot disappear Or appear from nowhere. Therefore oxidation and reduction processes MUST be paired. An oxidation reaction requires a reduction reaction and vice versa. During redox reactions, OXIDISING AGENTS BECOME REDUCED and REDUCING AGENTS BECOME OXIDISED. Examples of uses of redox reactions: Biological importance of redox reactions 1. Many important biochemical reactions involve transfer of electrons.  Energy Stored -CH2 ↔ -CHOH ↔ -CHO ↔ -COOH ↔ CO2 Reduced Oxidised Energy released C6H12O6 + 6 O2 → 6 CO2 + 6 H2O  Half Cell Reactions + 2H+ + 2e- → Note that these are the half cell reactions in their correct format and so the oxidations will go in the opposite direction. http://en.wikipedia.org Reduction of ubiquinone http://en.wikipedia.org/wiki/Oxidative_phosphorylation NADP+ has an extra phosphate group on the ADP. This allows enzymes to distinguish it from NAD+ but otherwise makes little difference. http://en.wikipedia.org/wiki/File:NAD_oxidation_reduction.svg "Citric acid cycle with aconitate 2" by Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins - http://biocyc.org/META/NEW-IMAGE?type=PATHWAY&object=TCA. Image adapted from :Image:Citric acid cycle noi.svg|(uploaded to Commons by wadester16). Licensed under CC BY-SA 3.0 via Commons - https://commons.wikimedia.org/wiki/File:Citric_acid_cycle_with_aconitate_2.svg#/media/ File:Citric_acid_cycle_with_aconitate_2.svg Transition metals often have many stable electron arrangements and therefore multiple oxidation states: Transition Metals Transition metals often have many stable electron arrangements and therefore multiple oxidation states: Many biochemical processes depend on this property. Fe 3+ + e ⇌ Fe - 2+ The Iron (III) is the oxidised form and has been reduced to Iron (II). The compound from which the electron has come has been oxidised by loss of the electron. Arrows indicate first transition series metals with biological roles involving redox reactions. https://en.wikipedia.org/wiki/Oxidative_phosphorylation#/media/ File:Complex_I.svg  F e C u F Mn e Fig. Mathews, van Holde, and Ahern Biochemistry 3rd edition. Redox couples are formally written using this notation: Oxidised + n e- → Reduced Form Form This is the case even if the reaction almost always proceeds from right to left e.g. Na+ + e- → Na Half Cell Reactions + 2H+ + 2e- → Note that these are the half cell reactions in their correct format and so the oxidations will go in the opposite direction. http://en.wikipedia.org  + 2H+ + 2e- → + H2O Note that these are the half cell reactions in their correct format and so the oxidations will go in the opposite direction. http://en.wikipedia.org  Half cell Na+ + e- → Na Zn2+ + 2e- → Zn H+ + e- → ½H2 Cu2+ + 2e- → Cu Fe3+ + e- → Fe2+ Each half cell reaction has an electrode potential. This is the voltage it generates when it is connected to a a standard hydrogen electrode. Strong oxidising agents have high (more positive) potentials and usually go from left to right. Strong reducing agents have low (more negative) potentials and usually go from right to left. Half cell Potentials Half cell Standard Potential Na+ + e- → Na -2.71 V Zn2+ + 2e- → Zn -0.76 V H+ + e- → ½H2 0.00 V Cu2+ + 2e- → Cu +0.34 V Fe3+ + e- → Fe2+ +0.77V ½Cl2 + e- → Cl- +1.36 V  A hydroxyl group (C-OH) can be oxidised to an aldehyde or ketone (C=O). An aldehyde can be oxidised to a carboxylic acid (COOH or COO-).  + H2O ⇌ + 2H+ + 2e- http://en.wikipedia.org Aldose Ketose http://commons.wikimedia.org/wiki/File:D-Glucose_vs._D-Fructose_Structural_Formulae_V.1.svg  + 2H+ + 2e- → + H2O Note that these are the half cell reactions in their correct format and so the oxidations will go in the opposite direction. http://en.wikipedia.org Electrons cannot disappear Or appear from nowhere. Therefore oxidation and reduction processes MUST be paired. An oxidation reaction requires a reduction reaction and vice versa. Reducing sugars http://jchemed.chem.wisc.edu/jcesoft/CCA/CCA5/MAIN/1ORGANIC/ORG18/TRAM18/B/0595723/ THUMBS.HTM http://www.chemie.uni-regensburg.de/Organische_Chemie/Didaktik/Keusch/D-Tollens-e.htm The two possible anomers are referred to as  and .  and  forms can interconvert when the ring opensMathews, van Holde, and Ahern Biochemistry 3rd Reducing sugars The C=O group of aldoses can be oxidised to a COOH group, reducing the ions or molecules with which they react. Cu2+ Cu Under alkaline conditions ketoses can form an aldose via an enediol intermediate. This is a tautomerisation reaction. http://www.chem.ucalgary.ca/courses/351/ Carey5th/Ch25/ch25-2-5.html Hemiacetal formation is readily reversible, opening a sugar’s ring and freeing an aldose to act as a reducing agent. Likewise hemiketals can revert to the original ketone, which can form an aldose under alkaline conditions. Therefore, under the alkaline conditions of Fehling’s, Benedict’s or Tollen’s solutions, aldoses and ketoses act as Unlike hemiacetals and hemiketals, acetal and ketal formation is not readily reversible. Therefore, if the OH group of the hemiacetal or hemiketal group is converted to an O-R group, then the sugar ceases to be a reducing sugar. This is the case for sucrose. http://www.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/hemiacet.htm & Mathews, van Holde, and Ahern Biochemistry 3rd edition. If there is an unsubstituted –OH on an anomeric carbon, the ring can open to give an aldehyde Lactose The sugar is a reducing sugar. http://en.wikipedia.org/wiki/Image:Lactose%28lac %29.png However, if the OH group of the hemiacetal or hemiketal group is converted to an O-R group, (where R is anything other than H) then the ring cannot open to free the aldehyde group and the sugar ceases to be a reducing sugar. For example, sucrose. 1 2 Mathews, van Holde, and Ahern Biochemistry 3rd Reducing sugars The C=O group of aldoses can be oxidised to a COOH group, reducing the ions or molecules with which they react. Cu2+ Cu Reducing sugars http://jchemed.chem.wisc.edu/jcesoft/CCA/CCA5/MAIN/1ORGANIC/ORG18/TRAM18/B/0595723/ THUMBS.HTM This means that if the –OH on C1 of an aldose or C2 of a ketose is substituted, Mathews, van Holde, and Ahern Biochemistry 3rd edition. 1.The sugar cannot act as a reducing sugar. 2.The a and b anomers cannot freely interconvert. MCQs 1)Which of the following statements is incorrect? a.Aldoses contain an aldehyde group on C1 b.Aldehyde groups can be oxidised to a carboxylic acid. c. Ketoses contain a ketone group on C2 d.Ketone groups can be oxidised to a carboxylic acid. e.Hydroxyl groups can be oxidised to a ketone or aldehyde group. 2) Which of the following is correct? a.Starch is a polymer of β-glucose b.Starch is a polymer of glucuruonic acid. c. Cellulose is a polymer of β-glucose d.Cellulose chains form a helical structure. e.Starch is a structural carbohydrate. 3) What is the defining feature of redox reactions? a.Addition of oxygen atoms. b.Addition of hydrogen atoms. c. A decrease in the number of molecules. d.Transfer of electrons from one species to another. e.Creation of electrons. 4) Which of the following are reducing sugars (there is more than one correct answer)? a.Lactose b.Sucrose. c. Starch. d.Glucose. e.Fructose. 5) Which of the following are incorrect? a. Oxidising agents tend to have more positive half cell potentials. b. Reducing agents tend to have more positive half cell potentials. c. Oxidising agents tend to have more negative half cell potentials. d. Reducing agents tend to have more negative half cell potentials.. e. The hydrogen half cell is defined as having a potential of 0 V under standard conditions.