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Fall 2014� HO OH O OH O OH Fall 2014! HO OH O OH O OH HO OH HO OH !-D-ribofuranose (Haworth) HO HO O O HO OH OH HO HO OH !-D-mannopyranose HO OH (Haworth) anomeric anomeric carbon carbon The anomeric monosaccharides, α-D-glucopyranose and β-D-glucopyranose, drawn as Fischer and Haworth projections, and as ball-and-stick models Upon cyclization, the carbonyl carbon becomes chiral and is referred to as the anomeric carbon. In the α-form, the anomeric OH (O1) is on the opposite side of the ring from the CH2OH group, and in the β-form, O1 is on the same side. The α- and β-forms are referred to as anomers or anomeric pairs, and they interconvert in aqueous solution via the acyclic (“linear”) form (anomerization). Aqueous solutions of D-glucose contain ~64% β-pyranose and ~36% α-pyranose. OH OH O HO HO H O HO OH OH CHO OH OH !-D-glucopyranose OH OH 37.6 % !-D-glucofuranose HO 0.11 % OH OH OH OH OH O HO H HO D-glucose O OH HO OH acyclic aldehyde OH 0.006 % OH "-D-glucopyranose OH 62.0 % "-D-glucofuranose -H2O +H2O 0.28 % CH(OH)2 OH HO OH OH OH D-glucose acyclic hydrate 0.004 % Monosaccharides that are capable of assuming a form in solution that contains a free carbonyl group can be oxidized by relatively mild oxidizing agents such as Fe+3 or Cu+2 (Fehling’s reaction). The saccharide is oxidized and the reagent is reduced.! CHO COO- OH OH HO cyclic and hydrate HO forms of D-glucose OH OH 2Cu+2 2Cu+ OH OH OH OH D-glucose D-gluconate acyclic aldehyde q " Phosphorylation can restrict the types of cyclization reactions of the acyclic carbonyl form. q " Phosphorylation enhances rates of anomerization. HO O O OH OPO3H-1 CHO OPO3H-1 OH OH O OH OH HO O OH HO OH CHO OH !-D-ribopyranose !-D-ribofuranose 20.3 % OH 7.4 % OH OH HO OH OH HO OH HO OH -D-ribofuranose-5P OPO3H-1 O O ! -D-ribofuranose-5P OH 35.6 % " HO OH D-ribose-5P 63.9 % OH ayclic aldehyde OH 0.1 % D-ribose HO OH ayclic aldehyde HO "-D-ribopyranose 0.04 % "-D-ribofuranose 13.3 % 58.9 % -H2O +H2O -H2O +H2O CH(OH)2 CH(OH)2 OH OH OH OH OH OH 3 -1 OH OPO H D-ribose hydrate D-ribose-5P hydrate 0.09 % 0.5 % pH 4.5! The two anomers show different sensitivities to pH, with the α-anomer generally more reactive than the β- anomer. Interestingly, the β-furanose is the more abundant furanose form in solution. If a similar study was done! on unphosphorylated D-ribofuranose, the kopen values would be considerably lower; phosphorylation enhances anomerization rates, which may have! physiological significance.! (solution conditions:! 2 o 0.03 M sugar in 15% H2O at 24 C)! CH2OH CH OH HO OH OPO H-1 2 OPO H-1 O 3 3 OH O OH OH HO O O O OH OH HO OH HO HO HO CH2OH HO "-D-fructopyranose OH OH ~0 % O "-D-fructofuranose OH 6.7 % HO HO HO OH "-D-fructofuranose OH !-D-fructofuranose-6P 16.1 % 81.8 % OH HO OH -1 O OPO3H O CH2OH OH HO OH D-fructose-6P OH OH ayclic ketone HO OH HO 2.2 % !-D-fructopyranose D-fructose -D-fructofuranose 67.2 % ayclic ketone ! 0.6 % 25.5 % -H2O +H2O -H2O +H2O CH2OH CH2OH HO OH OH HO HO HO OH OH OH OH OPO H-1 OH 3 D-fructose hydrate D-fructose-6P hydrate pH 4.5! ~0 % pH 5.5, 30 °C 0 % !-pyranose "-pyranose -1 41% 0.0043 s-1 0.0019 s 29% 7.8 s-1 2.7 s-1 aldehyde 0.046 s-1 0.03% 0.037 s-1 43 s-1 22 s-1 !-furanose "-furanose 18.5% 11.5% 2 50 mM Na acetate, pH 4.0, 15% v/v H2O, 28 °C PGI reaction! Endo-anomeric effect: a specific type of stereoelectronic effect; refers to the preferred orientation (axial vs equatorial) of the C-O bond involving the anomeric carbon of a monosaccharide Major factors that influence C1-O1 bond orientation in aldopyranosyl rings: q steric q stereoelectronic q solvation (hydration) The relative contributions of these three factors in dictating preferred C1-O1 bond orientation vary with the structure and configuration of the monosaccharide and environmental (e.g., solution) conditions. OCH3 controlled mainly by OCH3 steric factors favored OCH3 controlled mainly by stereoelectronic factors; O axial orentation more O OCH3 favored with increasing electronegativity of the ring favored substituent. O two dipoles approximately parallel; energetically unfavorable OCH3 !- !+ 1. Electrostatic (dipole-dipole) model! O two dipoles approximately perpendicular; energetically more stable !+ - ! OCH3 O CH3 no orbital overlap in the !-anomer O 2. Orbital interaction model orbital overlap possible in the "-anomer (nO5 #*O1); a stabilizing effect; O geometric implications are that (a) the C1- O5 bond shortens, (b) the C1-O1 bond lengthens, (c) C1 acquires more sp2- character, and (d) the O5-C1-O1 bond O angle increases relative to the -anomer. CH3 ! O5-C1-O1 angle: 111.4° O5-C1-O1 angle: 107.9° 1.442 1.429 1.426 1.416 1.389 1.396 methyl !-D-galactopyranoside methyl "-D-galactopyranoside Rings can adopt different three-dimensional structures in solution and in the solid state. Conformations of the cyclohexane ring. (a) Boat conformation; (b) chair conformation. In monosaccharides, the chair conformation is the most common conformation (3D form) of pyranose rings in solution. Different chair conformations interconvert spontaneously in solution, and the rate of interconversion is very rapid. More stable conformations orient bulky groups in equatorial (eq) rather than axial (ax) positions (excluding substituents at the anomeric carbon where stereoelectronic effects occur). 4 1 C1 C4 Pyranosyl Ring Drawing Hints HO (a) axial (ax) bonds (pink) are always orthogonal ax to the ring plane shown in blue. OH C6 (b) equatorial (eq) bonds are always parallel O5 C4 eq O to two endocyclic bonds. For example, HO C5 C2 the C2-O2 and C5-C6 bonds (red) are parallel to the C1-O5 and C3-C4 bonds (red). The C3-O3 bond (green) eq C1 C3 is parallel to the C1-C2 and C4-C5 bonds (green). eq ax OH Some of the exocyclic bonds are exaggerated in length in OH this rendering to emphasize their proper orientation. Normally these lengths are similar to those within the ring, as shown for the C1-O1 and C5-C6 bonds. 4 1 C1 C4! more stable less stable 5 equatorial substituents 5 axial substituents 0 axial substituents 0 equatorial substituents Note that equatorial and axial substituents exchange orientations! upon ring interconversion.! OH OH OH OH O O OH HO OH OH OH HO 4 1 C1! C4! 4 substituents axial! 1 substituent axial! 1 substituent equatorial! 4 substituents equatorial! ~20%! ~80% The ido(hexo)pyranosyl ring is observed in vivo in the L-configuration and with C6 oxidized to the carboxylic acid (α-L-idopyranuronic acid; IdoA); IdoA is a component of the proteoglycan, dermatan sulfate. In addition to chair forms, pyranose rings can assume! half-boat, half-chair, twist-boat and boat conformations.! HO OH OH H O Two boat conformations of ! O HO HO OH β-D-glucopyranose (6 are possible)! HO OH OH OH C = chair (2)! H = half-chair (12)! E = envelope (12)! B = boat (6)! S = skew or twist-boat (6)! Total: 38! A pseudorotational model describes systematic interconversion between pyranose conformers.!.
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