Lecture3_25 - Chapter 20 Four different groups are attached...

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3/24/2010 1 Chapter 20 Four different groups are attached to the alpha carbon atom in all of the standard amino acids except glycine In glycine R-group is hydrogen Therefore 19 of the 20 standard amino acids contain a chiral center Chiral centers exhibit enantiomerism (left- and right- handed forms) Each of the 19 amino acids exist in left and right handed forms
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2 The amino acids found in nature as well as in proteins are L isomers. Bacteria do have some D-amino acids With monosaccharides nature favors D- isomers The rules for drawing Fischer projection formulas for amino acid structures The COOH group is put at the top, the R group at the bottom to position the carbon chain vertically The NH 2 group is in a horizontal position. Positioning NH 2 on the left - L isomer Positioning NH 2 on the right - D isomer. Designation of Mirror handedness in standard amino acid structures In pure form amino acids are white crystalline solids. Not very soluble in water Amino acids exist as a Zwitterion: An ion with + (positive) and (Negative) charges on the same molecule with a net zero charge. Carboxyl groups give-up a proton to get negative
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Lecture3_25 - Chapter 20 Four different groups are attached...

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