Bioorganic Chemistry: A Chemical Approach to Enzyme Action by Hermann Dugas

By Hermann Dugas

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Free rotation does not readily occur about the C-N bond as this would destroyed the TC resonance overlap, with the trans geometry being preferable for steric reasons. 8 kJ/mol (10 kcal/mol), does not readily occur. 42 2: Bioorganic Chemistry of the Amino Acids The peptide bond is a strong bond, and energy is required for its formation. Mixing an aqueous solution of two amino acids, one with an unprotonated amino function (potentially nucleophilic) and the other with a protonated carboxyl function, at room temperature would only result in salt formation.

1. 1. 4 Methyl p-D-ribofuranoside 5-phosphate • Values refer to proton dissociation from the indicated structures. the carboxyl function. Acetylation of the amino group (eliminating the positive charge) reduces the acidity of glycine roughly by a factor of ten, but still the carboxyl of the acetylated glycine is significantly more acidic than that of acetic acid. Two effects are operative which account for the greater acidity of glycine relative to acetic acid. Inductive pull (Ie) of the positive ammonium and acetylated amine decreases electron density at the carboxyl so that the latter more readily gives up its dissociable proton.

Chemically, the carboxyl function must be converted to a good leaving group. Energetically, the carboxyl function must be activated to compensate for the work done during peptide bond formation. This is reflected in the free energy ofhydrolysis (LlGhydro) of the amide bond which is in the range of -12 kJ/mol ( - 3 to - 4 kcaljmol). 3 kJ Imol ( - 7 kcal/mol), and the chlorine atom is a good leaving group. It is therefore possible to convert the carboxyl of an amino acid to an acyl chloride (thionyl chloride, phosphorous pentachloride) and react this with the amino function of a second amino acid to form a peptide bond.

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