Carboxyl group model structure

However, when the X-ray crystal structure of the MoFe protein was examined, it was clear that homocitrate could not directly hydrogen bond to the histidine, since the carboxylate group and imidazole are stacked parallel to each other in the crystal. Nevertheless, as noted in the previous section, studies on model complexes have suggested that homocitrate can become monodentate during nitrogenase turnover, with the molybdenum carboxylate bond breaking to open up a vacant site at molybdenum suitable for binding N2. 

In the firefly, the carboxyl group of luciferin is activated. Key features of both compounds include an easily autoxidizable CH group, an activated carbonyl group in juxtaposition, and an oxidation product that is very fluorescent. Studies of the chemistry of the model acridan ester have led to a better understanding of firefly bioluminescence including correct prediction of the structure of the oxidized luciferin product. 

Model Studies. The major difficulty associated with separation and analysis of HPD arises from the fact that there are two hydroxyl and two carboxyl groups positioned unsymmetrically on the porphyrin ring. Statistically, there are 3 ether structural isomers and 4 esters possible between two hematoporphyrin molecules and each of these is a diastereomeric mixture. No doubt, this situation has made the isolation and characterization for a specific structure a nightmare. 

Koymans et al. argued that this 2 A difference could be explained by the interaction to one or other oxygen atom of the same carboxylate group (the distance between the two oxygen atoms in a carboxylate is about 2.2 A) (180). The structural features found for 16 substrates and 23 metabolic reactions were in good agreement with the previous models. All substrates exhibit a coplanar... 

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