Propionic acid disadvantages

Structural information on aromatic donor molecule binding was obtained initially by using H NMR relaxation measurements to give distances from the heme iron atom to protons of the bound molecule. For example, indole-3-propionic acid, a structural homologue of the plant hormone indole-3-acetic acid, was found to bind approximately 9-10 A from the heme iron atom and at a particular angle to the heme plane (234). The disadvantage of this method is that the orientation with respect to the polypeptide chain cannot be defined. Other donor molecules examined include 4-methylphenol (p-cresol) (235), 3-hydroxyphenol (resorcinol), 2-methoxy-4-methylphenol and benzhydroxamic acid (236), methyl 2-pyridyl sulfide and methylp-tolyl sulfide (237), and L-tyrosine and D-tyrosine (238). Distance constraints of between 8.4 and 12.0 A have been reported (235-238). Aromatic donor proton to heme iron distances of 6 A reported earlier for aminotriazole and 3-hydroxyphenol (resorcinol) are too short because of an inappropriate estimate of the molecular correlation time (239), a parameter required for the calculations. Distance information for a series of aromatic phenols and amines bound to Mn(III)-substituted HRP C has been published (240). 

One potential disadvantage is that the synthesis of the precursor 5 is via a four-step synthetic sequence from the benzyl ester TBDMS ether of 2,3-bis(hydroxmethyl)propionic acid (Scheme 3), and investigations are currently underway to develop a more efficient synthesis. 

The conversions of acetic anhydride to acetaldehyde and of ethylidene diacetate to ethyl acetate both involve hydrogenolysis of C—O bonds, whereas the hydrogenation of acetaldehyde to ethanol involves 0=0 reduction. An appropriate choice of hydrogenolysis versus hydrogenation catalyst functions should enable discrimination between the reaction pathways and the development of highly selective processes to both ethyl acetate and propionic acid, respectively. However, a clear disadvantage common to both is the requirement for recycling of stoichiometric quantities of acetic acid (see the next section). 

In mutagenesis the main condition of success is the development of a sensitive and rapid method of mutant selection. Vitamin B12 production by propionic acid bacteria correlates with acid production. But the method of selection based on picking colonies with the maximal diameter of dissolution zones of calcium carbonate (added to solid medium) did not give positive results, since the size of colonies is not identical while acid production is dependent on the quantity of biomass. Selection based on the intensity of pink color that correlates with vitamin B12 production has a disadvantage in that selection of small mutations is made step by step, when differences in color can hardly be noticed by eye. Another method used was based on direct determination of vitamin B12 in single colonies. This method is labor-intensive and severely limits the testing of large numbers of colonies. 

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