Methylorotic acid

Application of ultraviolet absorption spectroscopy119 (see p. 302-308) showed that the detailed spectrum of orotidine at different pH values is very similar to that for 1-methylorotic acid, which confirmed the assignment of the carbohydrate-pyrimidine linkage to N1 of the aglycon. 

Computational approach. Singleton, Beak and Lee explored the pathways of decarboxylation of 1-methylorotic acid 11 via the 2-protonated zwitterion 4b and the 4-protonated zwitterion 6b using Becke3LYP calculations with a 6-31 + G basis set.46... 

The barriers just described were calculated with 1-methylorotic acid (11) as a reference point to model the uncatalyzed reaction in solution. However, the computed free-energy barriers for decarboxylation of zwitterions 4b and 6b are 8.4 and 7.6 kcal mol-1, respectively. This difference of 0.8 kcal mol-1 is significantly smaller than the 6 kcal mol-1 difference calculated by Lee and Houk for the 2-protonation and 4-protonation pathways. This discrepancy arises from an internal hydrogen bond (12) between the Nl-H and the carboxylate that artificially stabilizes the 02-protonated zwitterion 4a, and renders its corresponding decarboxylation barrier too high. When the Nl-H is replaced by a methyl, the hydrogen bond is removed, and the ylide and carbene mechanisms become closer in energy nonetheless, 4-protonation is still favored. 

Warrener and Cain10 oxidized 6-hydroxymethyl-l-methyl-2-thiouracil (3) to 1-methylorotic acid (4) with 6 equivalents of Nakagawa s nickel peroxide. The reagent... 

For ODCase, non-covalent mechanisms have often been proposed, as reflected in three of the mechanisms shown in Fig. 2. This is the crux of the attention showered on ODCase how can this enzyme achieve its rate acceleration without the use of cofactors, metals, or acid-base catalysis From Wolfenden s measurements of the uncatalyzed reaction of 1-methylorotic acid in water, he calculated the rate enhancement (kcat/kun) in the enzyme to be 1.4x10, corresponding to a reduction of AG of 24 kcal/mol [1]. He also reported the catalytic proficiency to be 2x10 meaning that the enzyme-transition state complex is an impressive 32 kcal/mol more stable than the fi-ee enzyme and transition state in water (i.e., the effective binding free energy of the transition state out of water is 32 kcal/mol) [1] The experimental free energy of activation is 15 kcal/mol for this decarboxylation in ODCase. 

Enzymes accelerate chemical reactions. Figure 19.22 showvs an Arrhenius plot for the uncatalyzed reaction of 1-methylorotic acid (OMP) 71. 

Although it is reasonable to assume from the enzymic studies cited162 that orotidine possesses the /3 configuration at the anomeric center, unequivocal determination of this feature is to be desired. Of interest is the fact that 1,3-dimethylorotic acid,119 3-methylorotic acid,119 163 and orotic acid itself are decarboxylated to the corresponding uracil derivatives at elevated temperatures. If orotidine could be decarboxylated in some similar fashion to uridine, the complete structure of XXXII would be established. 

In several experiments, the chlorophyll mutation frequency in the barley test has been raised up to ten times by 5-bromodeoxyuridine (BUdR), with or without the addition of aminopterine similar results were found with 2-aminopurine (2-AP) (Ehrenberg, unpublished Natarajan et aL, unpublished). A similar effect is produced by compounds that cause metabolic disturbances such as nebularine, azathymine, azaguanine, theophylline (Ehrenberg et aL, 1956, and unpublished), and adenine + thymine (Arnason et aL, 1962), and by analogs of DNA base precursors such as methylorotic acid and carboxymethylidene hydantoin (Heslot and Ferrary, 1958). Thiou-racil and mercaptopurine are nonmutagenic. However, thiouracil, like azathymine, causes sterility. 

Methylorotic Acid (Z-Methylv/racHA-carb-oxyUc add),... 

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