ACID DISSOCIATION CONSTANTS AND COMPLEX FORMATION CONSTANTS OF SEVERAL PYRIMIDINE DERIVATIVES¹

5′-Nucleotidase from rat heart membranes. Inhibition by adenine nucleotides and related compounds

“…Computer analyses of the data in accordance with Cleland (1979) yield a pKa of 6.58+0.03 and pKb of 8.93 +0.03. The secondary ionization of AMP has a pK of 6.2-6.4 at 25°C (Tucci et al, 1961;Taqui Khan & Martell, 1967).…”

Organometallic complexes with biological molecules. X: dialkyltin(IV) and trialkyltin(IV) orotates: spectroscopic andin vivo investigations

“…At pH b 9.3 it is conceivable that the Hor 2À anion is formed. Only at strongly alkaline pH or with very strong Lewis acids can further deprotonation of N(3)H (pK a3 = 9.45) 4 occur, with orotic acid behaving as a triprotic acid and forming orotates containing the or 3À anion.…”

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“…43 The acidity constants of orotic acid, also known as 6-uracilcarboxylic acid, H(6Urca), are listed in Table 2 together with the corresponding values for 5-uracilcarboxylic acid, H(5Urca), and some other related compounds. 39,40,44 In Section 3.3 we have seen that the substituent effects in 5methyluracil and 6-methyluracil as well as in 5Umpa 2Ϫ and 6Umpa 2Ϫ are as expected for substituents with a weak inductive effect (ortho, para directing), 32 the uracil residue with the 5substituent being the more basic one. The corresponding expectation also holds for 5-uracilcarboxylate and 6-uracilcarboxylate, but the carboxylate group, being a deactivating meta directing substituent, 32 now makes the 5-substituted derivative more acidic (cf.…”

“…45 Since the negative logarithm of the measured acidity constant of 5-uracilcarboxylic acid, pK H H(5Urca) = 4.16 ± 0.10 (Table 2, entry 5) is much higher, there must be an effect which inhibits the release of the proton and this can only be the formation of a hydrogen bond between (C4)O and (C5)COOH. Indeed, Li et al 39 have already concluded that 5-uracilcarboxylic acid "is structurally favorable for intramolecular hydrogen bonding and resonance enhances (this) interaction between the carboxyl hydrogen and the adjacent oxygen . .…”