Ross STEWART and MADELEINE G. HARRIS. Can. J. Chem. 55,3807 (1977). The acidities (standard state, water) of the amino groups in 17 N-alkylated derivatives of adenine, guanine, cytosine, and isocytosine have been determined in aqueous DMSO containing base. Adenine methylated at the 9-position is a weaker acid than its 7-methylated isomer by 2.0 pK units and this difference is explained in terms of the known stabilities of the parent tautomers. A similar situation exists with 1-and 3-methylcytosine (ApK = 3.3). The position of alkylation of the isocytosine and guanine systems has a small or negligible effect on acidity, again consistent with known tautomeric effects. A modified ribosyl group attached to the 1-position of cytosine has an acid-strengthening effect of 1.9 units, relative to methyl. Of the amino groups involved in nucleotide base-pairing, that in the methyl derivative of guanine has the highest acidity, pKHA = 14.6; those in the cytosine and adenine derivatives have identical acidities, 16.7. An acidity function for purines and pyrimidines is reported for aqueous DMSO containing 0.011 M tetramethylammonium hydroxide (H-P).
23) Unpublished data from this laboratory. (24) Additional experimental details may be found in the Ph.D. thesis of J. A. Hautala, Northwestern University, June, 1971. (25) Prepared according to the procedure of A. C. Huitric and W. F. Trager, J. Org. Chem., 27, 1927 (1962). (26) Yu. N. Belikov, N. G. Faleev, and V. M. Belikov, Bull, Akad. Sci. USSR, Div. Chem. Sci. (Engl.), 1039 (1969); N. G. Faleev, Yu. N. Belikov and V. M. Belikov, ibid., 69 (1970). (27) Prepared according to the method of A. A. Griswold and P. S. Starcher, J. Org. Chem., 30, 1687 (1965). (28) N. Kornblum and H. E. IJngnade, Org. Synth., 38, 75 (1956). (29) No 3-nitro-I-butene resulting from direct displacement of chlorlde was formed. (1963). (30) We are indebted to Thomas G. Mecca for preparation of this compound. (36) We are indebted to Dr. W. J. Boyle, Jr. for preparation of this com-(37) P. West, J. L. Purmont, and S. V. McKinley, J. Am. Chem. Soc., 90, 797 (38) The present result is comparable to that observed for conversion of 1-(39) R. C. Kerber and A. Porter, J. Am. Chem. SOC., 91,366 (1969).Seventeen amino-substituted heterocycles, including pyridines, pyrimidines, cytosines, isocytosines, and adenines, have been compared with respect to acidity (~K H A , deprotonation at amino) and basicity (~K B H + , protonation at ring nitrogen). Unlike the analogous deaza compounds, in which protonation and deprotonation occur at the same site, there is no correlation between PKHA and ~K B H + .The ~K B H + for amino protonation of some of these compounds can be calculated, however, and in these cases the points fall very near the line for the deaza compounds. The displacement from this line can be regarded as a measure of the difference in basicity of the amino nitrogen atom and an aza nitrogen atom in the heterocycle in question.The basic nature of' organic amines is well known (eq l), (1) their acidic character much less so (eq 2 ) . RNH3+ * H+ + RNHz KBH+ = [H+][RNHz]/[RNH3+] RNHz * Hf + RNH-KHA = [H+] [RNH-]/[RNHz]
The acidities of the amino group in 18 heterocyclic compounds, nine pyridines, eight pyrimidines, and one triazine, have been measured. The pKHA values (standard state water) were calculated using the Bunnett–Olsen and Marziano–Cimino–Passerini extrapolative techniques from ultraviolet spectral changes accompanying ionization of the amino group in aqueous DMSO containing base. These compounds, together with seven anilines previously studied, constitute a well-behaved Hammett system (σ dependency) with a ρ value of 5.20, provided compounds containing nitro groups ortho or para to the amino group are excluded. The acid-strengthening effect of aza groups is additive and appears to be primarily due to induction rather than resonance, in contrast to the behaviour of the nitro group.
Evidence presented in previous papers supported the view that wool immersed in solutions containing hydrochloric lWit! combines stoichiometrically not only with the hydrogen ions of the acid but with chloride ions as well. As a consequence it appeared that the specific affinities for wool of the anions of different acids might vary considerably, and that therefore the positions of the titration curves of this protein with respect to the pH axis might vary by correspondingly large amounts according to the acid used.The present paper describes measurements of the combination of wool with 19 different acids, ranging in complexity from some of the mineral acids most commonly used through the simpler aromatic sulfonic, carboxylic, and phenolic acids to a soluble monoazo acid dye. It is shown that wide differences exist between the positions with respect to the pH axis of the titration curves of wool obtained with different strong acids, and that these differences may be ascribed to wide variations in the anion dissociation constants characterizing the corresponding protein-anion combinations. Equations previously derived to account for effects caused by variations in chloride concentration have been generalized for use in calculating these dissociation constants. A scale of relative affinities of anions for wool, based on these constants, and applicable to acid dyes, is proposed. Predic· tions as to the effects of variations of anion concentration and of temperature, based on the same generalized equations, have been tested and confirmed.Measurements of the combination of a number of the same acids with a soluble protein, crystalline egg albumin, have also been made. Since qualitatively similar differences in the positions, with respect to the pH axis , of the titration curves obtained with different acids are found with both proteins, it is concluded that the property of combining with anions is not restricted to insoluble proteins. The affinity of anions for proteins of both classes appears to increase with the dimensions of the anion, and is higher in aromatic than in aliphatic ions of the same size. The bearing of these relationships on the well known specific effects of ions on proteins and on the nature of the forces involved in the binding of anions by proteins is considered.
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