T'ol. 65
E. 12. TUCCI, E. DOODY AND X. C. LI
1570
ACID DISSOCIATION COSSTASTS A S B COMPLEX FORIlIATIOS CONSTASTS OF SEVERAL PYRIhZIDINE DERIVATIVES B Y EDMOND R. TUCCI, BR. E D W A R D DOODY AND
XORMM9N
c. LI
Duquesne University, Pittsburgh, Pa., and Christian Brothers College, Memphis, Tenn. Received A p r i l IO. 1961
Acid dissociation constants of uracil-5-carboxylic acid (isoorotic acid), 2-ethylthio-isoorotic acid, uracil-6-carboxylic acid (orotic acid), 5-nitroorotic acid and adenosine-5'-monophosphate have been determined a t an ionic strength of 0.1, 25 . The formation constants of the Cu(II), Ni(II), Co(II), Zn(II), Mn(If), Cd(I1) complexes of, some of these pyrimidine derivatives were determined using p H and ion-exchange methods. The binding sites in uracil-5-carboxylic acid and 2ethylthio-isoorotic acid toward metal ions are probably the carboxylate anion and the adjacent oxygen atom. The postulated sites in 5-nitroorotic acid toward metal ions and uracil-6-carboxylic acid toward Xi ion are the carboxylate and adjacent ring nitrogen anions. The formation constants of the zinc complexes of uracil-5-carboxylic acid and 5-nitroorotic acid, and of the sodium and manganous complexes of adenosine-5'-monophosphate obtained by the pH method are in agreement with the corresoonding values obtained by the ion-exchange method, indicating that these complexes are mononuclear.
Introduction The pyrimidine derivatives constitute a very importmalitclass of compounds because t'hey are components of nucleic acids and various enzymes, because they exert a pronounced physiological action, and because t,hey pose some interesting problems of st'ructure. As part' of a general program on studies of metal complexation with compounds of biological int'erest, this paper presents the results on t,he acid dissociation constants and formatmionconstants of metal complexes of a number of pyrimidine derivatives : uracil-5carboxylic acid (isoorot,ic acid), 2-ethylthio-isoorotic acid, uracil-6-carboxylic acid (orot,ic acid), 5 nitroorotic acid,2 and adenosine4'-monophosphoric acid. The outstanding struct,uralproblem of t'he pyrimidines is the tautomerism possible between t,he ketjoiiic and enolic forms (lactam-lactim). Coiisiderable dat'a on ultraviolet and infrared absorpt'ion spectra have been reported3t4 and there is general agreement that the preferred structure for the pyrimidines is the ketonic form rather t,han the enolic form. Thus Lacher, et aZ.,3afound t'hat a strong absorption band a t 1.43 p , corresponding to the first 0-H stretching overt'one in phenol, was lacking in uracil and its derivatives. Absorption Tyas found in the region corresponding to t'he first K-H overtone, 1.50 p , for uracil and 5-chlorouracil, and again a t 1.99-2.00 p indicating a second carbonyl overtone vibration which lies in this region. Browii and Short,3b from ultraviolet and pH data, conclude t,hat in aqueous solut'ion the 4-"hydroxypyrimidine" exists predominantly in the ketonic (lactam) form, I, rather than the enolic (lactim) form, 11. (1) Abstracted f r o m the 1'h.D. thesis of E. R. Tucci a t Duquesne University, and supported by National Science Foundation Grant No. NSF G7447 a t D.U. and by Atomic Energy Commission Contract No. A T (40-1)-2005 a t C.B.C. (2) In the Sigma Chemical Co. catalog, these compounds carry the names of 5-carboxy-2,4-dihydroxypyrimidine, 5-carhoxy-2-ethylmercapto-4-hydroxypyrimidine, 6-carboxy-2,4-dihydroxypyrimidine, a n d 6-carboxy-5-nitro-2,4-dihydroxypyrimidine, respectively. ( 3 ) (a) J. R. Lacher, D. E. Campion and J. D. Park, Sczence, 110, 300 (1949); (b) D. J. Brown a n d L. N. Short, J . Chem. ~ o c . ,331
(1953).
R.L o o f b o n r o x X. bl. Stimson and AI. J. Hart. J . Bm. Chem. 75, 148 (1953).
(4) ,J.
soi.,
fi
H I
I1
Experimental Materials.-The pyrimidine derivatives, gifts of Sigma Chemical Co., St. Louis, Mo., were dried in vacuo a t room temperature over calcium chloride. Stock solutions of these, always freshly prepared, were analyzed by determination of their neutralization equivalents using potentiometric methods. Solutions of metal salts were prepared and analyzed by conventional means. All chemicals were of C.P. grade. Carrier-free Mn-54 and Na-22 %-ere obtained from Nuclear Science and Engineering Corp., Pittsburgh, Pa. High activity Zn-65 was obtained from Oak Ridge National Laboratory in the form of zinc chloride in HC1 solution. For the cation-exchanger, Dowex-50, 8 7 , cross-linked, 100200 mesh, was used. The capacity of this type of resin has been found to be independent of pH over a wide pH range.5 Apparatus and Procedure.-The hydrogen ion concentration of solution S was determined with the cell Glass electrode /solution 91 R E (1) where RE is the reference half-cell Bg,AgClIO.l X KC1 satd. vith hgC1 ( s ) ] 0.1 M KCl The general design of the cell was that of Forsling, et al., A Becknian Type 40498 glass electrode was used. Ag6 AgCl electrodes were prepared by the method described by BiedermannP The potentials of the cell were read to 0.01 mv. with a Leeds and Korthrup Type I
