359
NOTES
Feb., 1962
THERMODYNAMICS OF PROTON DISSOCTATION IN DILUTE AQUEOUS SOLUTION. I. EQUILIBRIUM. CON STANTS FOR THE STEPWISE DISSOCIATION OF PROTONS FROM PROTONATED ADENINE, ADENOSINE, RIBOSE-&PHOSPHATE AND ADENOSZNEDIPHOSPHATE* BY REEDM. IZATTAND J A M EJ.~ CHRISTENSEN Department of Chemistry and Department of Chemaeal Enganeerzng, Brzgham Young Unzversztu, P r o w , Utah Received J u l y 14, 1961
Ilespitr: the vital importance of ribonucleotides in biological systems recent summaries2s3 show that few p K a data have been reported for proton dissociation from these or related substances. The available data have in nearly all cases been determined a t high ionic strengths and at a single temperature. No pK8 data have been reported previously for ribose-5-phosphate (RP). The present study is part of a program to determine equilibrium constants under identical experimental conditions for the stepwise dissociation of protons from ribonucleotides and related compounds a[$a function of ionic strongth, p , and temperature. pKa data are reported here a t 25’ and as a function of p for the dissociation of protons from protonated adenine, adenosine, adenosinediphosphate (ADP) and RP. The higher of the two observed pK values of protonated adenine (I) generally is agreed to be associ-
(1)
ated with the removal of a proton from Ns although tautomerism between N7 and N9 has been invoked4 to explain the p H change observed upoii formation of metal-adenine complex ions. Verification of the Ng-H bond is found in the fact that this dissociation is absent in adenosine, AMP, ADP and ATP, where the ribose group is attached to N9. There are conflicting reports, however, concerning whether the lower pK value is associated with the NL or the amino group. Bock (ref. 2 , p. 1) has pointed out that most workers assume the amino graup to be the proton acceptor and so interpret their data. Nakajima and Pullman5 and ZubaylG however, recently have presented data which indicate that the !VI is the more basic. (1) Supported in Part by N I H Grant A-3021, and a Research Corp.
Grant (2) R. M. Elook in “The Enzymes,” Vol. 2, 2nd Edition, edited by P. D. Boyer, $1. Lardy and Io study this dissociation furthor ushg niethods of ionic strerigth is in conformity wilh the theory capable of yieldirig quant,itnativ(:results in this high PS explained by L~itllcr.' pII region. Tho molal volume changes, (AV)2, involved in The increased acidities of adenosine and RP rela- these reactions are calculated for these solutions tive to that of adenine or I-12P04- ( p K = 7.20) in- by using the formula given by Ries, where the condicate that the ribose acts as an electron withdraw- centration has to be expressed in moles per e ~ . ing group. It is interesting to note in this connec- The values obtained for magnesium and manganous tion that pK1 is the same in ADP as it is in adenine (1) D.Ries, J . Chdm. Phye., 23, 428 (1955). (2) S. K. Kor and G. 8. Verma, ibid., 29, 9 (1958). and p K 2 is appreciably larger in ADP than it is in (3) M.Manes, ibid.. 41, 428 (1953). RP. The increased pKp value probably is due t o (4) D.Tabuchi. ibid., 26, 993 (1957). thc increascd negative charge on the ADP relative ( 5 ) H.R. Harned and B. B. Owen. "Physical Chemistry of Electo that on RP. This is further substantiated by trolyte Solutions," Reinhold Publ. Corp., New Pork, N. Y., 1950, 190,426. the increasing p& values observed in the series p. (6) M.Kriahnamurthl and M. Suryanarayana, J . Phys. SOC.Japan, adenosine mono-, di-, tri- and tetra-phosphate.12 16, 2318 (1960). 14.0 17.0 19.8
9.66 Q.6Q
I
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(12) R. M.Smith and R. A. Alberty, J. Pliys. Chem., 60,180 (1956).
(7) K. Laidler. "Chemical Kinetics," McGraw-Hill Book Co., New Pork, N. Y., 1950. p. 132.
