113’
Vol. SO
THOXAS R. HARKINS AND HENRY FREISER [CONTRIBUTION FROM THE
DEPARTMENT OF CHENISTRY,
UKIVERSITY OF PITTSBURGH]
Adenine-Metal Complexes’J BY THOMAS R. HARKINS AND HENRY FREISER RECEIVED -4l.iGWST 9, 1957 The reaction between adenine and certain divalent metal ions has been described. The acidic group of the imidazole ring apparently is involved in the reaction. A chelate structure is proposed for the complexes formed and formation constants of the metal complexes have been evaluated.
The Calvin3-Bjerrum4 potentiometric titration technique has been utilized in a study of the behavior of adenine (6-aminopurine) toward m e t d ims. €I
Such an investigation was thought desirable because of the occurrence of this purine in nucleic acids. ,4 very limited amount of work has been reported on the possible complexes formed between adenine and metal ions. Liquier-Milward5 has isolated a 1: 1 cobalt-adenine complex from aqueous solution. Albert6 has titrated adenine in the presence of copper(I1) and cobalt(II), concluding that a weak complex is formed with copper (log K , = 2 . 5 ) which is more likely to be of the type given by many other amines with this metal, rather than one involving the acidic group of the imidazole ring. I t is interesting in this respect that Laurer and Charney’ have found that purine compounds possessing an unsubstituted imino nitrogen (;.e., those purines having a potential acidic group) formed insoluble compounds with copper(1) , whereas those purines which had a substituent other than hydrogen in the 9-position did not form such insoluble compounds. Experimental T h e titration apparatus is the same as t h a t previously described.8 Potentiometric measurenients of $H were made using a Beckman model Cr $15 meter equipped with a glass-saturated calomel electrode pair. The purification of dioxane, standardization of sodium hydroxide and perchloric acid also have been described.8 Stock solutions of approximately 0.01 121 metal ions were prepared by dissolving their reagent grade perchlorates (G. Frederick Smith C o . ) in water. The copper(I1) and cobalt(I1) solutions were standardized by electrodeposition. T h e nickel(I1) solution was standardized h y precipitation with dimethylglyoxime. rldenine (Sutritionnl Biochemicals Corp.) was dried a t 110’. Anal. Calcd. f_orC5HaNs: C, 44.5; 11, 3.7. Found: C, 43.4, 43.9; H, 3 . 1 , 3.4. Adenosine (Xutritional Biochemicals Corp.) was dried in aacuo over calcium chloride.
Anal. Calcd. for C I O H I ~ S ~ O C ,~41.9; : H , 4.9. Found: C, 44.9, 44.2; H, 4.9, 4.8. D-Itibose (Nutritional Biochemicals Corp.) was dried in v a c m over calcium chloride. Anal. Calcd. for C5H1005: C, 40.0; H , 6.7. Found: C, 39.8; H, 6.5. The titration procedure is essentially the same as t h a t described previously*: 55 ml. of water (or dioxane), 50 ml. of 0.01 Xperchloric acid and 5 nil. of 0.01 M metal perchlorate \yere added to a weighed quantity of t h e reagent. Standard sodium hydroxide (0.1 N ) was added in small increments t o the stirred solution. For the titration of reagent alone 5 ml. of water was substituted for the metal perchlorate solutions. The method of calculation of t h e formation constants of the metal complexes has been summarized previously.8
Discussion Acid Dissociation Constants.-The P K a values determined are summarized in Table I ; p K a l refers to the dissociation of the imidazole N-H group. TABLE I SUMMARY OF ACID DISSOCIATION CONSTANTS Temp., Coniyoiind 25.
del I i i I
12
Solvent
‘(2.
PKai
fiK-2
50y0 dioxaiie Water
25 10 25 40 10 25 40 25
3.43 4.33 4.18 4.02 3.61 3.51 3.3;
