July 20, 1960
HEATSO F
IONIZllTION O F
The effect of optical activity of the peptides is again exhibited in the stability constants of the metal-peptide complexes. Since the metal complex is formed when the protonated amino group loses its proton, corresponding to pK2' and a linear form of the peptide, the R groups are cis to the peptide bond in the LD(DL)-isomer and trans in the LL(DD) -isomer. The LD(DL)-isomer should then form a more stable complex than the LL(m)-isomer, provided that the R groups are large enough to inhibit facile complexation with the metal ion. Hence, there will be a greater difference in the stability constants of the diastereoisomeric leucyltyrosines than in the diastereoisomeric alanylalanines complexes with a metal ion. This is borne out by the results given in Table 11. These considerations of the folding-unfolding processes of the dipeptides as a function of p H may be extended to predictions of the relative rates of acid or basic hydrolysis of diastereoisomers where attack at the peptide carbonyl is involved. In the $H range below pK1' the dipeptide exists in a linear form, the R group being trans to the peptide bond in the LL-isomer and cis in the DL-isomer. When the R groups are trans they flank to some extent both sides of the peptide bond. This should result in a decreased rate of hydrolysis by hindering attack a t the peptide bond. The DL-isomer, however, will be faster in this pH range since the R groups are both cis to the peptide bond. This is exemplified by the hydrolysis of the LL- and DLvalylvaline in 6 LV hydrochloric acid a t 100' in i t j effects are t h e same as those exhibited b y a comparison of t h e PKr' of tyrosine with t h e pKr' of various aminophenols a n d t h e PKI' of ethylamine with t h e PKr' of ethylenediamine.
[COKTRIBCTIOS S O . 1597 FROM
THE
3739
DEOXYNUCLEOTIDES TABLE 11" log kikt
log ki
Metal ion
D-LeUCyl-L-tyrOsine co++ Ni++ Zn*+
2.81 5.07 3.73 6.66 3.39 6.24 L-Leucyl-L-tyrosine 2.42 4.48 co++ Xi++ 3.23 5.99 Zn++ 2.98 5.66 ~-Alanyl-~-alanine eo++ 2.83' L- Alanyl-L-alanine Co'' 2 . 63h a Error in log kl is &0.03. Obtained from titration of equimolar mixtures of peptides and Co(NOa)*, 0.01 M .
'
..
which the rate of hydrolysis of the DL-iSOmer was greater than the LL-isomer.16 In the pH region between pK1' and pK2', the peptides exist in the folded form with the R groups cis in the LL- and trans in the DL-isomer. Thus, the LL-isomer would be expected to hydrolyze faster than the DL in this PH range. In the p H range beyond pK*', the behavior of the dipeptides on hydrolysis will be similar to that in the $H range below pK1' with respect to relative rates of diastereoisomers. Acknowledgment.-The authors are grateful to Mr. Paul Kelly for the data on the nickel-leucyltyrosine complexes. (16) J. W. Hinman, E. 72, 1620 (1952).
L. Caron
and H N Christensen, TRISJ O U R -
NAL,
PITTSBURGH 19, PESSA.
STERLING CHEMISTRY LABORATORY O F YALE USIVERSITY,
SEW
HAVES,CONNECTICUT]
The Heats of Ionization of Deoxynucleotides and Related Compounds1 B Y hIL4RY RAWITSCHER AND JULIAN
31.STURTEVANT
RECEIVEDJANUARY 4, 1960 The heats of ionization of the purine group of deoxyadenylic acid, deoxyguanylic acid and related compounds and of the pyrimidine group of deoxycytidylic acid and related compounds have been determined calorimetrically. A parallelism, which extends t o bases of other types, between heats of ionization and values of the apparent pK is noted.
The results of a recent calorimetric study2 of the acid denaturation of deoxyribosenucleic acid (DNA) were interpreted3 on the basis of the assumption that the heats of ionization of the pyrimidine and purine bases are very nearly zero in both native and denatured DNA. Titration data for DNA determined over a range of temperature^^-^ give conflicting evidence as to the magnitudes of the heats (1) This work was aided h y grants from t h e National Science Foundation (G-28Z.5) a n d t h e United S t a t e s Public Health Service (RG-4725). (2) J. 31. S t u r t e v a n t a n d E. P. Geiduschek, THISJ O U R N A L , 80, 2911 (1958). (3) J. M.S t u r t e v a n t , S t u a r t A . Rice a n d E. P. Geiduschek, Discirssions F a v a d a y Soc., 25, 138 (1958). (4) L. F. Caralieri and B. H . Rosenberg, THISJOURNAL,79, 5382 (1957). (5) R . A. Cox a n d A. R . Peacocke, J . Chem. SOC.,4724 (1957). (6) R. A. Cox and A . R . Peacocke, Discussions Faraday Soc., 26,211, 213 (1958).
of ionization. I t therefore seemed of interest to determine these quantities for the isolated mononucleotides and related compounds, even though it cannot be assumed that these heats are the same as those for the nucleotide units in DNA. The present paper reports values obtained by direct calorimetry, a method which gives results of considerably greater accuracy than that obtainable by application of the van't Hoff equation. Experimental Procedures and Materials The twin calorimetric apparatus and method employed have been described? previously. I n each experiment, the heat evolved when a solution of the base was mixed with a n equal volume of a solution containing less than one equivalent of HCl was determined. The amount of HC1 bound by the base was calculated from the amount added (7) A. Buzzell a n d J. M, S t u r t e v a n t , THIS JOURNAL, 73, 2454 (1951).
h f . 4 ~RAWITSCHER ~ AND JULIAN
3740
and the initial and final values of the PH of the base solution. The iuitial solutions of the bases and nucleosides were a t approximately PH 7, while those of the nucleotides were adjusted to PH values between 4 and 5. The final pH values of the nucleotide solutions were above 2.5, so that it can be assumed that no significant contributions from the phosphate ionizations are included in the observed heat effects. All solutions were adjusted t o an ionic strength of 0.1 M by the addition of Pl’aCl. Initial base 121 were employed. HC1 concentrations of 2 X 10-3 t o solutions were prepared by quantitative dilution of a constant boiling stock solution. The heat of dilution of the €IC1solution was negligible in all cases.8 The adenine sulfate was an Eastman Kodak “white label” preparation. ,411 the other bases were obtained from the California Foundation for Biochemical Research and were labeled “chromatographically pure.” These substances were used without further purification. The heats of dilution of some of the compounds were determined under the conditions of the ionization experiments. The ionization data were corrected for the heats of dilution, the corrections being of a magnitude comparable t o the uncertainty of the ionization heats themselve.,. All measurements were performed a t 25’.
x.STURTEVllNT
1-01, 82
It is interesting to note that for most of the compounds in Table I, the heat of ionization in kcal./ mole is approximately equal to the pK’. This generalization applies fairly widely to the ionizations of the acids conjugate to a variety of bases.’O Since AF’Bs = 1400 (pK’) cal./mole, it follows that AS029s = -1.2 (PK’) cal. deg.-’ mole-’ for these ionizations. Recent determinations,” by means of spectrophotometric observations a t 15, 25, 35 and 45”,of the heats of ionization of cytosine and deoxycytidine have given the values A H i o n i z . = 4200 500 and 4900 f 500 cal./mole, respectively. These values are in reasonable agreement with the calorimetric TTalues and thus lend support to the view that the ionization processes in a t least these two cases are probably not complex reactions.12 A number of interesting points can be made concerning the effect of change of structure on the heat of ionization. In the cytosine series, introduction Results and Discussion of the Yugar moiety has very little effect on the heat The experimental results are summarized in of ionization. The same is true in the adenine Table I. For each substance a minimum of four series, in which case it appeares that the ribose and independent determinations of the heat of ioniza- deoxyribose moieties produce practically the same tion (the heat change accompanying the dissociation effect, as would be expected. However, in the of protons from the conjugate acid) were made. other purine series there is a striking difference between the riboside and the deoxyriboside, which is TABLE I also manifested in the pK’ values. (Guanine HEATS OF THE IoxrzArIox OCCURRING AT THE INDICATED could not be measured because of its very low solupK‘ I’ALUES OF VARIOUS NUCLEOTIDES AND RELATED bility.) Those differences are so large that one CO’4POUNDS AT 25” suspects that the usual structures for the guanine Standderivatives do not adequately represent all the inard OH - @H a t error, tramolecular interactions which actually take AHionir., cal./ 0.005 M . Substance cal./mole mole pK’ cal./molea place. Phosphorylation of the deoxyriboside in the 5’200 4.1 -180 3990 Adenine position has practically no effect on the A H i o n i n . 4470 Cytosine 180 4.5 ( - 30) and the pK’ in the case of deoxycytidine, whereas in 60 3 6 (-150) Adenosine 3810 the cases of both the purine derivatives AHionis. is 80 3 8 -150 3870 Deoxgadenosine decreased by 1200-1800 cal./mole, with pK’ again 50 4.3 - 30 4300 Deoxycytidine not undergoing much change. 250 16 -220 990 Guanosine The data listed in Table I do not, of course, give Deoxy guanosine 1910 90 2 5 (-220) support to the assumption made in our earlier 120 4 0 (-180) 5’-Deoxyadenylic acid 2640 ~ o r kthat ~ , the ~ heats of ionization of the bases in 70 4.4 -2240 5’-Deoxycytidylic acid 4280 native and denatured DNA are negligible. On the 10 2.9 - 130 140 5’-Deoxyguanylic acid other hand it is conceivable that the bases have cona Values in parentheses are assumed values. siderably different heats of ionization in DNA than The mean values, after correction for heats of dilu- in isolated nucleosides and nucleotides. Further tion, are listed in the second column of the table, calorimetric experiments are under way on samples and the standard errors of the means are listed in of DNA containing markedly different adeninethe third column. A conservative estimate of the guanine ratios from that of the DNA (salmon testes) uncertainties in the heats of ionization is f (stand- used in our previous work. Such experiments may ard error 50) cal./mole. The data on heats of serve to clarify the situation with respect to the dilution are summarized in the fifth column, in the heats of ionization of the bases in DNA. form of values of the relative apparent molar heat Cox and Peacocke,6$6on the basis of titration contents a t 0.005 M , At these low concentrations, data, concluded that the heats of ionization of adethe heat contents are proportional to concentration. nine, cytosine and guanine in denatured DNA are The quantities in parentheses are estimated quan- -1, + 2 and +4 kcal./mole, respectively. These tities which were used in correcting the correspond- quantities are very different from those listed in ing heats of ionization. Table I. The apparent values of pK listed in the fourth (9) D. Shugar and J. J . Fox, Bio:/ii:u. Biophys. .Icid, 9 , 369 (1952). column of the table are average values taken from (IO) See, for example, d a t a listed b y J. T. Edsoll a n d J. R’gman, the literature, except that the value for deoxyadeno- “Biophysical Chemistry,” Vol. I , Academic Press, Inc., S e 8 v York, sine was arrived a t by adding 0.2 to the value for N.Y . , 1938, pp. 432, 464. (11) E. P. Geiduschek, private communication. a d e n o ~ i n e . ~Insofar as possible, values determined (12) For a brief discussion of possible discrepancies beta-een van’t a t ionic strengths of about 0.1 M are listed. Hoff a n d calorimetric values of A H for complex reactions see IV. W.
*
0
+
(8) J. M. Sturtevant, THISJOURNAL, 62, 3266 (1940).
Forrest and J . 11,Sturte\.ant, THISJ O U R N A L , 82, 585 ( 1 g c o ) .
