SODIUM SALT OF A DEOXYRLBONUCLEIC
ACID 407
gests the strong contribution of the interactions from hydrogen bonds and T electrons mentioned above. At full degree of neutralization, the P G A ion was found to be hydrated by three water molecules per monomer unit from the E.,, by the same method as that of the previous paper.** The extent of the electrostriction of PGA ions is seen to be about the same as that of PAA ions likewise having carboxyl groups.
of the length of the and this value was also found for PAA. PEI, and its analogs. O n the other hand, the Pig of PSt and P P were -6.0 and 0, respectively, being larger than -20. These large P?,, values wggested that the PSt and P P ions had the strong interactions with water molecules by K electrons of the benzene ring and the phosphate group, respectively. Similarly. the comparatively large P2,, value of PGA sug-
Acknowledgments. This work was supported in part by the grant-in-aid of the Ministry of Education.
(40) W . L. Masterton,J. Cheii~.Ph?,s., 22, 1830(1954).
Mean and Single-Ion Activity Coefficients and Transference Data of the Sodium Salt of a Deoxyribonucleic Acid in Aqueous Solution' Tsuneo Okubo and Norio Ise
Depurtment of Polj.nier Clieinistr),, K J v t o Unirrrsit)*,Kj'oto, Jupun. Receiced March I O , 1969 ABSTRACT: The mean activity coefficient of the sodium salt of a deoxyribonucleic acid, NaDNA, in two- and three-component systems, i.e.. NaDNA H 2 0 .and NaDNA NaCl H20,was investigated by the emf method. The transference and related measurements were also carried out for the determination of the net valency and the transference number of DNA anions. The polymer charge fraction, ,fip, was concentration insensitive, decreased with increasing concentration of added salt. and was larger in the denatured state than in the native one. The H,O system was about 0.5, being insensitive toward the transference number of DNA anions in the NaDNA polymer concentration. The mean activity coefficient of NaDNA increased sharply with polymer concentration (0.0007 0.003 equiv l.-I), indicating the presence of the strong short-range interactions between DNA molecule and solvent water. A similar tendency was also noticed for the single-ionactivity coefficient of DNA anions. The single-ion activity coefficient of gegenions slightly decreased with polymer concentration.
+
+
+
+
-
T
he solution properties of deoxyribonucleic acid (DNA) have been intensively investigated by using various experimental techniques, i.e., light scattering,2 v i s ~ o s i t y ,sedimentation,4 ~ Donnan equilibrium,j electrophoresis,6 etc. However, n o measurement has been reported o n the mean activity coefficient, which is one of the most fundamental thermodynamic quantities. We have measured this quantity for several synthetic polyelectrolytes by emf measurements of a concentration cell with and by isopiestic vapor pressure measurements. lO-1: Recently Dolar and Les( I ) (a) Presented a t the 18th Annual Meeting of the Society of High Polymers, Japan, May 1969, Kyoto. ib) Supported in part by the G r a n t in Aid of the Ministry of Education. ( 2 ) See, for example. P. Doty and B. H. Bunce, J . Amer. Chem. Soc., 74, 5029 (1952): G. L. BroLvn, M. B. M'ELven, and M . I. Pratt, ,Yature, 176, 161 (1955): J. Hermans, Jr., J . Phj,s. Chem., 63, 175 (1959). (3) See, for example, B. E. Conway and J. A. V. Butler, J . Po/.rm. Sci., 42, 441 (1960). (4) See, for example, A . R. Peacocke and H. I'
MtAh
TABLE II ACTIVITY COEFFICIENT O F -
NaDNA
Concii. equiv I -
d
' 0 4
0031
0302
0 0 0 0 0 0 0
0003
CZA Ccncentrat on imonorncle I ' )
Figure 3 Equivalent conductance of DNA ions at 25". curve 1. native. m3 = 0 001, 2. native, m3 = 0 003, 3, denatured. mi = 0 001, 4, denatured, md = 0 003 in the light of the ,ti,>values of the two conformations, for the hP of the native D N A is strikingly larger than that of the denatured DNA. From the transference experiments, the equivalent conductance of the hypothetically fully dissociated macroions, can be derived.20 Figure 3 shows the values calculated for the native and denatured D N A in the presence of added salt (m3 = 0.001 and 0.003). The figure shows that the helical D N A moves faster than the coiled one. This is clearly understandable since the rodlike native D N A would have a smaller frictional resistance than the expanded denatured D N A . From the A*,, values, one can calculate the mobility of macroions, Uz,,,by the relation
U,,
=
xQpf:,p/m2F= 1O-3f,&.bp,iF
(4)
wherz xlp is the macroion contribution to the specific conductivity and F the Faraday charge. The UQl, values of N a D N A ions thus obtained are given i n Table I (the last column). It is seen that the U,, values are larger for the coiled state than for the helix state, in contrast with the case. This result is due to the fact that the /., values of the denatured N a D N A are strikingly larger than those of the native one (see the ninth column of Table I). It should be noted, however, that the U,,, values observed by electrophoresiszs,29 were larger for the native state than for the denatured one, in contradiction with our results. The reason for this discrepancy is not clear at present; differences in the experimental conditions such as pH and concentrations of added salts might be responsible. The essential data of the mean activity coefficients of the denatured N a D N A obtained from the emf of a concentration cell with transference are given in Table 11. The first column gives the concentration of solution 2 in the cell I1 (equivalent per liter). that of solution 1 being the most concentrated solution. The -ye values (in the last column of Table 11) were the activity coefficients value at 1?7 = 0.001 by determined on the basis of a the same convention as adopted in the previous The strikingly increased with increasing polymer concentration. This tendency was also observed for sodium salts of poly(viny1 alcohol) partially acetalized with glyoxylic acid (NaPVAG),Rand for sodium polyphosphate (NaPP),I5 sodium poly(styrene sul(28) L. Constantino, A . iM. Liquori, 2 n d V. Vitagliano, Biopolymers, 2, 1 (1964). (29) B. M. Olivera, P. Bailie, and N . DaLidson, ibid., 2, 245 (1964).
-
-l
-
IU P U R E
THt
WATER
Emf.' mV 8 II 6 19
00144
4 45
00192 00216 00230 00287
3 17 2 28 1 89 0
DENATUR~D 25' __
Ya
f.,,h
-
~-
~
000719 000958
AT
0 0 0 0 0 0 0
523
532 515 511
499 490 549
~~
0 0 I I 1 2 3
95 98 24 49 95 26 33
The emf measurements were carried out with ml = 000287 From Table I of the present paper The valujs were determined using a y2g a value at 0 001 equiv 1 - I fonate) (NaPSt),'" and others,ll.li,liand is attributable to ion-solvent interactions such as hydrophobic, electrostrictional, 7i.-electron interactions, etc. Thus, it is highly probable that the D N A molecule interacts strongly with solvent molecules, since the increasing tendency of -ya is observed in a range of highly dilute concentrations, in which most of the synthetic polyelectrolytes show a decreasing trend of Y ~ .The D N A molecule is composed of monomer units (mononucleotides) having three groups-phosphate, ribose, and base. The phosphate groups can interact strongly with water by charge-transfer-type interactions as was discussed o n the basis of the osmotic and mean activity coefficients for polyphosphate.16 Ribose may also interact with water through a hydrogen bond by its O H groups, which is one factor that prevents the formation of icebergs in water. This has been suggested both from the solubility of naphthalene in aqueous solutions of soluble and from the mean activity coefficients of ~arboxymethylcellulose.3~These polymers are quite similar to the ribose and give rise to only weak hydrophobic interaction with water. Furthermore, the nitrogen atoms in the four kinds of bases, i.e., adenine, guanine, cytosine, and thymine, can interact strongly with water molecules by hydrogen bond formation. Thus all three groups in the mononucleotides interact with water, and d o not have strong hydrophobic effect on water structure. The polymer concentration dependence of ya of the denatured DNA, therefore, is quite logical. The single-ion activity coefficient of the sodium ions of the denatured N a D N A was measured, though its physical meaning is very obscure.3*,3 3 The stoichiometric and thermodynamic single-ion activity coefficients of gegenions are denoted by -y2r,s and -y2r,a respectively, and are interrelated to one another by 7..
.. = .f.,r.,.a
(5)
where /.;, denotes the fraction of effectively disassociated gegenions, and can be determined by the transference measurements. The definitions of these two coefficients were given in the previous paper. l 9 The obtained results are given in Table 111. Both ycp,z and -yPe,a are observed to decrease slightly with increasing D N A concentration. (30) T. Okubo and N . Ise, J. Ph1.s. Cheni., 73, 1488 (1969). (31) N . Isr, I
