May, 1967
SEDIMENTATION OF'
DEOXYRIBONUCLEATE IN MgCl2 SOLt7TIONS
in their behavior for the initial hydrogen adsorpt8ion. It, was shown in the previous paperb that the srirface of t.his germanium film behaves as though it is homogeneous in its properties for ammonia decomposition, and it is also the case, as will be shown i i i a later paper, for germane decomposition. Exchange Reaction between Hydrogen and Deuterium on the Germanium Film.-2.1 cm. of hydrogen and 2.9 cm. deuterium were admitted to a germanium film, with a surface area comparable with tlliose in the preceding adsorption measurements, at 302' for 130 minutes. The mass spectrometric analysis showed a large amount of hydrogen deuteridn produced in it, which supports the dissociative adsorption of hydrogen on the germanium film. The increased siirface area and the longer reaction times account for the deviations from the previous ohserva ti ons. l l 2 Activation Energy of Adsorption.-The activation energy of the adsorption was obtained from the dependence of the initial rate of adsorption at 0.065 mm. hydrogen pressuro on a bare germanium fiurface upon temperature. The results are shown in Fig. 5 and the activation energy was 14.6 kcal./ mole. The initial adsorption rate calculated from t.his activation energy and absolute rate theory,la is 4.2 X loQmdeculcs/sec. cm.2, while the observed rate is 3.0 X 10l1 moleculcs/sec. cix2, when the hydrogen pressure is 0.065 mm. Thc heat of ndsor tion on R bare surface is approximately 23.5 kcal./!mole and, consequently, the activation energy for desorption becomes 38 kcal./mole under this condition. Acknowledgment.-Thc assistance of Mr. B. W.
'1 40!8
Fig. 6.-Adsorption
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rates on bare siirfnce a t different temperatures.
Steiner in the mass spectromctric analyses i R gratefully acknowledged. The author is also indebted to Dean Hugh Taylor and Professor M. Boudart for their valuable suggestions and assistance, and to Yokohama National University, Japan, for a leave of absence. This work was carried out! on a post-doctoral fellowship kindly provided Princeton University by the Shell Fellowship Committee of the Shell Companies Foundation, Inc., New York City. We wish to express our apprcciation of this support. The work in question is also a part of a program of research supported by the Office of Naval Research NGonr-27018 on Solid State Properties and Catalytic Activity. Acknowledgment is made also to this research project for facilities used and for consultation wjt,h workers in the project.
THE SEDIMENTATION OF DEOXYRIBONUCLEATE IN MgCl, SOLUTIONS BY H. KAHLER ANI, J. SHACK National Cancer Institute, National Inatilutes of Health, U . R. Piihlac Ilealth Service, Belhenda 14, Maryland Received Deeembcr 12. 19.76
The sedimentation of DNA in MgClz solutions w& compared with its sedimentation in ICCI solutions. A t infinite dilution hoth sediment with similar velocity. A t finite concentrations the S coefficient in M K C Iis ~ hi her than in KCI and the hountlnries are less sharp. The viscosity in MgC12 is +so ICSE thrhn irl RCl. Addition of 0.16 $KCl to 0.05 Mg& diminkhes the sedimentation coefficient a t finite concentrations of DNA. Tho effcctjsnre not, the result of charge effects as o u t lined by Tiselius.' Mg++and K + compete for sites on the DNA structure, the resultant effect depending on the concentrations of the two ions.
The velocity of sedimentation of DNA (deoxyribonucleate) decreases rapidly as the concentration increases, This decrease in sedimentation velocity with concentration results in self sharpening during sedimentation, the sharpness of the boundary gradient being greater for greater changes in sedimentation velocity across the boundary. The attempt,s at a theoretical interpretation of the concentration effect in the sedimentfation of DNA presumably would be more successful if this effect were studied under as wide a range of conditions a8 possible. Numerous studies have been reported on the sedimentation of DNA in NaCl and KCl solut,ions, but only a single observation has been puhlished'.on its scdimentation in a bivalent salt, CaC12. The present work is conccrncd with (1) If. Knliler, T ~ r JOURNAL, e 68, 076 (1948).
a comparison of the sedimentation of DNA in KCl or NaCl and MgCll solutions. Materials and Methods One sample of DNA was prepared from calf thymus b the 8immons method.* Other samples prepared from cafi thymus and mouse lymphoma by a modified MirAkyPollister method previously described' were used. After preparation the fibers were cut into small pieces and thoroughIy mixed so that small portions taken for separate experiments would be truly representative of the whole preparation. The concentration of DNA in saline solutions was estimated by meaeuring the optical denRity at 260 mp and using the e (P) value determined for each preparation under the same conditions. All sedimentation experiments were performed in a
~(2)
.'h 8. Simmons,
9. A. Chavoa and H.
R. Orhnoh, U.C.L.A.
Report 184 (1953). (3)-J. Shack and J . M, Thompett, J . B i d . Chcm., 197, 17 (1952).
IT. KAHLERA N D J. SHACK
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10% greater than in 0.05 M MgC12.
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The sedimentattion coefficient for 0.1% DNA (calf thymus) in MgClz solutions is shown in Fig. 1. It appcars from this curve that the maximum is reachcd at 0.04 t o 0.05 M M g C h Since several of the studies report#edin the literature have been carried out nt 0.3 N I
