2862
M. A. FROMMER AND I. R. nfILLER
reaction occurs on that part of the electrode not covered by 0 type, as for HCOOH oxidation.28.2g
Summary and Conclusions (1) Anodic stripping accurately measures CH4 adsorption. (2) The CH4 adsorption rate is diffusionally controlled from 0.25 t o 0.35 V in a quiescent solution. Mixed diffusion-activation rate control is suggested at other potentials. (3) Both the calculated diffusion and the number of sites apparently occupied by the CH4adsorbate suggest that the adsorbate is in a highly oxidized state. This is confirmed by the &-e plot constructed from the adsorption data. (4) None of the steady-state adsorbate can be cathodically desorbed either on smooth or platinized
Pt. (5) Steady-state coverage is higher on smooth than on platinized Pt and increases with CHI pressure above -0.35 V. Below this potential, it is pressure independent.
(6) The steady-state adsorbate is similar to the O-type hydrocarbons for CaHg, n-C~H14, and to “reduced COz”; i ~ . ,it is an oxygenated C1 species. (7) This species is formed directly from CH4 and not from reduction of anodically evolved COz. (8) Because the initial CHI adsorbate (a C1 species, of course) is rapidly converted to 0 type, it is not possible to use CH4 to study this process. It does demonstrate, however, that C1 species, once produced from other hydrocarbons, will rapidly convert to the 0 type. (9) Since the O-type coverage produced from CH, i s higher than that from CO2,l2the former is more suitable to study the reactions of 0 type.
Acknowledgments. We are pleased to acknowledge support of this work by the U. S. Arrny Mobility Equipment Research and Development Center, Fort Belvoir, Va., on Contract DA-44-009-AMC-l408(T). (28) 8. B. Brummer and A. C. Makrides, J . Phys. Chem., 68, 1448 (1964). (29) S. B. Brummer, J . Electrochem. Soc., 113, 1041 (1966).
Adsorption of DNA at the Air-Water Interface1 by M. A. FrommeP and I. R. Miller Polymer Department, Weizmanm Institute of Science, Rehovoth, Israel
(Received January 90,1968)
The physical properties of DN4 adsorbed a t the air-water interface were determined by measuring directly the rate and the amount of adsorption of E8cherichia coli [SHIDNA a t the interface and by detecting the resultant changes in the surface pressure and potential. It was found that the rate of adsorption and the amount adsorbed increase to limiting values with an increase in either the DNA or the salt concentration. No measurable changes in surface pressure and potential could be detected. Arialysis of the Gibbs equation for polyelectrolyte adsorption reveals that a negative surface excess of a low molecular weight salt is expected either from the excluded-volume considerations or from the Donnan-equilibrium considerations. The negative surface excess of the salt balances the positive excess of the polyelectrolyte, and despite considerable adsorption, the surface tension is practically unaltered. Owing to the low surface activity of the DNA, only a small fraction of the adsorbed molecule is anchored a t the interface, and the surface potential remains practically constant. The decrease in eiecfrostsltic repulsion with the increasing salt concentration, in the monolayer and between the monolayer and the adsorbing polyelectrolyte, brings about an increase in the rate of adsorption and in the equilibrium surface concentration.
Introduction An understanding of the surface activity of nucleic acids is of great interest, since many biological processes take place at interfaces and many separation processes of RNA and DNA are based on their adsorption a t interfaces. Moreover, the surface activity of welldefined rodlike polyelectrolytes like DIC’A, as well as its variation when the rodlike tertiary structure is changed, is also of pure physicochemical interest. The Journal of Physical Chernistw
Even though the physical properties of polynucleotides adsorbed on various interfaces have been investigated,s-6 practically nothing has been reported on their (1) This work was sponsored by the National Institutes of Health under Research Grant No. GM-08-517 and under Research Agreement No. 615134. (2) This paper is part of a Ph.D. thesis submitted to the Feinberg Graduate School of the Wsizmann Institute of Science. (3) I. R. Miller, J . MOL Biol., 3, 229, 357 (19Gl). (4) I. R. Miller, Biochim. Biophys. Acta, 103, 219 (1965).
ADSORPTION OF DNA
AT THE
2863
AIR-WATERINTERFACE
behavior a t the water-air interface. The adsorption properties of any substance depend strongly on the standard free energy of adsorption (in the case of a polymer, it is the adsorptive energy per residue). This quantity depends on the nature of the residue as well as on the nature of the interface. The air-water interface differs essentially from water-mercury or w a t e r solid interfaces, and it is of interest to know to what extent this difference influences the adsorption properties. I n this study, the dependence of the adsorption rate and of the saturation surface concentration on the bulk concentrations of DNA and of salt, as well as on the nature of the counterion, are described. I n addition, it is shown to what extent the adsorbed DNA affects the surface tension and the surface potential of the water-air interface.
Experimental Section Materials. ‘I’ritium-labeled Escherichia coli DNA was purchased from Professor A. C. Paoletti’s laboratory in the Gustave Rousseux Institute, Villejuif, Paris. The tritium was incorporated into the DNA by growing E . coli in a culture medium containing labeled nucleotides. The E. coli [3H]DNAwas obtained in aqueous solution, and the hyperchromic jump measured, when it was received after shipment, decreased to 31% from the 40% value given in the specifications. Other specified values were : sedimentation coefficient Szo,w = 24; protein, l + RTPn cs + In (v4,cP + cd I
(3)
In c,)c, =
+ + ra(1 + +
r,[v4,ca/(V+,C,
CJl
[CS/(V4PC,
- (l/RT)(dy/b rP{
1
In c,)c, =
+ [V24P2C,/(~4,C, +
Cdl]
rs[v4PcP/(v4PcP
I n all the experimental cases v&cP
