differential capacitance of mercury in the absence and the presence of adenosine in aqueous...

Surface Technology, 18 (1983) 303 - 312 303

DIFFERENTIAL CAPACITANCE OF MERCURY IN THE ABSENCE AND THE PRESENCE OF ADENOSINE IN AQUEOUS ELECTROLYTES

H. A. GHALY

Chemistry Department, Faculty of Science, Assiut University, Assiut (Egypt)

F. EL-TAIB HEAKAL and A. A. MAZHAR

Chemistry Department, Faculty of Science, Cairo University, Giza (Egypt)

(Received August 16, 1982)

Summary

The differential capacitance of mercury in the absence and the presence of adenosine was traced in 1.0 N aqueous solutions of Na2SO4, NaC104, KNO3, KCI, KBr and H2SO4 over appropriate potential ranges at 1 kHz and 25 °C. The results are informative with respect to the adsorption, reorienta- tion and association of adenosine in Na2SO4 and NaC104, which ave assumed not to exhibit specific anion adsorption. The area occupied by the molecule in the vertical orientation was calculated from the relative surface coverage by a monolayer of adenosine and the associated minimum capacitance. The measurements provide evidence of the effect of both C104- and Br- anions on the stacking interactions between the adsorbed molecules, which prevent the formation of multilayers in these electrolytes. Measurements of the dif- ferential capacitance of mercury in the presence of adenine, ribose sugar and a mixture of both lead to the conclusion that the purine part plays the major role in the reorientation of adenosine.

1. Introduct ion

The adsorption of adenosine at the mercury-e lec t ro ly te interface was first mentioned in the investigations of Vetterl and coworkers [1, 2] and Janik and Elving [3] who reported that adenine nucleosides ave strongly adsorbed at about --0.6 V. Otherwise, no relevant results on the adsorption, orientation and association of adenosine have been reported. Recently, how- ever, Vetterl [4] has discussed the mechanism of the adsorption and associa- tion of cytosine on negatively charged surfaces; the reorientation to the vertical posit ion at more negative potentials was facilitated by the interac- tion of the electric field with the permanent and induced dipole moments of the molecules.

In the present s tudy differential capacitance measurements were extended to include neutral or acidic aqueous electrolytes, exhibiting or not

0376-4583/83/0000-0000/$03.00 © Elsevier Sequoia/Printed in The Netherlands

i E o

t,9

exhibiting specific anion adsorption. Anions such a s C 1 0 4 - which affect the association of adenosine were included in this study. Measurements of the differential capacitance of mercury in the presence of adenine, ribose sugar and a mixture of both were included. The results obtained proved useful in establishing the conclusions inferred from electrocapillary measurements.

2. Experimental details

Differential capacitance measurements were made using a simple a.c. bridge of the Wein type [ 5]. A full description of the bridge, capacitance cell and working procedure has been given previously [6]. A saturated calomel electrode was used as the reference electrode throughout. The preparation of 1.0 N solutions of Na2SO4, NaC104, KNO a, KC1, KBr and H2SO 4 and the prep- aration of the adenosine and adenine solutions was accomplished as described in the accompanying paper [7]. Ribose sugar (BDH, AnalaR grade) was used as provided. All measurements were made in an air bath adjusted to 25 °C using a frequency of 1 kHz.

3. Results and discussion

The set of differential capacitance curves obtained in 1.0 N N a 2 S O 4 in the presence of adenine (Fig. l(a)) bears a strong resemblance to that ob- tained by Vet ted [8] for adenine in 1 N NaC1. In the present work, however,

I

2 5

50 !

3 0 4

X

0 -0 .4 - 0 8 -12 - 1 6 0 - 0 4 - 0 8 -I 2 - [ 6 0 - 0 4 - 0 8 - 1 2 -I 6

(a) (b) E,V ¢sce) (C)

F i g . 1. D i f f e r e n t i a l c a p a c i t a n c e c u r v e s i n ( a ) 1 . 0 N N a 2 S O 4 + x × 1 0 - 3 M a d e n i n e ( c u r v e 1, x = 0 . 0 ; c u r v e 2, x = 1 . 0 ; c u r v e 3 , x = 2 . 0 ; c u r v e 4 , x = 2 . 5 ; c u r v e 5, x = 4 . 0 ; c u r v e 6, x = 6 . 0 ) , ( b ) 1 . 0 N N a 2 S O 4 + x × 1 0 -3 M r i b o s e ( c u r v e 1, x = 0 . 0 ; c u r v e 2 , x = 1 . 0 ; c u r v e 3, x = 6 . 0 ; c u r v e 4 , x = 1 0 . 0 ; c u r v e 5, x = 2 0 . 0 ) a n d ( c ) 1 . 0 N N a 2 S O 4 + x × 1 0 -3 M a d e n i n e + x × 1 0 -3 M r i b o s e ( c u r v e 1, x = 0 . 0 ; c u r v e 2, x = 1 . 0 ; c u r v e 3 , x = 2 . 0 ; c u r v e 4, x = 4 . 0 ;

c u r v e 5 , x = 6 . 0 ) .

305

the minimum capacitance is reached at a lower concentration of adenine because of the absence of specific anion adsorption. Thus the adsorption of adenine at lower concentrations is similar to that of simple organic molecules. The curves reveal that adenine is strongly adsorbed at the mercury drop elec- trode and that, with increasing adenine concentration, the capacitance decreases sharply, forming a very steep minimum or pit which widens with increasing concentration. Brab~c e t al. [9] have tentatively suggested that these anomalous pits are formed by a surface reorientation process but have admitted that their suggestion is without supporting evidence. It is more probable that this minimum, characteristic of the association of adsorbed molecules [10], is indicative of a two-dimensional condensation of the adsorbate [11] with the result that a different surface f i lm is formed owing to multilayer stacking.

Figure l (b) shows that the adsorption of D-ribose, the sugar component of adenosine, is very weak even at a concentration of 2.0 × 10 -2 M of ribose, which is the maximum concentration reached in the s tudy on adenosine. The same result was obtained for weakly alkaline solutions, i.e. for 1 N Na2HPO4.

A comparison of the set of differential capacitance curves of mixtures of adenine and ribose in 1.0 N Na2SO4 (Fig. l (c)) shows that they are identi- cal with those obtained for adenine alone. This reveals, as expected, that the presence of ribose sugar does not affect the adsorption of adenine. It should be noted that during the adsorption of adenine at the mercury drop elec- trode (Fig. l(a)) the transition from the non-associated to the associated state occurs over a small concentration interval. It has been pointed out by Vetterl [1] that this transition is manifested by an inflection point on the adsorption isotherm. This has actually been observed for the adsorption of adenine at the zero-charge potential (see accompanying paper [7], Fig. 8). The direct conclusion to be drawn from these results is that adenine forms a monolayer first but that on slightly increasing its concentration association occurs with the formation of multilayers.

As for the adsorption of adenosine, the results of differential capaci- tance measurements in the various electrolytes are shown in Figs. 2 - 7. The highest concentration of adenosine reached in this s tudy was 2.0 × 10-2 M, which is the maximum solubility of adenosine. The general shape of the set of differential capacitance curves obtained in 1.0 N solutions of Na2SO4, KNO3 and KC1 (Figs. 2, 4 and 5) shows a strong resemblance to that of the curves previously reported [1] for 0.5 N NaF with 0.1 M phosphate buffer, pH 7.0, which seem to be the only results published until now. A com- parison of the parameters of the electrocapillary maximum of mercury in the various pure electrolytes used [7] shows that Na2SO4 should exhibit the least tendency for specific anion adsorption. It was expected that the Na2SO 4 electrolyte would give the most explicit picture of the adsorption of adenosine. However, the curves obtained for 1.0 N NaC104 (Fig. 3), which is also known to exhibit weak anion adsorption, take a simpler course than those for other electrolytes. It will therefore be convenient to consider the results for 1.0 N NaC104 as the reference for those obtained in the other electrolytes.

60"

20

I E 4C u

L~

3

• 6 7 8,9

306

- o o -0.4 -o ,8 -~ z - t 6

E , V (sce)

Fig. 2. D i f f e r e n t i a l c a p a c i t a n c e c u r v e s in 1.0 N Na2SO4 + x x 10 -3 M adenos ine for various v a l u e s o f x : curve 1, 0 .0 ; cu rve 2, 1.0; curve 3, 2.0; curve 4, 4 . 0 ; cu rve 5, 6.0; curve 6, 8.0; curve 7, 10.0; curve 8, 12.0; curve 9, 15.0; curve 10, 20.0.

cu

E u

~9

6 0 -

4 0

-0 .0 - 0 4 -0 .8 - I .2 - I .6

E , V (sce)

Fig. 3. D i f f e r e n t i a l c a p a c i t a n c e c u r v e s in 1.0 N NaCIO4 + x x 10 -3 M adenos ine for various va lues o f x : curve 1, 0.0; curve 2, 1.0; curve 3, 6.0; curve 4, 10.0, curve 5, 15.0; curve 6, 20.0.

Figure 3 shows that at low concentrations of adenosine the capacitance decreases monotonical ly in the region of the electrocapillary maximum and then rises again to show adsorption-desorption peaks on the extreme catho- dic and anodic sides. At such low concentrations, i .e. up to 8 X 10 -3 M, the adsorption behaviour resembles that of simple organic molecules. (For

307

6 0 -

4 0

2 0

6 0

4 0

; tO

- 0 . 0 - 0 . 4 - 0 8 - I .2 - I . 6

E , V ( s c e )

Fig. 4. Differential capacitance curves in 1.0 N KNO3 + x x 10 -3 M adenosine for various values of x : curve 1, 0.0; curve 2, 1.0; curve 3, 4.0; curve 4, 6.0; curve 5, 8.0; curve 6, 10.0; curve 7, 12.0; curve 8, 15.0;curve 9, 18.0;curve 10, 20.0.

CJ I E u

L)

- 0 . 0 - 0 . 4 - 0 . 8 -1 ,2 - 1.6

E , V ( s c e )

Fig. 5. Differential capacitance curves in 1.0 N KCI + x X 10 -3 M adenosine for various values of x: curve 1, 0.0; curve 2, 1.0; curve 3, 4.0; curve 4, 6.0; curve 5, 8.0; curve 6, 10.0; curve 7, 12.0; curve 8, 15.0; curve 9, 18.0; curve 10, 20.0.

c l a r i t y , t h e d i f f e r e n t i a l c a p a c i t a n c e c u r v e s f o r t h e v a r i o u s e l e c t r o l y t e s d o n o t i n c l u d e al l t h e c o n c e n t r a t i o n s a c t u a l l y i n v e s t i g a t e d . ) A t a c o n c e n t r a t i o n o f 9 × 10 -3 M t h e c a p a c i t a n c e r e a c h e s a m i n i m u m Cm-p o f 1 1 . 7 # F c m -2 a t a p o t e n t i a l o f - - 5 0 0 m V . T h e c a p a c i t a n c e t h e n r i ses g r a d u a l l y t o r e a c h a m a x i - m u m o r p e a k a f t e r w h i c h i t f a l l s a p p r e c i a b l y i n t o a t r o u g h b e f o r e t h e a p p e a r a n c e o f t h e c a t h o d i c d e s o r p t i o n p e a k s . T h e s u p p r e s s e d a n o d i c d e s o r p - t i o n p e a k m e r g e s w i t h t h o s e b e l o n g i n g t o h i g h e r a n d l o w e r c o n c e n t r a t i o n s .

60"

r~ t E u

o

4C

2G

3 9

\ 308

- 0 . 4 - 0 . 8 - I . 2 - I .B

E, V ( s c e )

Fig. 6. Differential capaci tance curves in 1.0 N KBr + x × 10 -3 M adenos ine for various values of x : curve 1, 0.0; curve 2, 1.0; curve 3, 4.0; curve 4, 6.0; curve 5, 8.0; curve 6, 10.0; curve 7, 12.0; curve 8, 15.0; curve 9, 18.0; curve 10, 20.0.

With increasing adenosine concentration the capacitance trough formed at the more negative potentials becomes gradually lower and wider until it reaches a minimum value of 4.3 #F cm -2 at an adenosine concentration of 15 × 10 -3 M. A further increase in adenosine concentration leads to a widening of the capacitance trough formed at the more negative potentials, but no appreciable change occurs at the minimum formed at less negative potentials.

With 1.0 N Na2SO4 as electrolyte (Fig. 2) the differential capacitance curves show a more complicated trend. At a concentration of 4 × 10 -3 M the capacitance reaches a minimum Cm-p of 10.8 pF cm -2 at a potential of --400 mV. An increase in the adenosine concentration to 5 × 10 -3 M has a marked effect on the morphology of the differential capacitance curves. At less negative potentials the relatively symmetrical capacitance trough sud- denly changes to form a sharp minimum. At more negative potentials the capacitance trough widens on both the anodic and the cathodic sides whilst the minimum capacitance becomes lower. Meanwhile, the transitional peak and cathodic desorption peak become more accentuated, the former shifting to less negative potentials and the latter to more negative potentials.

A further increase in adenosine concentration to 6 X 10 -3 M leads to a widening of the minimum capacitance of the pit on the less negative poten- tial side of the curve, whilst the minimum capacitance value remains at 7.2 pF cm -2. At more negative potentials a f lat-bottomed trough is formed with a minimum capacitance Cm-v of 4.2 #F cm -2. However, the transition peak becomes lower whilst the height of the cathodic desorption peak becomes nearly constant.

309

An increase in the concentrat ion to values of between 8.0 X 10 -3 and 20.0 X 10 -3 M leads only to a widening of both pits wi thout any change in the values of the minimum capacitance. The transition peaks gradually become lower and at the two highest concentrations attain a somewhat flat configuration.

The appearance of the intermediate peak should obviously be con- nected with the orientation kinetics of the adsorbed molecules, since it is not an adsorpt ion-desorpt ion peak. The absence of this peak in very dilute adenosine solutions indicates the prevalence of single-orientation adsorption which is presumably in the planar form.

At relatively higher adenosine concentrations the capacitance trough on the cathodic potential side indicates a vertical orientation which is favoured by the high negative charge on the mercury surface. At potentials less cathodic than that corresponding to the reorientation peak, a planar orien- tation is favoured. It should be noted that reorientation of the molecules occurs at or above the concentration necessary for the capacitance at less negative potentials to reach Cm-p.

It is clear that the ribose group plays an important role in the reorienta- tion of adenosine molecules since in its absence, i.e. for adenine, no reorien- tation is observed. A folded structure [12] has been suggested for the vertical stacks at more negative potentials. Although the mode of arrangement of individual molecules in these stacks is still a matter of speculation, some models are favoured. For 9-alkyl purines Philips e t al. [13] proposed that a pyrimidine ring faces an imidazole ring, while for purine nucleosides it was proposed [14] that pyrimidine faces pyrimidine with the ribose moieties being opposite to one another in the stack. The presence of protonated nitrogen [1] would tend to favour alternate stacking (pyrimidine facing imidazole} because of the reduced electrostatic repulsion. The alternate stacking model would also explain bet ter the relatively strong concentrat ion effect, even at relatively low concentrations. This is actually the case in all electrolytes.

As with adenine, the f la t -bot tomed pit formed at less negative potentials may be explained by two-dimensional condensation, i.e. the formation of multilayers. For adenine, an effectively complete monolayer is formed (0 = 1) at concentrations of about 2 X 10 -3 - 3 X 10 -3 M at pH 9 [15]. Our results indicate that this monolayer is formed by adenosine molecules in 1.0 N Na2SO4 at 4 X 10 -3 M. Two adsorption stages may be distinguished: a rather dilute adsorption layer, and at higher bulk concentrations a condensed ad- sorption film where significant forces act between adjacent adenine base rings.

In NaC104, the differential capacitance curves of adenosine attain a minimum capacitance Cm_p of 11.7 pF cm -2. Even at the highest adenosine concentrations, no indication is found of the formation of multilayers, which is indicated by the steep minimum. There is sufficient evidence to show that CIO~ anions disturb the stacking interactions between adsorbed molecules of adenosine [16] and cytosine [4] and prevent the formation of multilayers. Also, the activity coefficient of adenosine decreases in the

310

presence of C104 ions [17]. These factors cause the formation of mono- layers of adenosine at the potentials at which planar orientation prevails. This fact was used in assessing the area occupied by an adenosine molecule in a vertical orientation when Frumkin's simple relation [18] was applied in the manner described in ref. 19, with the Cm-p and Cm-v values obtained in NaC104, i.e. 11.7 pF cm -2 and 4.3 pF cm 2 respectively.

Although nothing has been reported in the literature on the area of adenosine in the vertical orientation, it is quite probable that the relatively small area of 15.1 A 2 obtained in this study is a result of the partial overlap which occurs with vertical stacking [20]. This area is presumably that actually occupied by a vertically oriented molecule.

The differential capacitance curves in 1.0 N KNO3 and 1.0 N KCI reflect the same general features as those described above for 1.0 N Na2SO4. The main difference seems to be connected with structural changes in the elec- trical double layer due to specific anion or salt adsorption, particularly in the region of potential at which planar adsorption of adenosine occurs.

In these two electrolytes the concentration limit necessary to bring about complete surface coverage by a monolayer of planar-oriented mole- cules is 6 X 10 -3 M, whereas for complete surface coverage with vertically oriented molecules on the cathodic potential side a concentration of 10 X 10 -3 M adenosine is required. C m p and Cm -v values in 1.0 N KNO a are 11.8 pF cm 2 and 4.3 pF cm -2 respectively, whereas in 1.0 N KC1 they are 12.0 pF cm -2 and 4.4 gF cm -2 respectively.

In 1.0 N KBr electrolyte the differential capacitance curves are similar to those obtained in 1.0 N NaC104. The value of Cm-p, however, is higher compared with that obtained in other electrolytes. The concentrations neces- sary to attain complete coverage by molecules with planar and vertical orien- tations are also higher than those required in other electrolytes. Probably double-layer effects are critical in KBr. This may be deduced from the fact that at more negative potentials the curves are identical with those obtained in other electrolytes. It is quite possible that Br- ions, like C104- ions, dis- turb the stacking interactions between the molecules or lower the activity coefficient of adenosine and thus prevent the formation of the minimum which indicates the formation of multilayers.

In all electrolytes, Em~ is always markedly shifted towards less negative potential values from Eorient. This also excludes the possibility of vertical adsorption at the uncharged mercury interface in these electrolytes. From this point of view the adsorption behaviour in these electrolytes is thus iden- tical with that observed and discussed previously for Na2SO4 and NaC104.

If Cm_ v is assumed to correspond to complete surface coverage (0abs = 1), then an important conclusion emerges from the expected variation in 0 with potential. At potentials less cathodic than E~_p (on the anodic side) gradual desorption is expected to occur and 0 becomes progressively less than uni ty until complete desorption occurs. At potentials progressively more cathodic than Em p orientation effects are assumed to set in which deprive 0 of its exact physical significance as a clear-cut indicator of planar orientation.

311

(m

E

% L)

60"

o.o - 0 4 -o . s - I z - i . 6

E , v ( s c e )

Fig. 7. Differential capaci tance curves in 1.0 N H2SO4 + x × 10 -3 M adenos ine for various values of x: curve 1, 0 .0 ; cu rve 2, 1.0; curve 3, 4.0; curve 4, 6.0; curve 5, 20.0.

The above qualitative implications are in agreement with the adsorption isotherms obtained for the uncharged mercury surface in these electrolytes. The corresponding F values in KBr and KC1 did not attain the saturation limit except at relatively higher concentrations; this result is different from the results obtained in Na2SO4, NaC104 and KNO3 for the same adenosine concentration. This may be at t r ibuted to structural changes in the double layer owing to specific anion or salt adsorption.

The capacitance ratio gives a true measure of the area ratio only under the condit ion Cm-p + Cm-v ~ Co, which almost holds for NaC104 and KC1. It can be established from the various sets of differential capacitance curves in 1.0 N KNO3, Na2SO 4 and KBr that the greatest departure from the above equality is observed for KBr; such proport ionali ty between the Cm values and areas will then be invalid.

One remark needs to be made with respect to adsorption of adenosine in 1.0 N H2SO4. From Fig. 7 and our results reported in the accompanying paper [ 7 ], it can be seen that adenosine is apparently very weakly adsorbed in the presence of 1.0 N H2SO4, and E ~ is shifted to less negative poten- tials instead of to more negative potentials as observed in neutral electrolytes. Retter e t al. [2] noticed that, during the adsorption of adenosine in buffers of pH 5.06 - 8.98, maximum adsorption occurred at pH 7. It is quite possible that some kind of complex, similar to that formed between adenosine and H3PO4, H2C204 or HNO 3 [21], is formed between adenosine and H2SO4. Acid hydrolysis seems to be ruled out since this requires a high temperature [22]. Obviously the new compound will behave differently from adenosine, and this different behaviour was in fact observed in both the electrocapillary and the differential capacitance measurements.

312

References

1 V. Vetterl, J. Electroanal. Chem., 19 (1968) 169. 2 U. Retter, H. Jehring and V. Vetterl, J. Electroanal. Chem. InterfacialElectrochem.,

57 (1974) 391. 3 B. Janik and P. J. Elving, J. Am. Chem. Soc., 92 (2) (1970) 235. 4 V. Vetterl, Bioelectrochem. Bioenerg., 3 (2) (1976) 338. 5 M. Wein, Ann. Phys. (Leipzig), 58 (1896) 37. 6 A. A. Moussa, H. M. Sammour and H. A. Ghaly, J. Chem. Soc., (1958) 1269.

A. A. Moussa and H. M. Sammour, J. Chem. Soc., (1960) 2151. 7 H. A. Ghaly, F. E. Heakal and A. A. Mazhar, Surf. Technol., 18 (1983) 293. 8 V. Vetterl, Experienlia, 21 (1965) 9. 9 V. Brab~c, S. D. Christian and G. Dryhurst, Biophys. Chem., 7 (1978) 253.

10 W. Lorenz, Z. Elektrochem., 62 (1958) 192. 11 A. N. Frumkin and B. B. Damaskin, in J. O~VI. Bockris and B. E. Conway (eds.),

Sovremennye Aspekty Elektrokhimii, Mir, Moscow, 1967, Chap. 3. 12 B. Pullman and A. Pullman, Quantum Biochemistry, Wiley, New York, 1963. 13 R. Philips, P. Eisenberg, P. George and R. J. Rutman, J. Biol. Chem., 240 (1965)

4393. 14 A. D. Broom, M. P. Schweizer and P. O. P. Ts'O, J. Am. Chem. Soc., 89 {14) (1967)

3612. 15 H. Kinoshita, S. D. Christian and G. Dryhurst, J. Electroanal. Chem., 83 (1977) 151. 16 A. A. Maevskii and B. I. Sukhowkov, Biofizika, 21 (6) (1976) 1122. 17 D. R. Robinson and M. E. Grant, J. Biol. Chem., 241 (17) (1966) 4030. 18 A. N. Frumkin, Z. Phys., 35 (1926) 792. 19 A. A. Moussa, H. A. Ghaly, M. M. Abou-Romia and F. E. Heakal, Electrochim. Acta,

20 (1975) 489. 20 S. I. Chan, M. P. Schweizer, P. O. P. Ts'O and G. Helmkamp, J. Am. Chem. Soc., 91

{1969) 2843. 21 N. R. Dhar and G. P. Ghosh, Proc. Natl. Acad. Sci., India, Sect. A, 31 (1) (1961) 78. 22 A. Wacker and L. Traeger, Z. Naturforsch., TeiIB, 18 (1963) 13.