Radiation Chemistry in Organized Assemblies J. K. Thomas and T. S. Chen University of Notre Dame. Notre Dame, IN 46556
The past 30 years have seen gigantic strides in studies of the chemistry and physical properties of free radicals. This symposium concerns itself primarily with the chemistry of free radical species produced via the radiolysis of condensed systems. This paper is mainly concerned with the effects of complexity of organization on the radical processes, which can lead to signific'ant and useful modifications of the ohserved chemistry. Much of the chemistry of radiolysized systems is now understood, in particular the radiation chemistry of simple aqueous systems (1-7). The basic concepts require that the initial act of the high energy radiation produces ions, e.g., eand H20+, which rapidly (lo1' LM-' S-' in CTAB micelles (18).These large effects are satisfactorily explained by simple electrostatic re~ulsionor attraction effects of e-.,-* and the micelle surface (19),although an extended mechanism involving electron tunnelling has been suggested (20).Similar effects are observed with other complex systems such as hovine serum albumin, BSA (21). Hvdrated electrons react rapidly with this mol'ecule, hut addition of small amounts of ionic surfactantseither increases the reaction rate, as with CTAB which increases the positive charge on the protein, or decreases the rate as with NaLS which increases the negative charge (Fig. 2). This technique has been used to investigate the binding of drugs to BSA. Again, electrostatic effects effectively explain the rate data. Enveloping a solute by means of a neutral surfactant also decreases the rate of reaction of e-., -- with solutes such as DV.. rene (23);for the most part these smaller effects can he exDlained hv s i m ~ l esteric considerations.
Similar effects are observed for the transfer of electrons from one anion to another. For example, the transfer of e-,,, from C02- to pyrene is enhanced in CTAB micelles. In some cases the soluhilized molecule is remote from the micellel water interface and reaction of e-,, with the solute, although rapid in water, is very slow in the micellar system. For example, the reaction of e-,, and /%carotene in Triton X 100 micelles is slow, hut is enhanced on addition of CTAB to the micelles (26).The transport of e-,, to the 0-carotene may also he enhanced if biphenyl, $qis soluhilized in the micelle. The first reaction of e-,, produces $2- followed by transfer to
-
e-., + $2 $24%- + B carotene (,'-carotene)- + 6% 0-carotene, the d2 acting as an electron transport agent. @-caroteneis itself an efficient e - transport agent in vesicle systems (26).
-
Reaction of OH Radicals Polymers
Water soluble polvmers are imoortant models for more complex h i ~ - ~ o l y & esuch r s as D N A : T ~radiolysis ~ of aqueous solutions of polvmers follows two main ~ a t t e r n scrosslinkine : is observed k i t h polymers such as poiyvinyl p;rollidone or polyvinyl alcohol (27),while degradation occurs with polymers such as polymethacrylic acid (28).In both cases OH radicals or H atoms are the initiators of the damage via mechanisms OH
+ R-CH,R
vesicles (24).Contrary to ohserktions in homogenious solution, increasing the pyrene concentration does not produce a corresponding increase in the rate of formation of P-, hut the rate tends to a maximum. This is understood if the rate controlling feature of the reaction is the approach of e-,, to the vesicle. Once a t the vesicle the reaction with pyrene is rapid. This type of hehavior has been observed previously for polymers and is a feature of kinetics in aggregated systems (25).The rate of reaction of e-, with pyrene increases sharply a t the phase transition temperature of the vesicle.
---t
RCHR
+
Crosslinking
H,O
\
RHC-R CH,
I
/CH,
I
I
COOH
COOH CHz R-C
II + .C&-R
Degradation
I
COOH
The degradation reactions do not occur with smaller molecules as other radical processes are more prominent. Vesicles and Micelles
OH radical attack on micellar systems can lead to polymerization (29)and crosslinking. The rate constant for attack of OH radicals on a surfactant in a micellar form is much less than that for the monomeric form. Figure 3 shows the rate constant for OH reacting with several alkvl sulfates. The rate the surfactant NALS in the monomegcform. However, the rate constant k,i for OH attack on the micellar surfactant is cm much less. The k,,, is given hy 4 ~ y D where y is is the interaction parameter, and D, the sum of diffusion coefficients of OH a i d reactant. The rate constant for reaction of OH micelle is riven by a similar expression with apabout $0 proximately the same values of y and D. monomer units make up the micelle, so k,, in micelle form = h,iJ70, which leads to the lower k observed in Figure 4. Similar data have been observed for the OH reactions of e-., and with pyrene in lecithin vesicles (30,31). ~~~~
+
Figure 2. Effect o?surfactanton therate of reaction of e-, albumin. BSA.
with bovine serum
ow ever,
Volume 58
Number 2
February 1981
141
Figure 4. Effect of surfactant concentration on me rate of production, or yield of the nitroxide radical
Lo7 . 2
.
,
4
6
8
U
10
CARBON ATOMS
14
.
16
+
~ i g u r e3. Variation of rate constant for reaction of OH with alkyl sulfates.
+
Reaction of OH Counterion It is well known that OH radicals react with halide ions, X-, forming radicals of the type X2-, (e.g., 12-, Brz-, etc.).
.
the selective reactions of radicals that ~ r o v i d eisotone enrithmenr have heen rq~urted( 3 3 ) .Figure 4 illustrilre~a pro~ ~ u t l n wniiwilar d eiiect on the rearrim of a surfatrant frer radical and a spin trap.
X-+OH-X+OHx-+X-xzxp-+x2--x2+2x-
The formation and subsequent decay of these species via dimerization are all fast processes. Cationic micelles, e.g. CTAB has Br- counterions and the pulse radiolysis of these systems leads to BIZ-. The micelle also catalyzes the dimerization of Brz-, as these species are confined to the micelles leading to locally high concentrations of the radicals which, in turn, promotes dimerization. Reactions of Radicals Micelles often strongly promote the reactions of free radicals. Indeed, some remarkable effects of micellar system on
0.3M 10"
N
Radiolysis of the surfactant-spin trap system leads to surfactant radicals via OH radical attack, which under suitable contitions, gives rise to nitroxide radicals. This latter reaction, forming the nitroxide radical, is only evident at surfactant concentrations where micelles are present. In the absence of micelles radical dimerization is the dominent free radical reaction. In the presence of micelles the radical and spin trap are confined to the micelle space leading to catalysis or promotion of the radical-spin trap reaction of some 106fold. This protection of the free radical from reaction with solutes in the aqueous phase is demonstrated in many systems (35-37). For example, the surfactant radicals produced via OH attack react with ferricyanide ion (Fe(CN)&), with k = 3.2
J
U
so
0.1 M NaLS
16'BQ Oll M
0.06M NaLS 103sa
NaLS
3 x W'BQ
0.03M NaLS I@ ea 0 Ol M NaLS BQ
01.1 0% M NaLS
< low
Figure 5. Effect of surfactant and quinone concentrationson the rate of reaction of a surfactant radical with benzoqu8none.
142
Journal of Chemical Education
108LM-I seccl. In the presence of anionic micelles the rate drops off as the radicals associated with the micelles are prohibited from reaction with Fe(CN)& due to repulsion of the ion by the micelle surface. A competition exists between micelles and Fe(CN)c3- for the surfactant radical. those that react the micelle being protected. Similar data is available for reaction of the radicals with henzoauinone.. BQ, .. as shown in Figure 6 (37). Figure 5 shows the rate of growth of BQ- following reaction of BQ and R formed by OH-attack on monomeric or micellar NaLS (2,3).A t [NaLS] below CMC (CMC = 8.1 X loc3 M ) the rate is proportional to [NaLS] and to [BQ].Above CMC the growth of BQ- no longer shows simple kinetics. The traces clearly show two-step kinetic processes, a fast growth and a slow growth. On increasing the concentration of BQ, the extent of the fast growth increases, while increasing the concentration of NaLS causes the decrease of intensity of the fast growth; hut the total intensitv of ahsorntion remains constant. These observations lead to I%.At this stage the water bubble can cauture electrons ec, produced hv radiolvsis of the apGoaches a diffusion-cont.rolled rate at high water content (43). In the AOTIheptane system with 6%water (by (e-.,) is 0.5; in an oleate/hexanol/hexadecane microemulsion with 10% water G(e-,) is >3.0 (44). Thus, small amounts of water introduced in these systems via organized structures produced by the surfactant can dramatically change the nature of the
~
~
~~~
~
~~
~
~
~
~
~~~~~
(25) VonA.Rshulchi,Borgwardt,U.,Henglein,A..Schromberg. E..sndSchrebel.W.,Rer Bun. Phys Chem. 74,469 (1970). (26) Almgren,M.andThomas,3. K.,Pholocham.PhalaBiol., 31,329 11980). (27) Henglein, A,, and Schnakl,Ein Ful-g in die Strahlen Chemie? Weinhelmkrg Sir., Verlsg Chemie, 1969. 1281 . . Chan. T. S.. and Thomaa.J. K.. J. ofPafSci 17.1103(1979). (29l Hengloin, A ,and ~roake;~. ~.,~okranbl. Chem., 2279 119781. (30) Berber,D. J. W., and Thomas,J. K., Rod. Rsa , 74, 51 (19781. (31)Henglein. A,, Praake, J. L., and Schnecke, W., Be,. Bun. Phys Chsm., 82, 956
.
,.*,",.
,,n,o,
(32) (33) (34) (35) (36)
Frank. A.,Gdtlel.M.,sndKozeh,J..J. A m w Chem. Sm., 98.3317 (1976). Turro,N.,andKraeufler. B.,L Amar. Chem. Soc, 100,7432 (1978). Bakslik,O.,andThomas, J. K..J Phys. Chem, 81,1905(1977). Henglein, A.,andProske,Th., J Amer Chem Sor lW,3706 (1978). Almgren, M., Grieser, F., and Thomes, J. K., J. Chem. Sor. Farad. I., 75, 1674
.
1,974, ,. ...,.
(37) Chen. T. S., Pb.D. Theis Univ. of Notre Dsmr, 1977. (33)Wong, M., and Thomas, J. K.. "Micellirrttion,Solubilizationand Microemulsions," (Editor Mittal,K. C.) Plenum Press, New York, 1977, Vol. 2, p. 647. (39) W0ng.M..GrMzel, M.andThomas,J.K..J. Amer Chem S o c , 98.2391 (19761. (40) Wong,M.,Thomas,J.K., andNowar,T.,J. Amw Chem Sac., 99.4370 (19771. (41)Wong,M.. Grktml,M.,sndThomas,J.K., Chsm Phys L e t t , 30,329 (1975). (42) Wong, M., Grieser, F., and Thomas, J. K., Brrichle Bun. Phys Chm, 82. 950
,.",",. ,,o.,a>
(43) Beck, G.. Bakde,G.,and Thomas,3. K., Unpublished dafs.
(44) Thomas,J. K.. Unpublished dsta. (451 Wedlock,D. J. Phillips,G.o.,andThomas, J.K.,Polym~iFound., 11,681(1979). (46) Bisby,R. H., Cundell,R. B., and Wardman, P.. R Ba., 389,137(19751.
Volume 58
Number 2
February 1981
143
