J. Phys. Chem. 1984, 88, 95-99 be greater at lower ionic strength, where the masking of ions by a gegenion atmosphere would be less, so that the result that the measured volume of association of sodium 8-chlorotheophyllinate and pyridinium iodide is more negative at lower ionic strength is not unexpected. Acknowledgment. This work was supported by funds made
95
available by the Natural Sciences and Engineering Research Council of Canada. I am indebted to Dr. H. G. Drickamer for valuable advice on the design of high-pressure equipment. Registry No. Sodium 8-chlorotheophyllinate,46264-79-3; 1-methyl3-(carbomethoxy)pyridinium iodide, 4685-10-3; trans-cinnamic acid methyl ester, 1754-62-7.
Refractive Index Increment of a Colloidal Dispersion of Small Spheroidal Particles Yoh Sano and Masayuki Nakagaki* Institute f o r Plant Virus Research, Tsukuba Science City, Yatabe, Ibaraki 305, Japan, and Faculty of Pharmaceutical Sciences, Kyoto University, Sakyoku, Kyoto 606, Japan (Received: February 15, 1983; In Final Form: May 20, 1983)
Using the light scattering theory of Rayleigh-Gans for spheroids, we derive theoretical expressions for the refractive index increment, dnldc, of a dispersion in which small spheroidal particles are dispersed in a medium. The dn/dc depends on the axial ratio p , the intrinsic anisotropy p,, and the relative refractive index of the spheroid to the solvent m in the general case. Even if the particle is made of optically isotropic material (p, = l), the dn/dc depends on p as well as m, and the - l ) / ( m *+ 2 ) ] is correct only in the case of isotropic spherical particles well-known equation (dnldc), =1 = (3no/2p)[(m2 (pp= 1 ) . The deviation of (dnldc) from (dn/dc),p=l is larger, the larger the m is and the larger the p is.
Introduction The measurement of the refractive index increment for a colloidal dispersion or a macromolecular solution in which particles or macromolecules are dissolved in transparent liquid solvent is a relatively simple experiment for which several commercial refractometers are available.' Expressions for the refractive index increment based on theories of the refractive index of mixtures of transparent substances give generally fairly accurate refractive index increment values.' For a spherical particle, it has been shown theoretically by the Mie theory and verified experimentally for suspensions of monodisperse polystyrene lattices that the refractive index of colloidal dispersions is strongly size dependent2 The anomalous diffraction approximation agrees closely with the exact Mie theory and a general result of both these theories is that the refractive index increment of dispersed spheres tends to zero in an oscillatory fashion as the sphere diameter increase^.^-^ For spheroidal particles, Meeten has recently investigated theoretically the dependence of the refractive index of colloidal dispersions on shape and size using the Rayleigh theory and the Rayleigh-Gans-Debye theory, which involves the restrictive assumption that the refractive index of the particles is close to that of the surrounding medium ( m 3 l ) , and anomalous diffraction approximations, and shown that departures from particle sphericity modify the refractive index of the dispersion, both size and shape being generally of importance.6 However, he did not take the effect of the optical anisotropy on the refractive index increment into consideration. To our knowledge, no theory exists for calculating the refractive index increment from the axial ratio and the degree of anisotropy, as well as the density and the refractive indices of the colloidal and continuous phases. (1) M. B. H u g h , "Light Scattering from Polymer Solutions", Academic Press, New York, 1972, pp 165-332. (2) M. Nakagaki and W. Heller, J . Appl. Phys., 27, 975 (1956). (3) J. V. Champion, G.H. Meeten, and M. Senior, J . Chem. SOC., Faraday Trans. 2. 74. 1319 (1978). (4) J.'V. Champion, G. H. Meeten, and M. Senior, J . Colloid Interface Sci., 72, 471 (1979). (5) J. V. Champion, G.H. Meeten, and M. Senior, J . Chem. SOC., Faraday Trans. 2,-75, 184 (1979). (6) G.H. Meeten, J. Colloid Inferface Sci., 77, 1 (1980).
0022-3654/84/2088-0095$01.50/0
The subject of this work is to derive expressions whereby the refractive index increment may be related to the optical properties of the dispersed and continuous phases. This work is useful in the interpretation of light scattering measurements for colloidal dispersions of spheroidal particles, where light scattering theory generally requires either the refractive index increment or the relative refractive index of the scatterer to the solvent. Differing from the treatments quantitatively correct only for limiting conditions such as (a) for infinitely thin rods or infinitely thin disks or (b) for bodies whose refractive index differs very little from that of the surrounding medium, we derive here a general treatment by using the general theory given by Rayleigh7 and Gam* for the scattered light intensity of spheroidal particles. The treatment derived here will be referred to as the Rayleigh-Gans theory of spheroids (RGS theory) in order to avoid confusion with the well-known Rayleigh-Gans (RG) theory of scattering which involves the restrictive assumption that the refractive index of the particles is close to that of the surrounding medium.
Theory General Considerations. For small molecules, or flexible polymer chains, dissolved in a solvent of small molecules, a variety of expressions exist for the refractive index of the solution in terms of the refractive indices, polarizabilities, and densities of each component.' These expressions are derived from the assumption that each molecule is small enough to be regarded as a point dipole. The polarizability of each molecule is assumed to be independent of its environment and the induced electric dipole moment is assumed to result from the internal electric field. Van de H u h g showed that the real part of the refractive index n of an assembly of independent scatterers of any size or shape in a medium of refractive index no is given generally by m = n / n o = 1 + 2 x h w 3 Im S(O)
(1)
k = 2r/X
(2)
(7) L. Rayleigh, Philos. Mag., 44, 28 (1897). (8) R. Gam, Ann. Phys. (Leiprig), 37, 881 (1912). (9) H. C. Van de Hulst, "Light Scattering by Small Particles", Wiley, New York, 1957.
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. I , 1984
Sano and Nakagaki
where m is the relative refractive index, N is the number of particles per milliliter, X is the wavelength of light in the medium, Im indicates the imaginary part, and S(0) is the complex forward scattered amplitude function defined by Van de H ~ l s t . We ~ discuss below the application of eq 1 to calculate the refractive index increment dn/dc for a dispersion of spheroidal particles, the dispersion having a homogeneous real refractive index n and the particles being surrounded by a medium of real refractive index
for particles of arbitrary size and shape. The optical parameter gi used by Peterlin and StuartI3 in the study of the birefringence of a dilute solution of rigid macromolecules of ellipsoidal shape in a shear gradient is
96
110.
Small Particles with Random Orientation. A theoretical expression for the zero-angle scattered light function of spheroidal particles was first given by Rayleigh’ and which will be referred to as the Rayleigh-Gans theory of spheroids (RGS theory). The problem can, therefore, be treated in terms of light scattering by using the RGS approximation whose approach applied to particles of arbitrary size a and arbitrary relative refractive index m, where a = 2aa/X
(3)
Here a is the length of the semiaxis of symmetry for the spheroidal particle. The scattering of a single particle will give rise to a spherical wave which is obtained by a linear transformation of the incident wave. The general form of the scattered wave is given by the following equation which is used to discuss the depolarization of the scattered light by Van de Hulst9 with the help of the matrix S(0) (amplitude function): (4)
- l)/[l
g, = (1/4a)(m;
+ (m12- 1)LJ
where m, is the relative refractive index in the i direction (i = a or b) and the shape parameters L, in the i directions are functions of axial ratio p as given by Van de Hulstg
L,(P>l) = ( 1 / 2 ) [ P 2 / ( P 2- 1)1x [I - ( 1 / ( p ( p 2- W2)) In ( p + (p2 - 1)l4] (9) Lb(p< 1) = (1/2)[P2/(1 -P2)l[(l/dD(1 -Pz)’’21) arccosp - 11 (10) L,
+ 2Lb = 1
(11)
The optical polarizability of an ellipsoid in the i direction is given by ai
= vg,
(12)
where Vis the volume of the ellipsoid (4aab2/3), so that g, is the excess polarizability per unit volume. When the volume concentration is small
(rn - 1 ) / N V = (l/no)(dn/d@)
(5)
where 0 is the scattering angle, and a is a mean polarizability expressed by
= (a, + 2ab)/3
(6)
The nondiagonal elements of the scattering matrix in this case are zero, and the diagonal elements of the matrix at zero angle are equal. Therefore, the imaginary part of S(0) at zero angle in eq 1 is equal to k3a. Therefore
(m- 1)/N = 2aa
(7)
The equation within the assumptions of the RGS theory holds (10) M. Nakagaki, Bull. Chem. SOC.Jpn., 34, 834 (1961). (1 1) M. Nakagaki and Y.Sano, Bull. Chem. SOC.Jpn., 45,2100 (1972). (12) Y.Sano, J . Colloid Interface Sci., 11,319 (1980).
(13)
where = c / p , p is the density of the dispersed phase, and c is the concentration in g/mL. Equation 13 together with eq 7 gives dn/d@ = 2aano/V
The functions S I ( I = 1-4) are components of the scattering amplitude matrix S and each is a function of the two sets of angles defining the scattering and the orientation of the particle. E L o , Ello,E , , Ellare the components of the electric vectors of the incident and scattered wave perpendicular and parallel to the plane of reference containing the directions of propagation of the incident and scattered beams; r is the distance from the particle to the point of observation. To evaluate the components of S the optical properties of the particles must be introduced. Ellipsoids of homogeneous matter have a tensor of polarizability due to their nonspherical shape. For the sake of simplicity, a spheroidal particle of the excess polarizability with principal axes (a,, ab, ab), which is the difference between the polarizabilities of the particle and the medium, will be considered here.’0-12 If the particle is sufficiently small compared to the wavelength of light, it may be assumed that one dipole is induced in a particle and the oscillation of the dipole radiates the scattered light. The RGS theory gives the value of S for a spheroidal particle as follows, if the orientation of the particle is assumed to be completely random, so that the function is averaged over all possible directions of the orientation of the particle
(8)
(14)
Therefore, with eq 6 and 12, we obtain from eq 14 dn/dc = (2an0/3/3)(ga
+ 2gb)
(15)
This equation provides the dependence of dn/dc on the two functions goand gb and experimental quantities. It may be noticed here that the particle size or the volume of the ellipsoid V has been canceled in the RGS approximation. Anisotropic Particle. A particle may have two kinds of optical anisotropy: one is ”intrinsic” anisotropy and the other is “form” anisotropy. The intrinsic anisotropy occurs when the dispersed phase (that is the material composing the particle) itself has optical anisotropy because of, e.g., the orientation of valence bonds in the phase. If the dispersed phase is optically uniaxial, the excess polarizability can be expressed by a spheroid. For a spheroid of the excess polarizability with principal axes (a,, ab, ab),the optical anisotropy is given by the ratio pr pg
= ga/gb
(16)
It is seen from eq 8 and 9 that the optical anisotropy p g is given by the intrinsic anisotropy p m and the form anisotropy p . The form anisotropy is the optical anisotropy due to the anisometry. In other words it is due to the anisotropic shape of the particle made of the isotropic phase. If the shape of the particle is assumed to be a spheroid (it may, of course, be an ellipsoid in more general cases) and its semiaxes are (a, b, b), the anisometry is expressed by the axial ratio p = a/b
(17)
Even for the anisometric particle, the refractive index of the dispersed phase may be the same irrespective of the vibration direction of the electric vector, and the effect of the intrinsic anisotropy is not needed to be taken into consideration. The deformation of the colloidal particle from spherical shape is, however, mainly due to the anisotropy of the intramolecular forces within the dispersed phase and, therefore, it is reasonable to assume that the dispersed particle has intrinsic anisotropy and that the direction of the principal polarizability of the spheroidal particle is in agreement with that of the symmetry a axis of the spheroid. (13) A. Peterlin and H. A. Stuart, Z.Phys., 112, 1 (1939).
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984 97
Refractive Index of Spheroids In the following discussion, we consider the effect of the anisotropy of the optical polarizability, that is, the intrinsic anisotropy. If the shape of the particle is assumed to be a spheroid whose semiaxes are (a, b, b ) , the form anisotropy (anisometry) is expressed by the axial ratio p . If the principal refractive indices divided by the refractive index of the solvent no are (ma,mb,mb), the intrinsic (structural) anisotropy p m is defined by Pm
(m,2 - l)/(mb2 - 1 )
(18)
and the mean relative refractive index m is defined by
l / ( m 2 - 1 ) = [ l / ( m O 2- 1 )
+ 2 / ( m b 2- 1 ) ] / 3
(19)
Therefore, introducing eq 16, 1 8 , and 19 into eq 15 we obtain dn/dc = (3no/2p)[(m2- l ) / ( m 2 + 2)1(1 + @,)(I
+ 2 / ~ , ) / 9 (20)
where pg = { I
-
[ ( m 2- l ) / ( m 2+ 2 ) ] [ 1- ( 1
+
2pm)Lb/Pml)/{(l/Pm)+ 2 [ ( m 2- l ) / ( m z + 211 [ 1 - ( 1 + 2pm)Lb/Pml) ( 2 1 ) For the isotropic spherical colloidal particle the following equation can be derived from eq 20 by the use of p = 1 (Lb = and pm = 1 , and therefore p g = 1 according to eq 21: [dn/dcI, = (3no/2p)[(m2- l ) / ( m 2+ 211
Figure 1. Variation of p g with p values for the prolate spheroids of m = 1.20 and of various p m values. Open circles at p = 1 mean the values of spherical particles. Horizontal lines on both sides indicate the limiting values at p (infinitesimally thin rod) and p 0 (infinitesimally thin disk).
-
-
(22)
Equation 22 is identical with that given by Heller14and by Ewart, Roe, Debye, and MacCartney.I5 The correction factorfshowing the discrepancy from eq 22 is
f = [dn/dcl/[dn/dclo = (1 + 2PgM
+ 2/Pg)/9
(23)
A particle of isotropic material has the same relative refractive index m in all directions of electric vector of light independently of the particle form. Therefore, in the case of the anisometric particle made of isotropic phase, the optical polarizability is the same in all directions (p, = l ) , and we obtain from eq 21-23
pg = 1
+B/A
(24)
f = [(m2+ 2 ) / ( m 2- 1 ) ] ( A+ B / 3 )
(25)
dn/dc = (3no/2p)(A + B / 3 )
(26)
-
Here A and B are functions of m and p as defined by eq 27 and 28. For an infinitesimally thin rod (p a), for which L, = 0
A = ( m 2- 1 ) / 3 [ 1 B = (m2- 1 ) / 3 [ 1
+ ( m 2-
(27)
+ (m2 - 1)LJ - A
(28)
and Lb = I f 2 , the p g value obtained by eq 24 by using eq 27 and 28, pgm,is
and for an infinitesimally thin disk (p and Lb = 0 pg0 = l / m 2
-
(pm = 1 , p
0), for which La = 1 -c
(30)
0)
as already shown by one of us.1o See ref 16 Numerical Results and Discussion Variation of the p K Figure 1 shows the variation of the optical anisotropy parameter p g with the axial ratio p of the prolate ~~
~~~
ob
I
I
'.O ,P
I
20
Figure 2. Variation of p g with p m values for the prolate spheroids of m = 1.20 and of various p values. Open circles mean the values of spherical particles. Rectangles and triangles indicate the limiting values at p and p 0, respectively.
-
-
spheroids (p > 1 ) of various pmvalues and of a given m value, the case of m = 1.20 being shown here as an example. Open circles at p = 1 show the p g values of spherical particles. For the isotropic spherical particle the p g is equal to 1 because of B = 0. For the anisotropic spherical particle the p g increases with the increase of pm. For the prolate spheroids the p g depends strongly on p and pm. The p g increases monotonously with the increase of p , and the change is rapid up t o p = 5 , and for p > 5 the change is slow. The dependence of p g on p is larger, the larger the pm is. Horizontal lines in Figure 1 are limiting values for p m on 0 on the left side. If the particle is the right side and for p an infinitesimally thin rod (p a), the relations L, = 0 and Lb = I f 2 are obtained from eq 9 and 11, which give from eq 21
-
--
pgm = [(m2 + 2 ) / 3 1 ~ ,+
[ ( m 2- 1)/61
(31)
-
This agrees with eq 29 when pm = 1 . If the particle is an infinitesimally thin disk (p 0), the relations L, = 1 and Lb = 0 are obtained from eq 10 and 1 1 , which give from eq 21
~
(14) W. Heller, Phys. Reu., 68, 5 (1945). (15) R. H. Ewart, C. P. Roe, P. Debye, and J. R. McCartney, J . Chem. Phys., 14, 687 (1946) (16) The quantity p e used in ref 10 is the reciprocal of p g used in the present paper (p, = l/pg)
This agrees with eq 30 when pm = 1 . The p g values for the infinitesimally thin rods are almost equal to those for the spheroids at p = 14 as is seen in Figure 1. The
98 The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
Sano and Nakagaki
Figure 3. Variation of ps with p for the prolate spheroids of various m and pmvalues. Open circles indicate the values for spherical particles.
Figure 5. Variation of the coefficient f and refractive index increment dn/dc with p m values for the prolate and oblate spheroids of m = 1.20 and of various p values. Open circles mean the values of the spherical particles. Closed circles and triangles indicate the limiting values at p m and p 0, respectively.
5 p
10
-
-
20J 9
Figure 4. Variation of the coefficientfwith p values for the prolate and oblate spheroids of m = 1.20 and of various pmvalues. Open circles mean the values of spherical particles. Horizontal lines on both sides indicate the limiting values at p m and p 0. The variation of refractive index increment dn/dc (mL/g) with p is also plotted for the prolate spheroids of various pmvalues, where the density of the spheroids and the refractive index of the solvent are assumed to 1.35 and 1.340, respectively.
- -
p g values for the infinitesimally thin disk are much smaller than those for the Spherical particles at p = 1, and the difference between them is larger, the larger the pm is. Figure 2 shows the variation of the p g with pm for the prolate and oblate spheroids of various axial ratio p , together with the limiting conditions for infinitesimally thin disk and rod. The p g increases almost in a straight line with the increase of p,, and the increase for the rodlike particles is steeper than that for the disklike particles. Figure 3 shows the dependence of the p g on the axial ratio p of the prolate spheroids for various pm and m values. For the prolate spheroids the p g depends strongly on p , m, and p,, and the dependence of p g on p is larger, the larger the m is and the larger the pm is. Variation of the Correction Factorf. Figure 4 shows the variation of the coefficient f in eq 23 with the axial ratio p for the prolate spheroids of various pmand m = 1.20. The discrepancy from the isotropic spherical particle (f = 1 .OO) is less than 1% for isotropic (p, = 1) rodlike particles, but is larger, the larger the pm is. For the anisotropic spherical particle (p = 1 ) the deviation of thefvalue from 1 .O reaches about 8% for pm = 2.0 and runs to 15% for pm = 0.4. For the prolate spheroids (p > I ) of pm < 1 the f decreases monotonously with the increase of p , because the intrinsic anisotropy is partly compensated by the anisometry. The decrease is steeper, the smaller the pm is. On the contrary, for the prolate spheroids of pm > 1 the coefficient
Figure 6. Variation of the coefficient f with p values for the prolate spheroids of various m and pm values. The p m values in the figure are 0.4 (---), 1.0 (-), and 1.6 (---).
f increahes monotonously when the particles are deformed from the spherical shape, and the increase is larger, the larger the pm is. The change of the f is rapid up to p = 5, and for p > I the change is slow and thefvalues for the spheroids at p = 15 are almost equal to those for the thin rodlike particles having the limiting values of p a. Figure 5 shows the variation of the coefficient f with the pm for the prolate spheroids of various p values. In the case of the spherical particles shown with the open circles, the f takes a minimum point at an isotropical spherical particle (Le., pm = l), and the f increases when the particles are deformed from the isotropical shape, and its discrepancy is larger, the larger the intrinsic anisotropy is. For the prolate spheroids of p > 1, the minimum point offshifts from 1 to pm = 0.8 when the particles are deformed from the spherical shape to the infinitely thin rod shape. At the minimum point, the intrinsic anisotropy is compensated with the form anisotropy, where p g = 1 (see Figure 2) and, therefore,f= 1 (see Figure 5). The difference offfrom 1.0 becomes larger, the larger the p is in the case of pm> 1, the smaller the p is in the case of pm< 1. For the thin rodlike molecules having the limiting value of p shown with closed circles, the f is almost equal to that of p = 20. For the disklike molecules having 0 shown with triangles the minimum the limiting value of p point off shifts to pm = 1.6, where the p g = 1 (Figure 2 ) and therefore f = 1. The difference off from 1.O becomes steeper, the smaller the pm is in the case of pm < 1.5. Figure 6 shows the variation of the f with p for the prolate spheroids of various pm and m values. In the case of isotropic particles (p, = 1) thefis larger, the larger the m is and the larger
-
-
J . Phys. Chem. 1984, 88, 99-101
,
Figure 7 . Variation offwith p g for the prolate and oblate spheroids of various p , pm, and m values. .
the p is. The difference off from 1.O is less than 2% for m < 1.30, and less than 5% from m < 1.50 b r isotropic particles. For the anisotropic particles of p m > 1 the f is also larger, the larger the m is and the larger the p is, and the difference off from 1.O at p m = 1.6 reaches to about 19% for m = 1.50 and runs into about 10% for m = 1.20. On the contrary, for the anisotropic particles of p m < 1 the difference off from 1.0 is smaller, the larger the m is and the larger the p is, because the form anisotropy of the prolate spheroid (p > 1) partially compensates the intrinsic anisotropy of pm < 1. The difference off from 1.0 at p m = 0.4 reaches 14% for m = 1.10, reaches 10% for m = 1.20, and decreases to 2% for m = 1.50. Figure 7 shows the relationship between the difference off from 1.O and the p g value for the prolate and oblate spheroids of various axial ratios p , together with the limiting conditions for infini-
99
tesimally thin disk and rod, and of various m values. All data are fitted on one curve irrespective of the p and m values, whose curve is expressed by eq 23 as shown with the solid line in Figure 7 . This means that the f value depends unequivocally on the p g values. Variation of Refractive Index Increment. The variation of the refractive index increment dn/dc (mL/g), with the axial ratio p of the prolate spheroids is calculated by eq 20 and 26 and the results are shown in Figures 4 and 5 by taking the quantity dn/dc on the ordinate at the right-hand side. The numerical values are obtained by assuming that the medium is water, whose refractive index is 1.340, and the density of the spheroid is 1.35, a value based on the fact that the density of normal protein materials is generally believed to be about 1.35.17 An appreciable dependence of dn/dc on p is observed even in the case of isotropic particles (p, = 1). The value of the dn/dc at p = 1 increases with increasing the degree of the anisotropy. At any given m, the case of m = 1.20 being shown in Figure 4 and 5 as an example, the dn/dc increases steeply at first when a particle is deformed from a sphere to a prolate spheroid, and the increase is larger, the larger the p m is, but the dn/dc increases scarcely when p has exceeded 10 for small Pm.
As shown in Figure 5 , the dn/dc associated with a given p is single valued against p,, if we know separately whether the optical anisotropy parameter p m is larger or smaller than 1.O. As shown by eq 20, the dn/dc also depends on the density of the spheroids and the refractive index of the solvent. In general, the determination of the density of the compact spheroids is easy. Therefore, combined with the light scattering measurements and in conjunction with a knowledge of p which may be obtained by electron microscopy, the degree of anisotropy pm is obtained with the known quantity dn/dc and the refractive index of the solvent no. (17) C. Tanford, “PhysicalChemistry of Macromolecules”, Wiley, New York, 1961.
The Oxidizing Nature of the Hydroxyl Radical. A Comparison with the Ferry1 Ion (Fe02+) W. H. Koppenol* and Joel F. Liebman Department of Chemistry, University of Maryland Baltimore County, Catonsuille, Maryland 21 228 (Received: February 24, 1983; In Final Form: June 10, 1983)
For the standard reduction potential of the hydroxyl radical/hydroxide couple, two values are found in the literature, 2.0 and 1.4 V. Thermochemical data yield 1.98 V for EO(.OH,/OH-,,). Hydration of the hydroxyl radical by -5 kcal/mol changes this value to 1.77 V for Eo’(.OH,,/OH-,,), which is still considerably higher than 1.4 V. It is concluded that the latter value is incorrect. The following thermodynamic quantities are derived from, or consistent with, the new Eo’value: AGOd-OH,,) = +3.2 kcal/mol; Eo’(,0H,,/H201) = 2.59 V at pH 0; AGof(O-.,,) = 19.5 kcal/mol, Eo’(O-,,/OH-,,) = 1.64 V at pH 14; AGof(O~,) = 11.7 kcal/mol; Eo(03g/03-’aq) = 1.19 V, and Eo’(H2O2,,/-OHaq,H201)= 0.46 V at pH 7. The reduction of hydrogen peroxide by ferrous complexes might yield the ferry1 ion (Fe02+-chelate). If the chelating agent is a porphyrin (compound 11) the reduction potential of the couple ferryl/ferriporphyrin is estimated to be 0.9 V. For smaller chelating agents this potential is expected to be higher.
Introduction Irradiation of aqueous solutions produces hydroxyl radicals as well as hydrated electrons and hydrogen atoms. From such experiments the hydroxyl radical is known to be a strongly oxidizing radical, reacting indiscriminately with proteins and other biomolecules at rates which are close to diffusion contro1led.l In (1) Farhataziz and A. B. Ross, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 59 (1977).
0022-3654/84/2088-0099$01.50/0
biochemical systems and possibly in vivo it is thought to be formed through the one-electron reduction of hydrogen peroxide by metal ions and/or organic molecule^.^-^ Others have expressed doubt at the formation of the hydroxyl radical in such systems since small molecules, known to react with the hydroxyl radical at certain (2) C. Walling, Acc. Chem. Res., 8, 125-31 (1975). (3) R.L. Willson, Ciba Found. Symp., New Ser., 65, 19-24 (1979). (4) B. Halliwell, Clin. Respir. Physiol., 17, (Suppl.) 21-8 (1981).
0 1984 American Chemical Society
