Quenching of triplet states of aromatic ketones by sulfur-containing

Marta T. Ignasiak , Tomasz Pedzinski , Filippo Rusconi , Piotr Filipiak , Krzysztof Bobrowski , Chantal Houée-Levin , and Bronislaw Marciniak. The Jo...
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11937

J. Phys. Chem. 1993,97, 11937-1 1943

Quenching of Triplet States of Aromatic Ketones by Sulfur-Containing Amino Acids in Solution, Evidence for Electron Transfer Bronislaw Marciniak,+Krzysztof Bobrowski,t and Gordon L. Hug’ Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: April 6, 1993; In Final Form: September 14, 1993”

The mechanism for quenching triplet states of benzophenones by sulfur-containing amino acids in water/ acetonitrile solution was investigated by laser flash photolysis. The amino acids in the study were methionine, S-methylcysteine, and S-carboxymethylcysteine, and the eight aromatic triplets were those of benzophenone and its derivatives possessing electron-withdrawing or electron-donating groups. The presence of radical ions in the transient spectra and correlations of the quenching rate constants with the free energy change for electron transfer are strong indications that the process involves an electron transfer. These correlations were displayed as Rehm-Weller plots (logarithm of quenching rate vs free energy). Classical theoretical formulations of the Rehm-Weller correlations were used to estimate the intrinsic barriers and the transmission coefficients for the electron-transfer processes. Applying both “quadratic” Marcus and “asymptotic” Agmon-Levine free energy relationships led to the values of intrinsic barriers lower than the solvent reorganization energy calculated within the framework of the dielectric continuum model. These relationships also led to low electronic transmission coefficients. The low values of the intrinsic barriers for electron transfer were also obtained using the recently developed Tachiya approach which allowed for variable electron-transfer distance. Possible explanations were given for the resulting multiple Rehm-Weller plots with different plateau values for each of the amino acids studied.

CHART I

1. Introduction The interaction of triplet states of aromatic ketones with sulfurcontainingamino acids has been the subject of only a few studies.14 The nature of this interaction was suggested to be electron transfer in character. Since excited carbonyl compounds can be produced “in vivon,5,6their reactions with sulfur-containing amino acids are of biological significance because of the role that these reactions may have in the migration of electrons over long distances through peptides and protein matrices.’ Recently, a detailed electron-transfermechanismof the reaction of 4-carboxybenzophenonewith a series of 12 sulfur-containing amino acids was proposed, and experimental support for the scheme was given. The mechanism for neutral aqueous solution4 was resolved into an initial electron transfer from the sulfur atom to the triplet state of the substituted benzophenone followed by competitive dissociations of the charge-transfer (CT) collision complex. The competitive dissociations of this complex were identified as (1) the diffusion apart of the CT complex to form sulfur-centered radical cations and ketyl radical anions, (2) intramolecular proton transfer within the CT complex to form a ketyl radical and an a-alkyl-thioalkyl radical, and (3) back electron transfer to regenerate the reactants. The detection of such intermediates as a ketyl radical anion and an intermolecular S.-.S-bondedradical cation was direct proof of an electron-transfer process (transfer of a full unit of charge). A similar mechanism can operate in the quenching of excited triplet states of aromatic ketones by other organic compounds containing divalent sulfur atoms, e.g., thioethers.3J-lo In this paper we present further evidence to support the proposed mechanism for the quenching of excited triplet states of aromatic ketones by sulfur-containing amino acids. The quenching of a series of substituted benzophenone triplets possessing electronwithdrawing or electron-donating groups by a single amino acid containing a sulfur atom was studied by time-resolved techniques.

* To whom correspondence should bc addressed.

t FulbrightScholar,onleavefromtheFacultyof Chemistry,A. Mickiewicz

University, Poman, Poland. *On leave from the Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw, Poland. Abstract published in Aduance ACS Abstrocts, October 15, 1993.

0022-3654/93/2091- 11931%04.00/0

COO-

I

methionine

COO-

I

CH,-S-CH2-CH-NH$

CH,-S-CH,-CH~-CH-NH$

1

S-methykyrteinr

2

COO-OOC-CH2-S-CH,-CH-NH$

I

S-cwboxymrthykyrteine

3

Some contributions from radical ions were found in the transient spectra. The observed correlation between the experimental quenching rate constant and the free energy change for electron transfer, A G s was taken as additional evidence for an electrontransfer process. Such a treatment, first suggested by Rehm and Weller,’ 1,12 is generally acceptedI3and has been successfully used by various authors studying electron-transfer quenching, e.g., refs 14-20. A quantitative evaluation of theobserved correlations, using “quadratic” Marcus21J2 and “asymptotic” Agmon-Levine23 free energy relationships, led to the determination of intrinsic barriers and transmission coefficients for the electron-transfer processes. We have also tested our experimental results using the approach recently developed by T a ~ h i y a . ~ ~

2. Experimental Details Benzophenone and its derivatives were commercially available and purified according to published procedures.2s.26The sulfurcontaining amino acids (see Chart I) [methionine ( l ) , S-methylcysteine (2), and S-carboxymethylcysteine (3)] were obtained from Sigma as the best available grades and were used without further purification. Acetonitrile for HPLC Fisher was used as received, and water was purified by a Milli-pore Milli-Q system. The concentrations of the sulfur-containing amino acids in the quenching experiments werein the range l(P-10-2M while those of the benzophenoneswere about lW3 M. The experiments were performed in mixed solvents H2O/CH,CN (3:2 v/v) to compensate for the low solubility of the benzophenones in pure water. All solutions in the kinetics measurements were buffered in the 0 1993 American Chemical Society

11938

The Journal of Physical Chemistry, Vol. 97, No. 46, 1993

Marciniak et al.

TABLE I: Quenching of Substituted Benzophenone Triplets by Sulfur-Containing Amino Acids in HzO/CH&N (32 v/v) S-carboxymethionine S-methylcysteine methylcysteinc -ET - Ed,‘ k,/ 109,b AGCI,E k,/ lo9? GI? k,/lO*,b GI: kJ/mol M-l s-I kJ/mol compound M-I s-l kJ/mol M-* s-l kJ/mol -142.4 3,3’-ditrifluoromethylbenzophenone 2.50 -44.4 1.23 -39.5 3.37 -40.4 4-trifluoromethylbenzophenone -133.1 2.80 -35.2 1.22 -30.2 3.31 -31.1 -1 19.7 2.18 -21.8 4-chlorobenzophenone 0.92 -16.8 3.10 -17.1 -113.4 1.89 -15.5 0.70 -10.5 2.01 -1 1.4 benzophenone -1 10.0 1.28 -12.1 0.42 -7.06 1.16 -8.0 4-methylbenzophenone -106.2 0.96 -8.26 4,4’-dimethylbenzophenone 0.29 -3.26 0.28 -4.2 1.11 -6.66 4-methoxybenzophenone -104.6 0.14 -1.56 -0.010 -2.6 4,4’-dimethoxybenzophenone -97.6 0.036 +0.34 0.03 +5.34 C0.008 +4.4 a From refs 20, 25, and 26. Estimated error 110%. e From eq 3 taking E,, = 1.10 f 0.1 V, C = -0.032 eV, and solvent correction = -0.051 eV. From eq 3 taking Eox= 1.15 0.1 V, C = -0.032 eV, and solvent correction = -0.051 eV. From eq 3 taking Eo, = 1.06 f 0.1 V, C = 0, and solvent correction = 0. nitrile was determined for an 8 X 10-4 M benzophenone solution (purged with nitrogen) containing 0.1 M tetrabutylammonium perchlorate. The peak potentials obtained from the square-wave voltammetry were taken as the oxidation potentials E,, for amino acids 1-3.

3. Results and Discussion

I

I

I

1

I

2

4

6

8

I

Quenching of the triplet states of benzophenone and several of its derivatives, possessing electron-donating and electronwithdrawing groups, by three amino acids containing a sulfur atom, Le., methionine, S-methylcysteine, and S-carboxymethylcysteine, was studied by means of nanosecond laser flash photolysis in H I O / C H ~ C Nsolution. The quenching rate constants, k,, were obtained by monitoring the triplet-triplet absorption decays (kok)of benzophenones at fixed wavelengths for various quencher concentrations and by employing the relation

IO

[Q]x IO‘, M Figure 1. Plots according to eq 1 for benzophenonetriplet quenching by (a) methionine, (b) S-methylcysteine,and (c) S-carboxymethylcysteine in HzO/CH3CN (3:2 v/v) solution at pH = 6.8. Insert: experimental

trace for benzophenone triplet decay at 480 nm in the presence of methionine, 4.0 X 10-4 M.

presence of NaH2P04/Na2HP04 (0.025 M) and were deoxygenated by bubbling high-purity argon through them. UV-vis absorption spectra were recorded using a Cary 219 spectrophotometer. The nanosecond laser flash photolysis apparatus has been described in detail el~ewhere.2~ Laser excitation at 337.1 nm from a Molectron UV-400 nitrogen laser was used in a right-angle geometry with respect to the monitoring light beam. The laser was operated in the range 1-3 mJ and had a pulse width of approximately 8 ns. Rectangular quartz cells (0.5 X 1 cm) with a path length of 0.5 cm for the monitoring beam were used. The transient absorbances at preselected wavelengths were monitored by a detection system consisting of a double monochromator, a photomultiplier tube, and a pulsed xenon lamp of 1 kW as the monitoring source. The signal from the photomultiplier was processed by a 7912 AD Tektronix transient digitizer controlled by a LSI 11/2 microcomputer. The electrochemical experiments were done with an electrochemical analyzer from Bioanalytical Systems, Model 100. The measurements were made with a standard three-component cell that contained a platinum-disk working electrode, a platinumwire counter electrode, and a SCE (saturated calomel) reference electrode. Square-wave voltammetric28 and cyclic voltammetric techniques were used, and iR compensation was employed. To estimate oxidation potentials of the sulfur-containing amino acids, voltammetric measurements were performed on deoxygenated (purged with nitrogen) acetonitrile solutions of methionine (10-3 M), S-methylcysteine (10-3 M), and S-carboxymethylcysteine (4 X 10-4 M) with 0.1 M lithium perchlorate as the supporting electrolyte. The reduction potential of benzophenone in aceto-

where is the lifetimeof the benzophenone triplets in the absence of a quencher. A typical experimental trace for the triplet decay in the presence of an amino acid is presented in the inset of Figure 1. The longlived absorption was attributed to the formation of ketyl radicals in analogy to experiments in neutral aqueous s ~ l u t i o nThe . ~ higher ratioof the molar absorption coefficients of triplet to ketyl radical at 480 nm, compared to the corresponding ratio at 530 nm (maximum of the T-T absorption), made 480 nm a more favorable monitoring wavelength for triplet decays. The pseudo-first-order rate constants, kok, were calculated using eq 2, which takes into account a concomitant, underlying first-order growth of the photoproduct’s absorptionZ9

A - A” In-=-k

O bi

AO - A”

where AO, A, and A” are the absorbance changes a t time 0, t , and infinity, respectively. Some typical plots based on eq 1 are presented in Figure 1. The quenching rate constants obtained for the quenching of the triplet states of benzophenone and seven of its derivatives by methionine, S-methylcysteine, and S-carboxymethylcysteine are summarized in Table I. The k, values can be correlated with the standard free energy change for electron transfer, AGel, given by (3) where E , is the oxidation potential of the donor, Erd is the reduction potential of the acceptor, F is Faraday’s constant, and ET is the energy of the triplet state. Cis the difference (wp- wr) in work required to bring the products and reactants from infinity to the reaction radius in the complex. When computing it, only Coulombic terms were included. For a particular amino acid, the Rehm-Weller plot can be obtained by a correlation of log k, with the AG,l’s for a series of substituted benzophenones. The

Quenching of Triplet States of Aromatic Ketones

The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11939

10

- COmpOlPfIl

’

a

.

m

b

0

- . ..

Ob88Ned lIlNl*nl Methionine S S +

n

9

V

Y

Methionine a

S-Methylcysteine

S-Carboxymethylcysteine

7

limit

n -50

40

-30

-20

-1 0

0

10

AGJ / kJ/mol Figure 2. Dependence of the quenching rate constant, kp, on AG,] for quenching of substituted benzophenonetriplets by methionine (curve a), S-methylcysteine (curve b), and S-carboxymethylcysteine(curve c) in water/acctonitrilesolution (3:2 v/v) at pH = 6.8. The curves themselves

represent the best fits to eqs 14 and 15. The data points on curve c are marked by numbers indicating the following benzophenones: (1) 3,3’-

ditrifluoromethylbenzophenone,(2) 4-trifluoromethylbenzophenone,(3)

4-chlorobenzophenone, (4) benzophenone, (5) 4-methylbenzophenone, (6) 4,4’-dimethylbenzophenone,(7) 4-methoxybenzophenone, and (8) 4,4’-dimethoxybenzophenone.The data points on any particular one of the other three curves are associated with the same eight benzophenones, and those points occur in the same order with respect to AG,1 along any given curve. energies of the triplet states were taken from refs 20 and 26 and correspond to the 0-0 phosphorescence bands of the substituted benzophenones in ethanol/methanol(4:1v/v) at 77 K. Thevalues of Erd of the benzophenones were set equal to the half-wave reduction p o t e n t i a l ~that ~ ~ ,had ~ ~ been measured in acetonitrile vs a saturated calomel electrode (SCE). The values of E,, for 1-3 were determined in the current work using square-wave voltammetry. Similar to results of Glass et al.,30 two oxidation waves were observed in the cyclic voltammetry of the amino acids inacetonitrile. Thefirstbroadwavesat l.lOfO.lO, 1.15 fO.lO, and 1.06 f 0.10 V were observed for 1-3, respectively. To calibrate the electrochemical system, the Erd potential for benzophenone vs SCE in acetonitrile was determined. The value obtained (Erd = -1.83 V) was identical with that determined by Wagner et al.25 Since E,, and Erd were measured in pure acetonitrile, and not the solvent used in this work, these potentials were corrected by a difference in solvation energies (-0.05 1 eV for 1 and 2, 0 for 3) of the two solvents (Born formula: for procedure see for example refs 31 and 32). The electrostatic work term, C, in eq 3 was computed using the measured dielectric constant of 64 for a H20/CHsCN (3:2 v/v) mixture.33 This term was estimated to be -0.032 eV for 1 and 2 and 0 for 3. The AG,1 values calculated from eq 3 are in the range -45 to +5 kJ/mol for all combinations of 1-3 with the substituted benzophenones in Table I. These values, however, are uncertain by least f 1 0 kJ/mol (0.1 eV), due mainly to large errors in the E,, determinations. The dependence of the quenching rate constant on AG,I is presented in Figure 2. Typical Rehm-Weller plots were obtained for all three amino acids. As AG,l increases in algebraic value, the plateau region for exoergonic processes is followed by an intermediate region and finally yielded a straight line with a slope equal to 1/(2.3RT) in the endoergonic region. This can be taken as evidence for the operation of an electron-transfer mechanism in the quenching of the triplet benzophenones by the sulfur-containing amino acids. The involvement of electron transfer in this quenching mechanism is supported by the presence of radical ions in the transient absorption spectra. Such electron-transfer intermediates

were recently seen in analogous systems in neutral aqueous s ~ l u t i o n .Intermediates ~ such as the ketyl radical anion and the intermolecular S:.S-bonded radical cation were also found in this work in the aftermath of the quenching of the benzophenone triplet by 1 (see Figure 3) in water/acetonitrile solutions. The resolution of the observed spectrum into the component spectra was done using a linear multiple regression34of the form 4 u ( X j )

=

€i ( Xj ) Uj

0’ = 1,34)

(4)

i= 1

where M(X,) is the observed optical density change of the composite spectrum and where q(X,) is the molar absorption coefficient of the ith species at thejth wavelength of observation. The elements of the set (ai)are the regression coefficients which in this case are each equal to ci X I, where CIis the concentration of the ith transient and 1is theoptical path lengthof the monitoring light. For Figure 3, the number of wavelengths A, used in the regression was 34, namely, eq 4 stands for 34 equations. Of the component spectra and their molar absorption coefficients: the triplet absorption was collected in the current work in water/ acetonitrile with the emaxtaken to be 7220 M-’ cm-1 for benzene;35 the methionine S:.S spectrum36 was extended to the longwavelength region with a fitted Gaussian tail; and the spectra for the ketyl radical and its anion were taken from ref 37 with the ketyl spectrum renormalized with emax = 3220 M-1 cm-1.38 In an analogous composite spectrum, the radical anion of benzophenone was also identified as a minor transient product of the quenching of the benzophenone triplet by 3 in a mixed water/acetonitrile solution. The presence of the radical ions in Figure 3 and the correlations in Figure 2 are expected for electron-transfer processes. However, since the amino acids do not appear to differ significantly as electron donors and since their molecular structure is not very different, it was not expected that each of the “Rehm-Weller” plots in Figure 2 would have distinctly different plateaus in the exoergonic regime. A possible explanation of these results will be presented below. Multiple Rehm-Weller plots in electrontransfer quenching have been recently observed in nonpolar solvents.1*~39-40In these cases, one plateau value in the exoergonic region, common for all types of quenchers employed, was followed in the isoergonic and endoergonic regions by different linear dependencesvarying with the type of electron donor, Le., 7-donors or n-donors. This was explained18 by differing values of the electrostatic term that was more favorable for the n-donors. Similar multiple plots have been observed in the Marcus inverted region for the back electron transfer between ions in a polar solvent,

11940 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993

SCHEME I 3B*

+

kd

Q=

k*l

k

a+ ) P

3 ( B * * - * Q ) z 3( 8-

k-*I

k-d

1

product8

ka

B + Q

i.e., a ~ e t o n i t r i l e . ~This ~ was explained in terms of different solvation energies for the various sizes of molecule. Quenching of the triplets of the benzophenones (B) by sulfurcontaining amino acids (Q) in neutral aqueous solution can be described by Scheme 1: where kd is the diffusion rate constant, k d is the dissociation rate constant of the encounter complex 3(B-Q)*, kbt is the rate constant for back electron transfer, and k,l and k,l are rate constants for electron transfer. k, is the sum of two rate constants: one for the diffusion apart of the radical ions and a second one for a proton transfer within 3(B--Q+) to form a ketyl radical and an a-alkyl-thioalkyl radical. This approach requires the assumption that the sensitizers can be treated as a homologous series (with a constant intrinsic barrier and transmission coefficient) for which only AG,] varies for a particular quencher in the same solvent. Other parameters (kd, k 4 , and the sum (kbt k,)) are also assumed to remain fixed for each donor/acceptor pair formed from the given quencher with each member of the series of sensitizers. We think that the benzophenone derivatives chosen for this study fulfill these requirements as a series of sensitizers. Applying a steady-state approximation to the intermediates in Scheme I leads to

+

Marciniak et al. acids, alanine and glycine, whose diffusion coefficients in water were known.44 Of the remaining rate constants in eq 6 that would account for the differing plateau values, the most likely sources are those of k:l. They can be estimated from k y , kd, and k 4 . The value of kd (ignoring differences between amino acids 1-3) was calculated to be 7.5 X lo9 M-l s-I from the Debye equation42

kd = 8RT/30007 (9) using the appropriate viscosity data for water/acetonitrile 3:2 (v/v) at room t e m p e r a t ~ r e .k~4~ was estimated to be 8.7 X lo9 s-l from the Eigen equation46 k4 = 3000kd/4ar3No (10) taking r = 7 X 10-8 cm. With these rate constants and with the k y s estimated from the fits in Figure 2, the k:;s could be calculated from eq 6. Taking k y values for amino acids 1-3 equal to 2.6 X lo9, 1.2 X lo9, and 3.3 X lo8 M-l s-I, the resulting values of k:l are displayed in Table 11. It can be seen that the rate constant k:l for 2 is about one-half that of 1, whereas k:’ for 3 is about one-tenth that of amino acid 1. A quantitative description of electron transfer with classical theories requires that AGel be kn0wn.11912914J922Since there is a large uncertainty of at least 10 kJ/mol in AG,I (Table I), the results did not allow for an accurate quantitative evaluation of the electron-transfer quenching in the systems studied. However, proceeding with an analysis similar to that used in refs 16, 17, and 19 led us to an estimation of the values of k$ (kbt kp),and AGII(O) for amino acids 1-3 (Table 11). Following the classical treatment of electron-transfer processes suggested by Balzani et a1.,14

+

In anticipation of the free energy relationships to be discussed later, it is expected that the maximum value of k, (or the k:s on the experimentally observed plateaus in Figure 2) will correspond to the limit when k,l>> k,l and AG*,I = 0. In this limit eq 5 can be simplified to eq 6 describing the maximum k, value corresponding to the experimentally observed plateau where k:l can be interpreted as the preexponential factor in the relation k,l = k:l exp(-AGiI/RZ‘). Thus, the different kmsx values for amino acids 1-3 (that do not reacha diffusion-control(ied value) can be rationalized in terms of the different values of the rate constants in eq 6 for these amino acids. Since the molecular structures of the sulfur-containing amino acids 1-3 are very similar, one would not expect kd or k 4 to vary much between these compounds (at least for 1 and 2). However, the possibility for significant variations in kd can be easily checked. The diffusion rate constants for the amino acid-benzophenone systems can be estimated from the Smoluchowski equation42

+

kd = 4 x 10-3aNo(DB Dq)r (7) if the diffusion coefficients D are known. In eq 7, NOis Avogadro’s number, and r is the sum of the van der Waals radii of B and Q (in units of centimeters). The diffusion coefficients of 1-3 and of benzophenone in aqueous solution were estimated from the Stokes-Einstein equation with empirical correction factors n,43 D = kT/nar)r,Cf/fo) (8) where r ) is the viscosity of the solvent, r, is the van der Waals radius of thediffusing species, andf/fo is the ratioof the frictional coefficient of the diffusing species to that of a spherical one. Values of kd, computed from eqs 7 and 8, for benzophenone with each of the amino acids 1-3 do not change significantly. kd for 3 was about 2%lower than that for 2. This suggests that variations in kd are not responsible for the different rate constants kg““ for the amino acids 1-3. Equation 8 was checked against two amino

k,, = k!l exp(-AG:JRT)

(12)

where AG:, represents the free energy of activation and ~ $ 1 represents the transmission coefficient for electron transfer. It follows from eqs 5, 11, and 12 that

r;

k, = kd 1

+ 0exp(AG:l/RT) +

The free energy of activation AG:l can be related to the free energy change of the electron transfer process AG,I by two types of relationships: (1) “asymptotic” forms such as Agmon and Levine,23

and (2) the theoretical “quadratic” form of the classical Marcus theory,21.22 AGL1 = (AGel

+ X)*/4X

(16) where AG:l(0) (the intrinsic barrier) is the free energy of activation when AGel = 0 and where X is the reorganization energy related to AG:l(0) by X = 4AG,:(O) (17) Fitting Procedure Based on the Agmon and Levine Free Energy Relationship (Eq 15). From the classical theory of electron transfer outlined above, there should be a specific functional

The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11941

Quenching of Triplet States of Aromatic Ketones

TABLE Ik Best-Fit Parameters for Electron-Transfer Quenching of Triplet Benzophenones by 1-3 amino acid

ktl/ io9,"

1

4.7 1.7 0.40

S-1

2 3

AG:(o)!

(kbt

kJ/mol

Kc1

7.7 X lo-' 2.8 X lo-' 6.6 X le5

5.5 (4.0-7.8) 5.3 (4.3-7.5) 1.3 (1.2-1.7)

+ kp)!

v4,C

S-1

kJ/mol

1.4 X lo8 (2 X l e - 3 X lo9) 3.5 X lo8 (3 X 106-5 X 10") 6.6 X lo6 (6 X 105-3 X 109)

8 .O 6.8 7.5

k;/ 109,d

V4P

kJ/mol 8.3

S-l

5.8 2.1 0.49

7.0 7.7

0 Calculated from K" and eq 6. b Best-fit parameters of solid curves in Figure 2 calculated using the Agmon and Levine eq 15 as the free energy relationship; in parentieses are ranges of the fitted parameters taking into account the estimated experimental errors in Eox = hO.1 V. e Best-fit parameters calculated using the Marcus free energy relationship (eq 16); one-parameter fitting procedure taking ktl from column 1 of this table and k b + k , = k 4 (see text). d Best-fit parameters calculated using the Marcus free energy relationship (eq 16); two-parameter fitting procedure taking k b + kb = k 4 (see text).

dependence of kqon AGel (see eqs 14 and 15). There are still two free parameters in these equations, namely, AG:~(o)and ( k b t + k,), since the other rate constants have already been estimated (vide supra). Estimates for these two remaining parameters were obtained by employing a nonlinear least-squares fitting procedure to the relations in eqs 14 and 15. The main FORTRAN routines were taken from Bevington34 and use the Marquardt alg0rithm.4~ The best fits for the log k, vs AG,I correlation from this procedure are presented in Figure 2, and the estimates of ( k b t k,) and AGtI(O) for 1-3 are listed in Table 11. The fitting strategy adopted in this workis somewhat different from those of earlier works. In previous studies,16J7J9where the plateau rate constants were near lo9 M-' s-l and above, it was found that for the rate constant analogous to ( k b k,) any value above 108 s-l did not affect the computed kq (from eqs 14 and 15). Preliminary calculations were done in this fashion, namely, by fixing ( k ~ k,) to k a and by taking as fitting parameters AG:l(0) and k:l. As in all previous related studies, ( k b t + kp)was assumed constant for all the benzophenones and a particular quencher. It is common4I to take k, independent of AG,l. The independence of kbt on AG,I can be explained taking into account the large negative values for the free energy change for back electron transfer; AGbt = Erd - E , for all members in the three seriesstudiedin thiswork(AGh = -(2.7-3.2) eV). Ifthiselectrontransfer process can be described by an Agmon-Levine relation, then the back electron transfer would, therefore, be an almost activationless process. Values obtained for k:, were almost identical to those computed from k,maX and eq 6, but the fits for 1 and 3 were unacceptable. On checking, it was found that for the systems under study, ( k b t + k,) can have a large effect on the goodness of the fits to the Weller plots. Since k:l seemed to be well-determined by computing it from the well-defined k,maX and eq 6, it was taken to be fixed, and the fitting parameters were taken to be AGtI(O) and ( k b t + k,). The best fits are presented as the solid curves in Figure 2, and the resulting parameters are given in Table 11. The estimated values of the intrinsic barriers AG:l(0) for 1-3 are very small, in the range 1-6 kJ/mol, and indicate small values for the solvent reorganization energy in the electron-transfer processes studied. The large uncertainty in the determination of AG,l and, as a consequence, the large uncertainty in AGzl(0) make definitive conclusions concerning the influence of amino acid structure on the value of the intrinsic barrier untenable. Fitting Procedure B W on the Marcus Free Energy Relationship (Eq 16). The fitting procedure in this case requires finding the best values for three parameters A, k:,, and kbt + k, to fit the data according to eqs 14 and 16. As previously, the values of k:' can be obtained from values and eq 6. With only two variable parameters the results can be obtained with a higher degree of confidence. In this case, however, the optimization was found not to be sensitive to reasonable values of ( k b t kp)> 108 s-I. This was also noted before by other authors, e.g., in refs 16 and 19. For simplicity, ( k b t k,) = k d was used for the final calculations. Thus, the fitting procedures were required to find only one parameter, A. The results of such calculations for amino acids 1-3 are summarized in the sixth column of Table

+

+

+

+

11.

+

10

9

w

Y

7

-

Methionine S-Methylcysteine

'

0

.,-50

limil

S-Carboxymethykysteine

40

-30

-20

-10

0

10

A G e l / W/mol Figure 4. Dependence of the quenching rate constant, 4, on AGd for quenching of substituted benzophenones by methionine (curve a),

S-methylcysteine (curve b), and S-carboxymethylcysteine(curve c) in water/acctonitrilesolution (3:2 v/v) at pH = 6.8. Thecurves themselves represent the best fits to eqs 14 and 16, and the obtained parameters for particular amino acids are presented in the last two columns of Table 11. The dashed line (curve d) represents the best fit obtained for S-carboxymethylcysteine using a fixed value of X = 108 kJ/mol, calculated from eq 18. The experimental data points of any particular one of the other three curves correspond to the various benzophenones as indicated in Figure 2. Similar results were also obtained using a two-parameter fitting procedure for k:' and h with fixed values of kbt k,. As previously, the results were found not to be sensitive to the values of (kbt k,) > lo8 s-l. These results are presented in Figure 4 (curves a+) and are summarized in last two columns of Table 11. The values obtained for the intrinsic barriers are small, and as for the calculation with the Agmon and Levine free energy relationship, the intrinsic barriers are in the range of several kJ/ mol. The solvent reorganization energy, within the framework of the dielectric continuum model for spherical molecules, is described (in SI units)

+

+

where Ae is the electronic charge transferred, €0 is the permittivity of free space, nand t, are the solvent refractive index and dielectric constant, re and rQare the radii of reduced acceptor and oxidized donor, and rBQ is their center-to-center separation distance. Applying in eq 18 the proper values for HzO/CH$N as the solvent (n2= 1.78, cs = 64) and donor and acceptor radii (re = rQ = 3.5 A, r m = 7 A), the solvent reorganization energy was calculated to be 108 kJ/mol. This is equivalent to the value of AG:,(O) = 27 kJ/mol. Thus, the value obtained is about 4 times higher than those presented in Table 11. Using the value of h / 4 = 27 kJ/mol as a fixed parameter, the fitting procedure led to the value of k:l in the range of 1011 s-I for amino acids 1 and 2 and 1010 s-l for 3, but the fits were unacceptable (see curve d in Figure 4).

11942 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993

The values of k:l (and transmission coefficients ~ ~ presented 1 ) in Table I1 (obtained from the k? and eq 6 as well as those obtained from the best fits using the Agmon-Levine and Marcus free energy relationships) decrease from amino acid 1 to 2 to 3. It seems likely that the structure of the excited complex between the benzophenone triplet and the amino acid, namely, how the triplet benzophenone orients itself with respect to the sulfur of the amino acid, is responsible for this effect. One possible explanation for this could be an increase of the distance between the carbonyl group of the benzophenone and the sulfur atom as a result of the hydration of the COO- and NH3+ groups in the amino acids. This effect can be observed much easier for 3 with an additional COO- hydrated group. As shown in ref 43, the van der Waals increments for COO- and COO-/hydrated change from 29.5 to 71 A3, respectively (for NH3+ and NH3+/hydrated from 4.6 to 11.5 As). A second possible explanation for the different plateaus in Figure 2 could involve the differential interaction of the donor/ acceptor pairs with the two-phase structure of water/nonelectrolyte solutions,4* and waterlacetonitrile mixture, in this case.49 According to the Naberukhin-Rogov two-phase structure for solutions of nonelectrolytes in water,48 the ion S-carboxymethylcysteine (3) would prefer to be inside the water globules, and the hydrophobic benzophenones would prefer to be inside the random phase that is rich in acetonitrile. The protective shell of the water globule around amino acid 3 could make the distance between the carbonyl group of the benzophenones and the sulfur atom of 3 much larger than with 1or 2, and consequently, a much weaker complex would result with a much lower value of k:l ( ~ $ 1 ) . An additional observation that points to the possible influence of the two-phase structure of water/acetonitrile is that the ratio of k y v a l u e s for 1and 3 in water/acetonitrile (equal to 8) is larger than the ratio of k, values for the quenching of the 4-carboxybenzophenone triplet by 1 and 3 in water (equal to 3).4 Thus, the hydration of the polar groups of the amino acids 1-3 in combination with preferential solvation of the donor/acceptor pair between the two phases of water/acetonitrile solutions may be the origin of the different plateau values in the Rehm-Weller plots for 1, 2, and 3. Consequently, different k’:s would result. The observed results show a very similar experimental picture to those obtained for the quenching of the triplet states of organic compounds by various iron(II1) and chromium(II1) tris( 1,3diketonates)50.5l where the replacement of methyl groups in the ligand by tert-butyl groups led to the reduction in the transmission coefficients and were explained by a “steric” effect. A similar dependence of the quenching rate constants on the donor-acceptor distance was found by Rau et a1.52,53for the quenching of excited R~[(C,H~+~)~bpy]3~+complexe~ with various hydrocarbon chains by methylviologen in water/acetonitrile solutions. The small values of k:, (and transmission coefficients K ~ I ) indicate a nonadiabaticcharacter of the electron-transfer process studied. This can be related to molecular parameters by calculation of the transfer integrals which are determined by the overlap between the initial-state and the final-state electronic wavefunctions. In the semiclassical theory of electron transfer,s4 the transmission coefficient can be written as

GQ

Marciniak et al. k, of the same order of magnitude. There is no information about the relative values of kbt vs k, for the current series. The results presented in Table I1 indicate that the intrinsic barriers obtained are much smaller than those obtained from eq 18 using a fixed separation distance between B and Q equal to the sum of their van der Waals radii. Since X, calculated from eq 18 strongly depends on the distance r w , and as a consequence, the rate constant kclalso depends on the donor-acceptor distance, we have tested our experimental results using the theory recently developed by Tachiya and Murata.24 The theoretical scheme of Tachiya and Murata allows for the possibility for electron transfer tooccur at any distance. Formally, they allow for this by keeping the distance dependence in the semiclassical electron-transfer theory using the Marcus free energy relationship kel(rBQ) =

The exponential dependence with the characteristic reciprocal distance, fl, is the conventional distance dependence of the electronic matrix element.55 Instead of the phenomenological kinetics of Scheme I, Tachiya and Murata used a kinetics formalism that was based on the diffusion equation with the uclosurena p p r ~ x i m a t i o n .The ~ ~ reciprocal of the quenching rate constant is given by l/k, = l / k D + l/k,

(21)

where k, is the second-order electron-transfer rate constant given in terms of the first-order rate constant for electron transfer by

and the diffusion rate cosntant is given by

k,,(R?R’’ dR’sR:kcl(R)R dR] (23) where R and R’are dummy variables of integration and where D’= DE + DQ. The rate constants k, and kD were computed numerically with a double-precision FORTRAN subroutine from Digital Equipment’s scientific subroutine package. The integration subroutine uses Simpson’s rule together with Newton’s 3/8 rule or a combination of the two r ~ l e s . 5 7 , The ~ ~ parameters taken were /3 = 1 A-’; rB= 3.5 A; rQ = 3.5

A; D’= 1.4 X

cm2/s

When A, was computed from eq 18 with Ae equal to one electronic charge, the best fit from a nonlinear least-squares procedure resulted in curved of Figure 5 with = 26.5 cm-1. Since the match between curve d and the data points is unsatisfactory, several other fitting schemes were tried. The one that gave the most satisfactory results was one that varied both and the transferred charge Ae (Figure 5, curves a-c). The interaction energies and the Ae’s from the fits are listed in the captions to Figure 5. The (Ae)% give A i s by eq 18 which are about 4 times smaller than those computed from the dielectric continuum model for the transfer of a single charge. These lower than expected results for the reorganization energies are consistent with the results of the other two models. The interaction energies are also small and are consistent with the small electronic transmission coefficients of the other models, all of which indicates that the electron transfer is nonadiabatic.

GQ

eQ

Taking the ~~l values from Table I1 and the nuclear frequency Y, to be 6 X 1Ol2 s-l, the values of were obtained in the range 1-3 cm-I. At the present stage the results obtained do not allow us to make any quantitative description of (kbt k,) values for the amino acids 1-3. However, based on the photochemical quantum yields of ketyl radicals (called @’ in ref 4), the 4-carboxybenzophenone/amino acid systems in water appear to have kbt and

GQ

+

Quenching of Triplet States of Aromatic Ketones

The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 11943 York, 1990; pp 389-399. (8) Inbar, S.; Linschitz, H.; Cohen, S. G. J . Am. Chem. Soc. 1982,204,

lo

1679-1682. (9) Guttenplan, J. B.; Cohen, S.G. J . Org. Chem. 1973,38,2001-2007.

(10) Bobrowski, K.; Marciniak, B.; Hug, G. L. J. Photochem. Photobiol. A, submitted. (1 1) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969,73,834-

't

6 [ -50

S-Methylcysteine

"

-40

.

'

-30

'

'

-20

'aJumr

*'.,

S-Carboxymethylcysteine

'

limn

'

-1 0

1 0

10

A G d / kJlmd Figure 5. Dependence of the quenching rate constant, k,, on AGel for quenching of substituted benzophenones by methionine (curve a), S-methylcysteine (curve b), and S-carboxymethylcysteine (curve c) in water/acetonitrile solution (3:2 v/v) at p H 6.8. The curves themselves represent the best fits for particular amino acids obtained using the Tachiya-Murata approach according to e q s 20-23 and 18 (curve a: V& = 4.7 cm-l, (Ae)2 = 0.27 e2; curve b: V& = 2.8 cm-l, (Ae)z = 0.23 e2; curve c: V!& = 1.4 cm-1, (Ae)Z= 0.26 eZ). The dashed line (curve d) represents the best fit obtained for S-carboxymethylcysteine taking Ae = e and V& = 26.5 cm-1. The experimental data points of any particular one of the other three curves correspond to the various benzophenones as indicated in Figure 2.

4. Conclusions Quenching of triplet states of aromatic ketones by sulfurcontaining amino acids in water/acetonitrile solution was shown to occur via the electron-transfer mechanism. This was directly confirmed by the observation of radical-ion products (in the transient spectra) and indirectly by the Rehm-Weller correlations of k, vs AGel. Quantitative description of the Rehm-Weller correlations using various free energy relationships (AgmonLevine and Marcus) gave the values of the reorganization energy for electron transfer lower than the solvent reorganization energy calculated within the framework of the dielectriccontinuum model for spherical molecules. This behavior was also seen applying the Tachiya-Murata approach which allowed for variable electron-transfer distance. Acknowledgment. The work described herein w a s supported by the Office of Basic Energy Sciences of the U S . Department of Energy, and this paper is Document No. NDRL-3590 from the Notre Dame Radiation Laboratory. B.M. thanks the Fulbright Foundation for the Research Grant (1991-1992) and thanks Professor R. H. Schuler for the hospitality shown during author's stay at the Radiation Laboratory. The authors thank Mr. Di Liu for his assistance and advice on the electrochemistry measurements. References and Notes (1) Cohen. S.G.: Oianoera. S. J. Am. Chem. SOC.1975.97.5633-5634. (2j Bhattacharyya,-S. N.; Das, P. K. J. Chem. SOC.,Faraday Trans. 2 1984,80, 1107-1 116. (3) Encinas, M. V.; Lissi, E. A.; Olea, A. F. Photochem. Photobiol. 1985, 42. -347-352. (4) Bobrowski, K.; Marciniak, B.; Hug, G. L. J. Am. Chem. SOC.1992, 114, 10279-10288. (5) Cliento, G. Pure Appl. Chem. 1984,56, 1179-1 190. (6) Cliento, G. In Chemical and Biological Generation of Excited States; Adam, W., Cliento, G., Eds.; Academic Press: New York, 1982, pp 277-307. (7) Priitz, W. A. In Sulfur-Centered Reactive Intermediates in Chemistry

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