J. Phys. Chem. 1982, 86,3470-3403
3478
-
-
Acknowledgment. Dr. Petr6 of the University of Brussels is acknowledged for the help with the oscillating-jet apparatus. E.R. is greatly indebted to the Belgian I.W.O.N.L. for financial support during the course of this work.
Making now the long-time approximation, k t >> 1, erf (kt)'I2 1,exp(-kt) 0, and considering C, as a constant, then eq A.2 and A.3 approximate to
Appendix It will be shown that the assignment of a surface age to a surface element (the method of Hansen, Defay, and Pet$ yields substantially the same result. If one uses the Laplace transformation on eq 1 2 and accounts for the conservation of mass at the surface
AC = r[r/(4Dt)I1/' (k 0) (-4.5) The corresponding jump in surface tension is for micellar solutions
(A.1)
d r / d t = D(dC/&),
AC = I'/[(kD)1/2t]
-
and for simple diffusion
results, after rather tedious calculations, in
r=,
( 2
2k1I2
s,
t1/2
(-4.7) exp(-kt)] -
:)'jz
If one compares relation A.6 with eq 17, it is seen that 9 = l / t rather than 9 = 1/2tas for simple diffusion. From eq A.6 and A.7 it follows that
C,(t - 7 ) e-kr d7lI2 -
k=
(k7)'I2 d71/2
It is shown that eq A.2 reduces for k of Ward and Tordai:
-
0 to the relation
c,(t - 7 ) d71/2 ( ~ - 3 )
-[ 4
it1" C, (t erf 7)71/2
64.4)
T
da*/d(l/t1/2) da/d(l/t)
]
2
(A.8)
Hence, k calculated from eq A.8 differs only by k obtained from eq 20 by a numerical factor (k from eq 20 is a factor 2 too small as calculated from eq A.8). The introduction of the concept of diffusion penetration depth simplifies much of the mathematical analysis and gives results at least of the correct order of magnitude.
Photoionization of Aromatic Amino Acids in Aqueous Solutions. A Spin-Trapping and Electron Spin Resonance Study Magdl M. Mossoba, Kelsuke Maklno, and Peter Rlesr" Laboratory of PathophysMogy, National Cancer Instltote, Natbnal Instnotes of Health, Bethesda, Maryland 20205 (Received: February 26, 1982; In Flnal Form: May 3, 1982)
The wavelength dependence (280-334 nm) of the photoionization of tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe) in aqueous solution was investigated by means of ESR and spin trapping. Chloroethanol, glycine (Gly), and alanine (Ala) were used to scavenge the hydrated electrons in the pH range 7-10. The dechlorination radical from chloroethanol and the deamination radicals from glycine and alanine were spin trapped with 2-methyl-2-nitrosopropane(MNP) and identified by ESR. From these observations it was inferred that photoionization could occur at 280 f 10 and 313 f 7 nm but not at 334 f 10 nm.
Introduction The photochemistry of the aromatic amino acids plays a dominant role in the inactivation of enzymes by UV light.lP2 Continuous irradiation and flash photolysis studies of tryptophan (Trp) below 275 nm indicate that an efficient initial process consists of ejecting an electron which results in the formation of a radical ~ a t i o n l that -~ could deprotonate giving a neutral r a d i ~ a l . ~A flash photolysis study of Trp with 300-360-nm radiation by Pailthorpe et al.e indicated fission of the N-H bond of (1) Grossweiner, L. I. Curr. Top. Radiat. Res. Q. 1976, 11, 141. (2) Vladimirov, Yu. A.; Roshchupkin, D. I.; Fessenco, E. E. Photochem. Photobiol. 1970, 11, 227. (3) Amouyal,E.; Bemas, A.; Grand,D. J. Phys. Chem. 1977,81,1349. (4) Bangher, J. F.; Grwweiner, L. I. Photochem. Photobiol. 1978,28,
175. (5) Santus, R.; Grossweiner, L. I. Photochem. Photobiol. 1972,15,101.
indole followed by rearrangement to the neutral 3-indolyl radical. In an investigation by Amouyal et al.3 of the photoionization of Trp in deaerated neutral aqueous solutions as a function of wavelength, in which nitrous oxide was used as the solvated electron (eaq-)scavenger and the N2 yield was monitored by gas chromatography, it was concluded that a photoionization threshold exists at 275 nm. In contrast, a study by Steen,' in which Trp solutions containing ferrous sulfate and oxygen were irradiated and the Fe3+yield was measured spectrophotometrically, revealed that the e, - yield was observed up to 300 nm. The yield markedly aecreased around 260 nm and was constant in (6)Pailthorpe, M. T.; Nicholls, C. H. Photochem. Photobiol. 1971,14, 135. (7) Steen, H. B. J. Chem. Phys. 1974, 61, 3997.
This article not subject to US. Copyright. Published 1982 by the American Chemical Society
Photoionization of Aromatic Amino Acids
the range from 260 to 300 nm. Moreover, Dillons has obtained the same photoproducts above and below 275 nm in a photolysis study of Trp-containing peptides. Evidence for the nonoccurrence of a photoionization threshold for Trp between 254 and 280 nm has been discussed recently by Zechner et al.9 In view of the conflicting reports regarding the photoionization threshold of Trp, the purpose of this investigation was to examine the generation of photoejected electrons from aromatic amino acids in aqueous solutions as a function of wavelength by spin trapping1@13and electron spin resonance. This method consists of scavenging eaq; with appropriate substrates which give rise to free radicals that are subsequently spin trapped and detected by ESR. In principle, a spin trap such as 2methyl-2-nitrosopropane (MNP), (CH3)3CN=0, reacts with short-lived radiqals, R-, to produce longer-lived spin adducts, (CH,),CN(O)R, whose structures can be conveniently identified by ESR. A preliminary spin-trapping and ESR study by Lion et al.14 on the photoionization of Trp a t 295 nm with glycine as the eaq-scavenger has been reported recently.
Experimental Section L-Tryptophan, L-tyrosine, L-phenylalanine, DL-alanine, and glycine were purchased from Sigma Chemical Co. 2-Methyl-2-nitrosopropane (MNP) and 2-chloroethanol were obtained from Aldrich Chemical Co. and were used without further purification. MNP was stored under nitrogen at 5 "C in the dark. Aqueous solutions of 5.7 X lo9 M (0.5 mg mL-') MNP were prepared by stirring overnight at room temperature in the dark. This low concentration of MNP was found to be advantageous to prevent excessive formation of the undesirable product di-tert-butyl nitroxide (DTBN). The concentration of aromatic amino acids in the MNP solution was 3 X 10-2-5 X M for experiments in the pH range 7-10 while that of DL-danine or glycine varied from 0.2 to 2.0 M. The volume of 2chloroethanol used was 50 pL mL-l (about 0.8 M). The pH was adjusted by addition of NaOH and its value was measured on a Corning pH meter. The photolyses were performed in situ at room temperature, in the presence of dissolved air, in the standard aqueous quartz cell (60 X 10 X 0.25 mm) placed in the ESR cavity, using a Schoeffel 1000-W high-pressure Hg-Xe lamp coupled with a grating monochromator. The emerging beam was filtered with a Corning filter CS7-54 to remove the scattered light. The excitation light had maxima centered at 280, 313, or 334 nm. The monochromator calibration was verified by measuring the light intensity as a function of wavelength using an IL 500 Research Radiometer (International Light, Inc.) and comparing the results with the standard spectral emission curve for the Hg-Xe arc lamp supplied by Canrad-Hanovia, Inc. The desired spectral bandwidths were obtained by adjustment of the monochromator slit widths. Under our experimental conditions a biphotonic ea; production may be disregarded.16 The samples were exposed to UV irradiation for about 2-15 min. (8) Dillon, J. Photochem. Photobiol. 1981, 33, 137.
(9) Zechner, J.; Kohler, G.; Getoff, N.; Tatischeff, I.; Klein, R. Photochem. Photobiol. 1981,34,163. (10) Lagercrantz, C.J. Phys. Chem. 1971, 75, 3466. (11) Janzen, E.G. In "Free Radicals in Biology";Pryor, W., Ed.; Academic Press: New York, 1980; Vol. IV, pp 11654. (12) Rieaz, P.; Rustgi, S. Radiat. Phys. Chem. 1979, 13, 21. (13) Perkins, M.J. Adu. Phys. Org. Chem. 1981,17, 1. (14) Lion, Y.; Kuwabara, M.; Riesz, P. Photochem. Photobiol. 1982, 35,53. (15) Grabner, G.; Kohler, G.; Zechner, J.; Getoff, N. Photochem. Photobsol. 1977, 26,449.
The Journal of Physlcal Chemktty, Vol. 86,No. 17, 1982 3479
ESR measurements were made on a Varian E-9, X-band spectrometer using a magnetic field modulation frequency of 100 kHz. The microwave power was maintained at 10 mW to avoid saturation, and the scans were traced with a modulation amplitude equal to 0.2 G. The magnetic field was calibrated with an NMR gauss meter. In those experiments where the spectral bandwidths were small (*7 or *3 nm), the UV irradiation time had to be increased (between 5 and 15 min) in order to obtain sufficient signal intensity, and this resulted in the formation of large amounts of DTBN. In those cases where the DTBN ESR lines (labeled S in the figures; uN = 17.2 G, triplet) interfered with other ESR signals, DTBN was extracted with petroleum ether.16 The observation that spin-adduct ESR spectra show postphotolysis growth as well as decay as a function of time prevented a quantitative determination of the yield of hydrated electrons as a function of wavelength. Moreover, the scavenging of ea; by MNP, the monomer-dimer equilibrium of MNP, and the complex chemistry of MNP in aqueous solutions would further complicate a quantitative measurement of eaq-yields. It should be noted that, when a Trp solution (pH 8) containing MNP and Ala (1M) was photolyzed (A = 280 nm) under nitrogen, the resulting ESR spectrum was similar to that obtained in the presence of oxygen. This experiment was carried out in a specially designed glass U-tube" in which MNP crystals were placed in one side and an aqueous solution of TRP and Ala in the other side. After Nz gas was bubbled for 20 min through a fine needle inserted into a rubber septum, the solution was transferred to the side arm containing MNP. The spin trap was then dissolved in the aqueous solution by warming to 45 "C for 15 min while stirring. Finally, the apparatus was inverted to fill the ESR flat cell which was already connected to the U-tube by means of a tapered joint.
Results and Discussion The reactions of the photoejected electrons from Trp, Tyr, and Phe with aliphatic scavengers, which do not absorb UV light at the excitation wavelengths, 280 and 313 nm, used in this work, have been investigated by spin trapping. The hydrated electrons generate free radicals from 2-chloroethanol, glycine, and alanine which could be spin trapped with MNP and detected by ESR. The ESR signals of the spin-trapped radicals from the aromatic amino acids did not prevent the detection and identification of the ESR spectral lines of the spin adducts obtained from the scavengers. In nitroxyl radicals the spin density is mostly localized on the nitrogen and oxygen atoms of the nitroxyl group and the interaction of the unpaired electron with the 14N ( I = 1)of the nitroxyl group generates a primary triplet. Secondary splittings, due to the spin density on the magnetic nuclei of the trapped radicals, are also observed in the ESR spectra and are most useful in identifying the structure of the spin adducts. The a,/3, and y positions are defined with respect to the nitrogen of the nitroxyl group as illustrated for the spin adduct generated from chloroethanol: (CH3)3CN(0)C"Hf-CoHZr-OH M solution of Trp containing UV photolysis of a 5 X 0.8 M chloroethanol and 5.7 X M MNP at 280 f 10 nm in the presence of air resulted in the ESR spectrum shown in Figure la. The unlabeled lines in this spectrum (16) Lion, Y.; Kuwabara, M.; Riesz, P. Photochem. Photobiol. 1981, 34, 297.
(17) Evans, C. A. Aldrichimica Acta 1979, 12, 23.
3480
The Jownal of Physical Chemistry, Vol. 86, No. 17, 1982
+
TRP CICH2CH20H A =2&lnm
(a)
Mossoba et al. GLYCINE
(a)
TRP+ GLY
A =280nm
t
pH 9.9 +S
ALANINE
TRP+ALA 1 =280nm
Figure 1. (a) ESR spectra of a UV-photolyzed (A = 280 f 10 nm) solution (pH 8.8) containing MNP, Trp, and chloroethanol. (b) ESR spectrum of spln-trapped deamlnatlon radicals from polycrystalline a-alanine y-lrradiated at room temperature and subsequentty dissohred in an MNP solutbn. (c) ESR spectra of a UV-photolyzed (A = 280 f 10 nm) solution (pH 8.6) containing MNP, Trp, and w-alanine. The DTBN signal is labeled S.
consist of a secondary 13.1-G 1:2:1 triplet which is further split into a 0.5-G 1:2:1 triplet. This spectrum is assigned to the spin adduct of .CH2CH20H. The middle lines of the 13.1-G triplet from chloroethanol overlap with the lines labeled A. The broad triplet (labeled A) is suggestive of the C(3) radical of the indole ring14 (structure I) which was H +
H3N
I
- C - COOI
CH,
I
H
I
TRP + GLY
A =280nm pH 5.7
Flgure 2. (a) ESR spectrum of spin-trapped deamination radicals from polycrystalline glyclne y-Irradiated at room temperature and subsa quently dlssolved In an MNP solution. (b) ESR spectra of a UV-photolyzed (A = 280 f 10 nm) sohdlon (pH 9.9) containing MNP, Trp, and glycine. (c) Same as b but at pH 5.7. The lines labeled H were obtained only during photolysis and are assigned to the H adduct of MNP. The DTBN signal is labeled S.
produced by UV photolysis of Trp in the presence of MNP at 280 nm. In a similar experiment using alanine as the scavenger, the deamination radical of alanine .CH(CH,)COO- was formed. Figure ICshows the resulting ESR spectrum in which the lines of the deamination radical of alanine consist of a secondary 5.3-G doublet, due to coupling to the p hydrogen, which is further split into a 0.4-G 1:3:3:1 quartet due to the interaction with the three equivalent hydrogen atoms of the methyl group. The identity of the spin-trapped deamination radical of alanine was verified by generating the same radical (Figure lb) by y-irradiating polycrystalline alanine in the solid state to a dose of 10 Mrd at room temperature and subsequently dissolving it in an MNP solution.18 In another photosensitization experiment with Trp in which glycine was the scavenger, the deamination radical (18)Minegishi, A.; Bergene, R.; Riesz, P. Int. J.Radiat. Bid. Relat. Stud. Phys., Chem. Med. 1978, 33, 305.
Photoionization of Aromatic Amino Acids
The Journal of Physical Chemistry, Voi. 86, No. 17, 1982 3481
TABLE I: Wavelength Dependence of the Photoionization of Aromatic Amino Acids As Measured by the Reaction of eaq- with Alanine, Glycine, and Chloroethanol in the pH Range 7-10
(a)
TYR+ALA
h =313nm
.CH,CH,OH CH,CH,OH CH,CH,OH *CH,COO' a CH,COOCH(CH,)COO- .CH(CH,)COO- a 313 t 7 .CH,COOCH,COOa CH(CH,)COO- CH(CH,)COO- (I 313 t 3 CH,CH,OH CH,CH,OH CH,CH,OH 3 3 4 t 10 none none none a Because of the presence of several overlapping spectra attributed to DTBN as well as radicals originating from Phe,16 only chloroethanol was used as a scavenger. 280
2
10
of glycine was similarly found (Figure 2b). The observed 8.5-G 1:2:1 secondary triplet of CH2COO- (Figure 2a) was independently generated by y radiolysis in the solid state.'* Identical photosensitization reactions could be carried out at 313 nm but not at 334 nm (see Table I). In a photolysis experiment at 280 f 10 nm of a solution containing Trp, glycine, and MNP at pH 5.7, an ESR signal, which appeared only during UV illumination, characteristic of the hydrogen adduct of MNP, (CH,),CN(O)H, was observed (Figure 2c) (uN(nieor1) = 14.7 G; auH= 14.2 G ) . The light dependence of the adduct is due to its short lifetime. The ESR spectrum obtained upon photolysis of an MNP solution containing Tyr, p-OH(Ph)CH2-CH(NH3+)COO-, at 280 nm consisted of a broad primary triplet due to the nitrogen of the nitroxyl group. This result has been attributed to a radical on the aromatic ring of Tyr,19but a detailed assignment of the radical structure could not be made. Figure 3a shows an ESR spectrum, obtained when Tyr was photolyzed at 313 nm in the presence of alanine, in which the DTBN signal obscures the lines from both the spin adducts of the Tyr radical and that of the deamination radical of alanine. When the photolyzed solution was subsequently treated with petroleum ether, the DTBN signal was removed, and the spectrum obtained (Figure 3b) clearly shows the spin adduct of the deamination radical of alanine. The results of experiments with Tyr and the different scavengers are summarized in Table I and indicate that photoionization could occur at 280 and 313 nm but not at 334 nm. UV photolysis of Phe at 280 nm in the presence of MNP leads to the formation of the decarboxylation radical (structure 11) as well as the benzyl radical16(structure 111).
(b)
TYR+ALA AFTER PET. ETHER EXTRACTION
IS
kS
k
t
H3N
H I
C(CH3I3
I
I
I
-C-N-
C ( CH3)
0'
N
I
The generation of hydrated electrons from Trp by UV light and the reactions used to detect eaq-in the present work are summarized in eq 1-6. Subsequent to pho-
I
(19)Lion, Y.;Kuwabara, M.;Riesz, P. Photochem. Photobiol. 1982, 35,43.
hu
Trp+.
eaq- Trp
TRP+. H+
Trp--
eaq- H30+
+ eaq- + H3N+CH&OOeaq - + H3N+CH(CHJCOO-
eaq- ClCH2CH20H
I11
The h y p e f i e coupling constants of both adducts are given in Table 11. When chloroethanol was also present during photolysis, ita dechlorination radical waa formed, but the ESR lines due to its spin adduct overlapped with those from the benzyl radical (lines marked A in Figure 4a). However, a t 313 nm the benzyl radical was not observed (Figure 4b) under our experimental conditions and the signal from the spin adduct of .CH2CH20Hcould then be detected.
- + - + + + - + - + - + - + + + + Trp
- 0'
CH2
I1
Flgure 3. ESR spectra of a UV-photolyzed (A = 313 f 7 nm) solution (pH 9.8) containing MNP, Tyr, and ol-alanine: (a) before petroleum ether extraction and (b) after petroleum ether extraction. The DTBN signal is labeled S.
(CH,),CN=O R spin trap (MNP)
eaq- MNP
MNP-.
H30+
ea;
(la)
Trp-
Ob)
HsO+
H20 C1-
Trp.
H.
(3)
.CH2CH20H (4a)
NH3
CH2COO- (4b)
NH3
CH(CH3)COO(44
(CH,),CN(O)R spin adduct MNP-.
(CH,),CN(O)H H adduct
(5)
(64
+ H20
(6b)
toionization (reaction la), eaq- will follow competing pathways (reactions 2,3,4, or 6) depending on the product of the concentration ( c ) of the substrate with which it might react and the rate constant ( k ) for that particular
3482
The Journal of Physical Chemistry, Vol. 86, No. 17, 1982
Mossoba et al.
TABLE 11: Hyperfine Coupling Constants (gauss) of t h e Spin-Trapped Radicals compd L -Trp L-Tyr L-Phe
ClCH,CH,OH GlY DL-Ala
secondary splittings
primary 14N splitting
pap^
radical
yaH
PaH
I
16.3 16.5 16.0 16.6 16.6 16.0 16.0
a
1.6
1.1
10.6 13.1 (2H) 8.5 (2H) 5.3
0.4 ( 2 H )
I1 I11
0.5 (2H)
CH,CH,OH CH,COOCH(CH,)COO-
0.4 ( 3 H )
* The observed primary triplet was broad and no secondary hyperfine structure could be detected, suggesting a radical o n the aromatic ring of Tyr. I PHE +CICH2CH20H I. =280nm
(a)
I A
r5G-
i/
PHE + CICH~CHZOH 1 =313nm
I
employed in our experiments. Similar conclusions apply to Tyr and Phe. Although the rate constant for the reaction of eaq-with H30+is 2.4 X 1O1O M-'s-l (ref 23) (diffusion controlled), for experiments at pH >4 the conversion of eaq-to Ha can be neglected in comparison with the reaction of eaq-with other scavengers (reaction 4). Hence, the formation of (CH,),CN(O)H from H. and MNP can be ruled out. Several other pathways leading to the formation of the H adduct of MNP will be considered next. One possible mechanism involves the reduction of MNP followed by protonation (reactions 6a and 6b). Alternatively, it is also possible to produce the H adduct from tert-butylhydroxylamine, (CH3)3CN(OH)H,24 which upon reaction with HN02 gives (CH3)3CN(OH)N=0.25 Both nitrous acid and tert-butylnitrosohydroxylaminewere found to exist in UV-photolyzed aqueous solutions of MNP.26 While the chemistry of MNP is not fully understood, the different reaction pathways presented above show that the formation of the H adduct provides no definitive evidence for the photoionization of aromatic amino acids. Hydrogen atoms, which can be formed from Trp above 275 nm,6 may react with OH- (k = 1.5 X lo7 M-' s-l (ref 27)) to give ew-. However, it can be shown that this reaction is negligible compared to that of Ha with Trp (k = 2.3 X lo9 M-' s-l (r ef 28) at concentrations used which therefore effectively acts as a hydrogen atom sink. The attack of He on glycine or alanine leads to H-abstraction radicals12which were not observed in the present work. On the other hand, two reactions occur between H. and chloroethanol, namely H.
+ ClCHzCHzOH Hz
Flgure 4. (a) ESR spectra of a UV-photolyzed (A = 280 f 10 nm) solution (pH 10.0) containing MNP, Phe, and chloroethanol. The lines due the radical generated from chloroethanol overlap with the benzyl spin-adduct slgnal (labeled A). (b) ESR spectra of a UV-photolyzed (k = 313 f 3 nm) solution (pH 10.0) containing MNP, Phe, and chloroethanol. The DTBN signal is labeled S, while denotes the 4.8-0 doublet due to the 13C satellites of DTBN.12 The MNP concentration was 5 mg mL-'.2e
reaction. Considering the rate constants of the reactions e,; + CEtOH, of e,; (ew- Trp, k = 3.1 X lo8 M-l k = 4.1 X lo8 M-' s-l; 21 ea; + Gly, k = 5.5 X lo6 M-' s-l; 22 e, - + Ala, k = 5.9 X lo6 M-l dn), one can show from the values of the products ck for these reactions that the photoejected electrons can be scavenged by chloroethanol, glycine, or alanine (reactions 4a-c) under the conditions
+
(20) Braams, R.Radiat. Res. 1966,27, 319. (21) Anbar, M.; Hart,E. J. J. Phys. Chem. 1966,69, 271. (22) Davies, J. V.; Ebert, M.; Swallow, A. J. In 'Pulse Radiolysis"; Ebert, M., Keene, J. P., Swallow, A. J., Baxendale, J. H., Eds.; Academic Press: New York, 1965; pp 165-70.
+ CICHCHzOH + ClCH2CHOH (7) +
k = 1.5 X lo7 M-l s-l (ref 29) H.
-
+ CICHzCHzOH
HC1+ CH2CHz0H
(8)
k = 1.5 X lo6 M-l s-l (ref 29) I t should be noted that reaction 8 generates the same radical as reaction 4a, which is used in the present investigation as a measure of photoionization of Trp. However, the contribution of reaction 8 can be ignored since ck products for the reaction of H. Trp or for reaction 7 are 1 order of magnitude greater. It should be pointed out that the possibility of the formation of the
+
(23) Anbar, M.; Neta, P. Znt. J. Appl. Radiat. Zsot. 1967, 18, 493. (24) Makino, K., unpublished results. (25) Caeon, J.; Prout, F. S. J.Am. Chem. SOC.1949, 71, 1218. (26) Makino, K.; Suzuki, N.; Moriya, F.; Rokushika, S.; Hatano, H. Radiat. Res. 1981,86, 294. (27) Nehari, S.;Rabani, J. J.Phys. Chem. 1963, 67, 1609. (28) Neta, P.; Schuler, R. H. Radiat. Res. 1971, 47, 612. (29) Anbar, M.; Neta, P. J. Chem. SOC. A 1967, 834.
J. phys. Chem. iga2, 86, 3483-3489
same radical by H* or eaq-arises only for Trp with C1CH2CH20Hsince the deamination radicals of glycine and alanine can only be formed by the reaction of ea;.*33 The current results indicate that at 313 nm photoionization of the aromatic amino acids Trp, Tyr, and Phe could (30) Sevilla, M. D. J. Phys. Chem. 1970, 74, 2096, 3366. (31) Neta, P.; Fessenden, R. W. J. Phys. Chem. 1970, 74, 2263. (32) Garisson, W. M. Rodiat. Res. Rev. 1972, 3, 305. (33) Rustgi, S.; Joahi, A.; Rieez, P.; Friedberg, F. Znt. J.Radiat. Biol. Relut. Stud. Phye., Chem. Med. 1977,32,533.
3483
occur. This conclusion is consistent with the results of Steen,' Dillon: and Zechner et aL9even though the nonquantitative nature of the spin-trapping experiments may have a bearing on any comparison with the results of other techniques. Finally, the possibility of a direct electron transfer from the triplet state of Trp to the scavengers remains to be investigated. If such a process could be shown to occur, it might account for the apparent difference between the present results and those of Amouyal et a1.3
Effect of Solvent on CaCi,-NH3 Equilibria in Suspensions W. E. Wentworth;
C. F. Batten,
Department of Chemistty, Univefs#y of Houston, Houston, Texas 77004
J. A. Ayale,+ and Rlcardo Alcala I.T.E.S.O., c)uedeb~re, Jaikco, Mxlco (Received: November 10, 1981; I n Final Form: April 2, 1982)
The equilibria CaC12.(8/4)NH3and CaC12.(4/2)NH3have been studied with the solid phases suspended in 1-heptanoland 1-pentanol. The equilibrium NH3 pressures are consistent with those obtained from the direct gas-solid reaction. The temperature dependence for the CaC12.(8/4)NH3equilibrium has been studied in a high-pressure system to -6000 torr. A nearly linear log P vs. 1 / T relationship was obtained with a heat of reaction AH = 41.0 kJ/(mol of NH3) at 298 K. A slight nonlinearity observed at high temperatures can be attributed to the change in heat capacity and to the deviation from ideal gas behavior. The equilibrium vapor pressure can be adequately represented by In P = -109.8953 - (1.15966 X 104)/T+ 21.24888 In T - 0.0279337T + (2219.06 In T)/T.
Introduction In a previous publication' it was suggested that metal salt-gas complex equilibria should be the same whether equilibrium is between the gas and solids directly or between the gas and solids suspended in an appropriate solvent. Support for the concept was obtained by comparison of the equilibrium ammonia pressure for the gas-solid reaction with that obtained from the gas-suspension reaction. The SrC12-NH3 complexes' and the CaC12-NH3 complexes2 were investigated, and the equilibrium ammonia pressures from the suspensions agreed with those for the gas-solid reaction. In fact, because of faster reaction rates, it was possible to detect the equilibrium SrC12.2NH3(s)+ SrCl2.NH3(s) + NH3(g)
(I)
which we will designate by SrCl242/1)NH3,in addition to the well-known SrC12.(8/2)NH3 equilibrium. It is this increase in rate of reaction that would allow these reactions to be used for solar thermal storage in a chemical heat pump cycle.2 The increase in reaction rate presumably arises from (1)the greater surface area of the solids when suspended in the inert solvent and (2) increased NH3 concentration in the solvent over that in the gas phase. In the previous work,'f a minimum was observed in the t Present
address: Instituto de Investigaciones Electricas Cuernavaca Mor. Mexico.
pressure vs. NH3/salt mole ratio in the vicinity of the transition from one complex to another. It was suggested that this minimum was a kinetic effect. Data supporting this hypothesis are presented in this paper. 1-Heptanol and the was used as the solvent in the previous SrC12- and CaC12-ammonia complexes formed a finely divided suspension in this solvent. In this paper we present additional data for the CaC12.(8/4)NH3 equilibrium in 1-heptanol at higher pressures (up to 8.5 atm) and at higher temperatures (-75 "C). It waa also suggested's2 that other solvents could be used provided they are sufficiently polar to suspend the solids but do not interact so strongly as to complex with the salt and displace complexed ammonia. It was noted that CaC12-NH3 complexes in decane, a nonpolar hydrocarbon, did not form suspensions, whereas, in ethylene glycol, CaC12 apparently formed a sufficiently stable solvent complex so as to prevent the formation of the ammoniate complex. In this paper we report similar studies for CaCl2-(8/4)NH3in 1-pentanol. The equilibria are essentially the same as those measured in 1-heptanol. In measurements of NH3(g)-solid salt complex equilibria, it is customary to plot NH3(g) pressure vs. (moles of bound NH3)/(mole of salt). In measurements of NH3(1) W. M. Raldow, D. W. Johnston, and W. E. Wentworth, J.Phys. Chem., 84, 2599 (1980). (2) W. E. Wentworth, D. W. Johnston, and W. M. Raldow, Sol. Energy, 26, 141 (1981).
0022-3654/82/2086-3483$01.25/00 1982 American Chemical Society
