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The Journal of Physical Chemistry, Vol. 83, No. 26, 1979 T. Shimanouchl in "Physical Chemistry An Advanced Treatise", Vol. 4, Academic Press, New York, 1970, Chapter 6. J. Overend, Annu. Rev. fhys. Chem., 21, 265 (1970). H. H. Nielsen, Rev. Mod. Phys., 23, 90 (1951). A. K. Jameson, K. Schuett, C. J. Jameson, S. M. Cohen, and H. Palker, J. Chem. fhys., 67, 2821 (1977). C. J. Jameson, A. K. Jameson, H. Parker, S.M. Cohen, and C. L. Lee, J. Chem. fhys., 68, 2861 (1978). C. J. Jameson, A. K. Jameson, and S. M. Cohen, J. Chem. fhys., 67, 2771 (1977). A. K. Jameson, J. W. Moyer, and C. J. Jameson, J. Chem. Phys., 68, 2873 (1978). C. J. Jameson, A. K. Jameson, and H. Parker, J. Chem. fhys., 68, 2868 (1978). C. J. Jameson and A. K. Jameson, J. Chem. fhys., 69, 1655 (1978). C. J. Jameson, A. K. Jameson, and H. Parker, J. Chem. Phys., 69, 1318 (1978). C. J. Jameson and A. K. Jameson, J. Chem. fhys., 69,615 (1978). C. J. Jameson and A. K. Jameson, to be published. C. J. Jameson, A. K. Jameson, and S. M. Cohen, J. Chem. fhys., 62, 4224 11975). C. J. Jameson, A. K. Jameson, and S. M. Cohen, J. Chem. Phys., 65, 3397 (1976). R. W. Rudolph and R. W. Party, J. Am. Chem. Soc., 89, 1621 (1967). D. D. Des Marteau and G. H. Cady, Inorg. Chem., 5, 1829 (1966). L. Maler and R. Schmutzler, J. Chem. Soc., Chem. Commun ., 96 1 (1969). The temperature-dependent line shape In "NF, has been observed by E. L. Muettertiesand W. D. philllps, J. Am. Chem. Soc., 81, 1084 (1959). Landoit-Bornstein,"Zahlenwerten und Funktionen", Springer-Verlag, New York, 1967. G.Heckmann and E. Fluck, Z.Naturforsch. B, 24, 1092 (1969); Mol. fhys., 23, 175 (1972). For a critical review of the aDDlication of ,P ' NMR shifts to studies of biological reactions, see M.'Cohn and B. D. N. Rao, Bull. Magn: Reson., 1, 38 (1979). M. Toyama, T. Oka, and Y. Morlno, J. Mol. Spectrosc., 13, 193 [ 1964). A. D.'BuckIngham and W. Urland, Chem. Rev., 75, 113 (1975). D. R. Herschbach and V. W. Laurie, J. Chem. fhys., 35,458 (1961). An example of a calculation in which higher order terms are Included is that by W. T. Raynes, A. M. Davles, and D. 8. Cook, Mol. fhys., 21, 123 (1971). C. J. Jameson, J. Chem. Phys., 66, 4983 (1977). I. Suzukl, Appl. Spectrosc. Rev., 9, 249 (1975).
Ichikawa, Kevan, and Narayana (31) C. J. Jameson, J . Chem. fhys., 67, 2614 (1977). (32) C. J. Jameson, J. Chem. Phys., 66, 4977 (1977). (33) For example, see the plot of (Ai-) 'versus Tfor a diatomic molecule in Figure 1 of ref 25. (34) K. Kuchitsu and Y. Morlno, Bull. Chem. SOC.Jpn., 38, 814 (1965). (35) M. L. LaBoda and J. Overend, Spectrochim. Acta, Part A, 32, 1033 (1976). (36) K. Kuchitsu and L. S. Bartell, J. Chem. Phys., 36, 2470 (1962). (37) W. Stricker and J. C. Hochenblelcher, Z. Naturforsch. A, 26, 27 (1973). (38) D. A. Gilbert, A. Roberts, and P. A. Griswold, fhys. Rev., 76, 1723 (1949). (39) Isotope effects on mean displacements have been discussed by L. S. Bartell, J. Chem. Phys., 38, 1827 (1963); K. Kuchitsu and L. S. Bartell, /bid., 36, 2460, 2470 (1962); L. S. Bartell, ibid., 42, 1661 (1965); E. A. Halevl, Trans. Faraday Soc., 54, 1441 (1958). (40) H. Batiz-Hernandezand R. A. Bernhelm, Prog. Nucl. Magn. Reson. Spectrosc., 3, 63 (1967). (41) A. K. Jameson and C. J. Jameson, J. Magn. Reson., 32,455 (1978). (42) J. Joklsaarl, K. Ralsanen, L. Lajunen, A. Passoja, and P. Pyykko, J. Magn. Reson., 31, 121 (1978). (43) J. Jokisaari and K. Raisanen, Mol. fhys., 36, 113 (1978). (44) Y. Morino, K. Kuchtsu, and S. Yamamoto, Spectrochim. Acta, Part A, 24, 335 (1968). (45) C. J. Jameson, unpublished calculations. (46) K. Kuchltsu, J. Mol. Spectrosc., 7, 399 (1961). (47) Approximate anharmonic force fields for PF, and NF, have been used to calculate observed spectroscopic constants. See, for exampie, C. Small and J. G. Smith, J. Mol. Spectrosc., 73, 215 (1978); A. C. Jeannotte I1 and J. Overend, Spectrcchim. Acta, fad A, 33, 1067 (1977). (48) J. H. Letcher and J. R. Van Wazer, J. Chem. phvs., 44, 615 (1966). (49) Some theoretical estimates of (a d a Ar) are available In calculations In which u has been determined for more than one internuclear separation In diatomic molecules. See, for example, CO: R. M. Stevens and M. Karplus, J. Chem. Phys., 49, 1094 (1968); NP: E. A. Laws, R. M. Stevens, and W. N. Llpscomb, ibEd., 54, 4629 (1971); :H ; T. 8 . Garrett and D. Zeroka, Int. J. Quantum. Chem., 6, 663 (1972); HP: W. T. Raynes, A. M. Davles, and D. B. Cook, Mol. fhys., 21, 123 (1971); T. W. Marshall and J. A. Pople, ibM., 3, 339 (1960); E. Ishlguro and S. Kolde, Phys. Rev., 94, 350 (1954); D. Zeroka, J. Chem. Phys., 59, 3835 (1973); HF: R. M. Stevens and W. N. Lipscomb, ibid., 41, 184 (1964); LIH: R. M. Stevens and W. N. Lipscomb, Ibid., 40, 2238 (1964). (50) J. Ridard, E. Levy, and Ph. Millie, Mol. fhys., 36, 1025 (1978). (51) C. J. Jameson, to be published.
Electron Spin-Echo Modulation Studies of Silver Atom Solvation in Methanol. Geometrical Model for the Methanol Solvation Shell Tsunekl Ichikawa, Larry Kevan," Depattment of Chemistry, Wayne State University, Detroit, Michigan 48202
and P. A. Narayana School of Physics, University of Hyderabad, Hyderabad, India (Received July 16, 1979)
Electron spin echo modulation studies have been carried out for Ago in CD30H and CH30D at 4.2 K. The data have been analyzed by using ratio analysis and complete simulation. The results indicate that Ago is surrounded by four methanol molecules with their oxygen ends pointing toward Ago. The average distance between the silver atom and both the hydroxyl and methyl protons is about 3.3 A. There is no observable geometrical change in the solvation shell around Ago on warming the sample to 77 K in contrast with the ice matrix. This is consistent with weaker hydrogen bonding interactions in methanol than in water.
Introduction Recent magnetic resonance studiesl-4 of the molecular environment of Ago in ice have yielded a facinating picture of solvation. It was found that when AgO is initially formed at 4.2 K by reaction of Ag+ with electrons it is surrounded by four water molecules with their oxygens pointing toward Ago. However, brief warming of the sample to 77 K in-
duces an environmental change toward equilibrium, or solvation, involving the rotation Of a single Water molecule about its OH bond. The driving force for this solvation probably involves a hydrogen bonding interaction. To contrast Ago interactions in ice, we here investigate the molecular environment of Ago in methanol matrices in which the hydrogen bonding interactions may be
0022-3654/79/2083-3378$01.00/00 1979 American Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 26, 1979 3379
ESE Study of Ago in Methanol ELECTRON SPIN ECHO SPECTROMETER
I---= RECORDER
I
Rth(7)= [ V m a ~ h ( ~ ) / V m = i ~(rth)" h ( ~ ) ] n (3) where the theoretical maxima and minima are given by
20db PHASE ATTENUATOR SHIFTER
I
RESET 9GNAL FOR THE PULSE
respectively. Experimentally the ratio Rex= Vm,y/ VmineX is determined for the same value of T as a function of T . The advantage of this method is that ReX(s)is independent of the echo decay function. This is compared with the theoretical ratio
I
GENERATOR
where
Figure 1. Block diagram of the electron spin echo spectrometer.
weaker. Electron spin echo spectrometry has proved successful for determining solvation structure in frozen solutions5 and has been applied to the present problem.
Theory and Data Analysis The theory and applications of electron spin echo modulation has been discussed in detai1516and so only the final formulae used to calculate the modulation are given. The normalized echo modulation as a function of 7, the separation between the two pulses, is given by'
where
F(x)= ( k COS (AX)), F'(x)= ( k 2 COS (AX)), For a system with n equivalent nuclei Rth
A=--(3 cos2 0 - 1) - 27ra hr3 B = -3ggnPPn sin 0 cos 0 hr3 In the above expressions wIis the nuclear Larmor frequency in the field Ho, o, and obare the hyperfine frequencies associated with the electron spin levels 11/2) and 1-1/2), a is the isotropic hyperfine coupling constant, r is the electron-nuclear distance, and 0 is the angle between Hoand the r vector. For a disordered system the total modulation is obtained by averaging over all the orientations 19. Further, if the electron interacts with n identical nuclei the overall modulation is given by Vmod = [(V(r,a)>ln (2) where the angular brackets indicate the averaging over all the orientations. It is this expression which was programmed numerically to calculate the modulation. The experimental data were first analyzed by using the ratio analysis described by Ichikawa et a1.8 This method is briefly described. First, two smooth curves joining all the maxima and all the minima of the echo modulation are drawn. These curves are denoted by Vmex and Vmhex,
[rth(a,r)ln
log (log Rth) = log n
+ log (log rth)
(5)
Thus, a convenient way of analyzing the modulation curves is to plot log (log Rex) and log (log rth) vs. T . Such a plot gives two parallel curves displaced along the y axis by log n provided correct values for r and a are chosen. Thus, a and r are varied till nearly parallel curves are obtained, The shift needed to bring these curves into coincidence gives log n and hence n. The parameters obtained from the ratio analysis are used to simulate the modulation. In order to compare the calculated modulation with the experimental curve, we multiplied the normalized modulation with a decay function g ( T ) of the form g(T)
ggnPPn
N
= exp(Ao
+ A ~ +T A2r2 + A373)
(6)
The coefficients A, are determined by a least-squares method described by Ichikawa et ala8
Experimental Section The electron spin echo spectrometer used is shown in Figure 1. The basic spectrometer is similar to that described earlier.2 However, there are some important modifications. Microwave pulses were produced by a klystron and amplified by a l-kW traveling wave tube (TWT) amplifier (Litton Model 624). The input to the TWT was pulsed by a General Microwave (ADM 864) PIN diode. The TWT itself was turned on about 100 ns before the microwave input was given. The reflected power from the cavity was amplified by a Watkins-Johnson 491-55 low noise TWT amplifier which also acts as a limiter. The output from the low noise TWT was detected by a crystal and amplified by a Hewlett Packard 462A pulse amplifier. The output of the amplifier was fed to a PAR boxcar integrator (Model 162 with 164 sampling head) and the data were transferred to a recorder or to a signal averager to obtain the experimental modulation data. The signal averager (Tracor Northern NS-570) output goes to a recorder and to a Data General Eclipse S-130 computer for processing the data. The necessary signals to pulse the microwave switches and to trigger the TWT, boxcar, and the signal averager were produced by a pulse programmer
3380
The Journal of Physical Chemistry, Vol. 83, No. 26, 1979
"
Ichikawa, Kevan, and Narayana
L
*, PS Flgure 2. Comparison of the experimental (- - -) and calculated (-) modulation for Ago in CD30H. The parameters for the calculated curve are r = 3.3 A, n = 12, and a = 0. The decay function used is exp(2.53 - 2.347 4- 0 . 5 3 ~- ~0.06~~).
built around Berkeley Nucleonics Corp. Models 7010 and 7030 digital delay generators. The home-built microwave cavity has a Q 500. In the present studies relatively large pulse widths of 100 ns were used to suppress modulation from protons. Two specifically deuterated methanols, CH30D and CD30H, were used in these experiments. These chemicals were obtained from Stohler Isotope Chemicals. Solutions of 0.5 M AgC104 with 0.2 M NaF in CH30D and CD30H were prepared in 3-mm 0.d. Suprasil quartz tubes. The fluoride ion acts as a hole trap and enhances the yield of silver atoms. The samples were degassed several times by the freeze-pump-thaw method and sealed. The samples froze into nice glasses when submerged in liquid nitrogen. Samples were X irradiated at 4.2 K to a total dose of 0.5 Mrd a t 50 kV and 40 mA with a Siemens X-ray tube (AGW61). The helium dewar in which the sample was irradiated was directly inserted into the cavity. Thus, the samples were always maintained at 4.2 K except for the brief period when they were deliberately warmed to 77 K to induce solvation shell reorientation.
T,
-
-
Results and Discussion The ESR spectrum of Ago in methanol consists of two doublets separated by 1740 M H z . ~Each of the doublets arises because of the approximately equiabundant isotopes lo7Agand lOsAg. The spin echoes were observed by monitoring the high field line of lo9Ag. The experimental modulations from Ago in CD30H and CH30D are shown in Figures 2 and 3, respectively. These modulations were recorded at 4.2 K after irradiating the sample at 4.2 K. The samples were then briefly warmed to 77 K by inserting into liquid nitrogen and the modulation was again recorded at 4.2 K. No substantial difference in the modulation pattern was found between the annealed and unannealed samples for both Ago-CD,OH and Ago-CH30D. The data were analyzed by using the ratio analysis described above. The results are shown in Figure 4 for Ago-CD30H. The top curve shows the variation of -log (log rth)with T while the bottom curve shows the variation of -log (log Rex)with 7. The upper curve was calculated for r = 3.3 A and a = 0. The two curves are reasonably parallel to each other as would be expected for a correct set of parameters. The displacement of these two curves along the ordinate gives n 12. These parameters were used to simulate the modulation shown in Figure 2 (dashed N
-
PS
Flgure 3. Comparison of the experimental (- -) and calculated (-) modulation for Ago in CH,OD. The parameters for the calculated curve are r = 3.2 A, n = 4, and a = 0. The decay function used is exp(2.34 - 1.847 - 0 . 1 1 ~ '4- 0 . 0 3 ~ ~ ) .
t Ago IN CD30H
V
I
U
I
:: IC J
0'
2
I T?
I
PS
Flgure 4. Ratio analysis of two pulse electron spin echo modulation data for Ago in CD,OH. The upper curve (-- -) is the calculated curve for r = 3.3 A and a = 0. The lower curve represents the experimental data. The two curves superimpose for n = 12.
line) together with a least-squares analysis for the decay. The decay constants are given in the Figure 2 caption. Since there are three deuterons per molecule, the analysis implies that there are four methanol molecules Surrounding Ago with their methyl deuterons located a t a distance of 3.3 f 0.1 A. In order to determine the orientation of the methanol molecules with respect to Ago the modulation data from Ago-CH30D were also analyzed. The results of the ratio analysis are shown in Figure 5. This analysis yields I" = 3.2 f 0.1 A, n = 4, and a = 0. The simulated modulation obtained by using these parameters is shown in Figure 3 (dashed curve) where the decay constants are given. This analysis again implies that there are four water molecules surrounding Ago. The analysis also implies that all four methanol protons are approximately equidistant from Ago. This suggests that the orientation of methanol is such that the oxygen end points toward Ago. This structure deduced for the Ago solvation shell in methanol is expected to be similar to that for Ag+ existing before the ion is reduced to the atom. The Ag-D distance is 0.1-0.2 A greater than the similar distance in a water matrix determined by electron spin echo modulation method^.^*^ While this is barely experimentally significant,
The Journal of Physical Chemistty, Vol. 83, No. 26, 7979 3381
ENDOR and ELDOR Studies of Dipeptides
pattern. No change was seen in striking contrast to Ago in ice matrices. Thus, the initially stabilized Ago solvation shell in methanol at 4.2 K seems to be close to an equilibrium configuration. Since no geometrical change, like rotation about a C-O bond, to produce a hydrogen bonding interaction with Ago occurs in methanol, we suggest that this hydrogen bonding interaction is weaker in methanol than in ice. This is consistent with the less acidic character of the hydroxyl proton in methanol as compared to water.
Acknowledgment. This work was supported by the U.S. Department of Energy under Contract EY-764-02-2086.
; 0 ’ 2
I
7-j
ps
Figure 5. Ratio analysis of two pulse electron spin echo modulation data for Ago in CH30D. The upper curve (- -) is the calculated curve for r = 3.2 A and a = 0. The lower curve represents the experimental data. The two curves superimpose for n = 4.
-
it is consistent with the expected slightly greater effective radius for methanol compared to water. The stability of the initially produced Ago solvation shell in methanol at 4.2 K was investigated by briefly annealing to 77 .K followed by reexamination of the modulation
References a n d Notes
(1) L. Kevan, H. Hase, and K. Kawabata, J. Chem. Phys., 66, 3834 (1977). (2) P. A. Narayana, D. Becker, and L. Kevan, J . Chem. Phys., 66, 652 (1978). (3) L. Kevan, J. Chem. Phys., 69, 3444 (1978). (4) P. A. Narayana, T. Ichikawa, and L. Kevan, J. Chem. Phys., in press. (5) L. Kevan in “Time Domain Electron Spin Resonance”, L. Kevan and R. Schwartz, Ed., Wiley-Interscience, New York, 1979, Chapter 8. (6) W. B. Mims in “Electron Paramagnetic Resonance”, S. Geschwind, Ed., Plenum, New York, 1972, Chapter 4. (7) W. B. Mims, Phys. Rev., 5, 2409 (1972). (8) T. Ichikawa, L. Kevan, M. K. Bowman, S. A. Dikanov, and Yu. D. Tsvetkov, J. Chem. Phys., 71, 1167 (1979). (9) J. Michalik and L. Kevan, J . Mag. Reson., 31, 259 (1978).
ENDOR and ELDOR Studies of X-Irradiated Polycrystalline Dipeptides, Myosin, and Actomyosin James S. Hwang,+ Alan C. Dickinson,l and Lowell D. Klspert” Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35486 (Received August 17, 1979) Publication costs assisted by the U S . Army Natick Research and Development Command
ENDOR and ELDOR studies have been carried out for nine dipeptide powders as well as powders of myosin and actomyosin X-ray irradiated at 77 K in an attempt to characterize the final radical stable upon annealing between 183 and 260 K. The dipeptides studied were glycylglycine, L-alanylglycine, glycyl-L-alanine, L-alanyl-L-alanine, glycyl-L-aspartic acid, glycyl-L-glutamicacid, glycyl-L-methionine,glycyl-L-serine, and L-lysyl-L-lysine. Nitrogen ENDOR spectra have been observed between 1and 8 MHz for each powder and the nitrogen hyperfine and quadrupole tensor has been estimated. Analysis of the ENDOR, ELDOR, and ESR spectra indicates at least one of the final radicals in the dipeptide powders (except Gly-Gly, and possibly Gly-Glu, Gly-Ser) to be the decarboxylation product NH,CHRCONHCHR’ rather than just the abstraction type (NH3+CHRONHCR’COO-) previously identified in irradiated dipeptide ices. A decarboxylation type radical is also present as a final radical in the irradiated myosin and actomyosin.
Introduction In recent years many contributions have been made toward an understanding of the radiation chemistry of biomo1ecules.l The study of radiation damage to proteins, for example, has been greatly aided by experiments on amino acid and peptide model systems.lI2 For single crystals studied as a function of temperature, radiation products are characterized as oxidation and reduction product^.^,^ The major oxidation products in many amino acids and dipeptides such as glycine, DL-serine, L-histidine, &alanine, L-cysteic acid, acetylglycine, and glycylglycine, Department of Chemistry, University of Petroleum and Minerals, Dhahran, Saudi Arabia. t Chemistry Department, American International College, Springfield, MA 01109.
at 77 K or lower, arises from decarboxylation. For a-amino acids with a C-OH bond, e.g., serine, another radical is formed from the loss of hydrogen from the hydroxyl group. For amino acids containing conjugated rings the oxidation products can be different because the spin densities are distributed in the conjugated system. The initial reduction products in amino acids at low temperature result from the addition of an electron to the carboxyl oxygen. At higher temperatures the reduced amino acids can undergo deamination to produce the deamination radicals. For peptides the addition of an electron can occur a t the carboxyl group or the carbonyl group of the peptide bond. The radicals produced by reduction are usually stable only a t low temperatures and are not observed at temperatures a t which the abstraction-type radicals remain. This abstraction type radical
0022-3654/79/2083-3381$01.00/00 1979 American Chemical Society
