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Communications to the Editor (5) J. -P. Dodelet and G. R. Freeman, Can. J. Chem., 50, 2667 (1972). (6) K. Horacek and G. R. Freeman, J. Chem. Phys., 53,4486 (1970). (7) Y. Hatano, K. Takeuchi, and S. Takao, J. Phys. Chem., 77, 586 (1973). (8) Y. Hatano and J. L. Abboud, Euii. Chern. SOC.Jap., 47, 238 (1974). (9) P. P. lnfelta and R. H. Schuler, J. Phys. Chem., 76, 987 (1972); G. W. Klein and R. H. Schuler, ibid., 77, 978 (1973). (10) M. 0. Robinson and G. R. Freeman, Can. J. Chem., 51, 650 (1973). (11) G. R. A. Johnson and J. M. Warman, Trans. faraday Soc., 61, 512 (1965). (12) Y. Hatano and H. Shimamori, J. Phys. Chern., submitted for publication.
Laboratory of Physical Chemistry Tokyo Institute of Technology Meguro-ku, Tokyo, Japan
Kenji It0 Yoshihiko Hatano*
Received October 26, 7973
Figure 1. Disulfide stretching wave number vs. maximum of longest wavelength absorption band. Compounds for experimental points starting at the left side are described in the text. The straight line is the relation proposed in ref 3.
Invariance of Disulfide Stretching Wave Numbers to Disulfide Dihedral Angles Publication costs assisted by the National Science Foundation
Sir: In research performed in this laboratory disulfide vibrational spectra of about 20 compounds as revealed by Raman and infrared spectra were subjected to analysis and interpretations.1 In contrast to earlier proposals2 no correlation was found between the Raman intensity ratio for C-S and S-S stretching bands and the CSS angle nor was attribution of bands at 634 and 684 cm-I in thioctic acid to symmetric and antisymmetric C-S stretches supported. A more recent study3 while agreeing with our above two conclusions also claims, as its main point, that a linear relation exists between the wave number of the disulfide stretch and CSSC dihedral angle. This claim contrasts with our earlier conclusion that such a correlation does not exist. Since such a correlation, if it occurs, would be of value in estimating disulfide dihedral angles from Raman wave numbers in complex molecules, it is reexamined in this note with the result that our original view is reaffirmed. The results considered in ref 3 are presented in a plot of disulfide stretching wave number obtained on a solid us. the CSSC dihedral angle determined from diffraction methods on a solid or from the wavelength at the maximum of the longest wavelength ultraviolet disulfide absorption band in solution. Consistent with several earlier studies a linear plot is also shown for the wavelength of maximum absorption us. dihedral angle. However, a more direct approach that permits use of a wide range of results without the intermediacy of the dihedral angle is to plot the disulfide stretching wave number us. the wavelength of the ultraviolet absorption maximum. Such a plot is shown in Figure 1 where the straight line represents the relationship proposed in ref 3. In gathering results for compounds to plot in Figure 1 we consider (with one exception) only Raman and ultraviolet spectra that have been obtained in the same phase, which is that of liquid or solution. In this way tenuous arguments in ref 3 such as that which result in the intense peak at 511 cm-I in solid thioctic acid not being assigned to the disulfide stretching wave number are avoided. For the seven points bunched at the left of Figure 1 from 242 to 248 nm both kinds of spectra were taken on aqueous acid so1utions.l s 4 , 5 The compounds all derivatives
of L-cysteine are hexamethylcystine, cystine, homocystine, mixed disulfide of cysteine and ethanethiol, cystine dimethyl ester.2HC1, N,N'-dimethylcystine and oxidized glutathione (bullseye representing two identical points). The next two points at 249 and 252 nm are for diethyl disulfide and dimethyl disulfide, respectively, where the Raman spectra were taken on neat liquids and the ultraviolet spectra on dilute solutions in hydrocarbon solvents. trans-2,3-Dithiadecalin absorbs maximally at 289 nm in hydrocarbon solvent^.^ The Raman spectra was taken on a sample of the solid.1 This is the only compound of Figure 1 for which the two kinds of spectra were recorded on different phases. The pair of points at 329 and 331 nm represent 3,4-dithiaspirodecane and 4,4-diethyl-1,2-dithiolane. The ultraviolet spectra were procured in hydrocarbon solv e n t ~ and ~ , ~the Raman spectra on freshly distilled neat 1iquids.l The last point in Figure 1 at 333 nm is for D,L6,8-thioctic acid with both kinds of spectra observed in methanol.3 The authors of ref 3 point out that their correlation concerning the CS-SC dihedral angle is valid only for a gauche conformation about the CC-SS bond. Consistent with this limitation all the results plotted in Figure 1 are thought to correspond to values for the favored gauche conformation.' The conclusions are based upon extensive studies over a period of years.'?* Even if all the points grouped on the left of Figure 1 correspond to the anti conformation, then the corresponding points for the favored gauche conformation would occur at about 14 cm-I lesser wave number, at about 495 cm-l, even further off the line of Figure 1. The points in Figure 1 fall into three groups depending upon the disulfide dihedral angle. For values near 90" the cluster of nine points on the left of the figure is obtained. More points that fall within this cluster could be obtained by a more extensive survey of the literature. The single point at 289 nm is for trans-2,3-dithiadecalin where the dihedral angle is near 60". Finally the last three points near 330 nm are for five-membered ring disulfides where the disulfide dihedral angle is about 30". It is evident from Figure 1 that the proposed3 linear relationship between disulfide stretching frequency and the ultraviolet absorption maximum or dihedral angle is not verified by the experimental points. Indeed, only a single point is acceptably near the proposed line. With limited results on a single phase of compounds with small diheThe Journal of Physical Chemistry, Vol. 78.No. 8, 1974
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dral angles, it is uncertain whether the experimental points in Figure 1 deviate significantly from a horizontal line drawn at about 509 cm-I. It is possible that a line with a small negative slope (not shown) will be indicated when more results are available. However such a small slope, if it does occur, will be of no utility in estimation of disulfide dihedral angles from disulfide stretching wave numbers. Moreover, any correlation is of limited value if it depends upon prior knowledge of gauche from anti conformations about the CC-SS bond. A further consequence of the linear relationship proposed in ref 3 is that the dihedral angle in crystalline L-cystine with a gauche conformation about the CC-SS bond is about 40", less than that for the cyclic trans-decalin. Finally the authors of ref 3 propose a disulfide dihedral angle of but 6" for the cyclic pentapeptide malformin A on the basis of an absorption peak at about 365 nm.9 However this absorption peak was observed on crude malformin A that had been shaken in concentrated HCl for 2 days in order to induce thiazoline ring formation.1° In the same figure of ref 9 a peak also appears at about 365 nm for reduced malformin A and glutathione, neither of which contains a disulfide bond. Furthermore purified malformin A did not yield an absorption maximum at 365 nm under the same conditions.9 Purified malformin A does not show absorption above 350 nm and does show two strong CD bands at 235 and 280 nm.4 The intensity
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
Communications to the Editor and wavelength of these bands are not consistent with a dihedral angle near 10". They may be accommodated by a disulfide dihedral angle of 70-80" with M screw sense as proposed earlier4 or by a dihedral angle near 130" for a P helix.11 References and Notes (1) E. J. Bastian, Jr., and R. B. Martin, J. Phys. Chem., 77, 1129 (1973). (2) R. C. Lord and N. T. Yu. J. Mol. Bioi., 50, 509 (1970). (3) H. E. VanWart, A. Lewis, H. A. Scheraga, and F. D. Saeva, R O C . Nat. Acad. Sci. U.S., 70, 2619 (1973). (4) J. P. Casey and R. B. Martin, J. Amer. Chem. SOC., 94, 6141 (1972);J. P. Casey, Ph.D. dissertation, University of Virginia, 1968. (5) J. P. Casey, E. J. Bastian. Jr., and R. B. Martin, unpublished observations.
(6) G. Bergson, G. Claeson. and L. Schotte, Acta Chem. Scand., 16, 1159 (1962). (7) D. W. Scott and M. 2 . El-Sabban, J. Mol. Spectrosc., 31, 362 (1969). (8) H. Sugeta, A. Go, and T. Miyazawa, Chem. Lett., Chem. SOC.Jap.,
83 (1972); Boll. Chem. SOC.Jap., 46, 3407 (1973). (9) S.Marumo and R. W. Curtis, Phytochemistry, 1, 245 (1961). (10) R. 8. Martin, S. Lowey, E. L. Elson, and J. T. Edsall, J. Amer. Chem. Soc., 81, 5089 (1959);R. 8 . Martin and J. T. Edsall, Boll. SOC.Chim. Bioi., 40, 1763 (1958). (11) M. Ptak. Biopolymers, 12, 1575 (1973). Chemistry Department University of Virginia Charlottesville. Virginia 2290 7 Received December 17, 1973
R. Bruce Martin
