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Agreement with the Disulfide Stretching Frequency-Conformation Correlation of Sugeta, Go, and Miyazawa (Raman Spectroscopy) HAROLD E

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Agreement with the Disulfide Stretching Frequency-Conformation Correlation of Sugeta, Go, and Miyazawa (Raman Spectroscopy) HAROLD E Proc. Natl. Acad. Sci. USA Vol. 83, pp. 3064-3067, May 1986 Chemistry Agreement with the disulfide stretching frequency-conformation correlation of Sugeta, Go, and Miyazawa (Raman spectroscopy) HAROLD E. VAN WART* AND HAROLD A. SCHERAGAt *Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, FL 32306-3015; and tBaker Laboratory of Chemistry, Cornell University, Ithaca, NY 14853-1301 Contributed by Harold A. Scheraga, December 16, 1985 ABSTRACT Two hypotheses have been advanced to ex- about the two C-S bonds of the CCSSCC moiety were plain the conformational dependence ofthe disulfide stretching assigned S-S stretching frequencies of approximately 510, frequency of the CCSSCC moiety in unstrained disulfides. 525, and 540 cm-l, respectively. Such compounds can adopt conformations that exhibit At about the same time, we had been investigating the disulfide stretching bands near 510, 525, and 540 cm-'. Sugeta effects of rotation about the C-S and S-S bonds of the et al. [Sugeta, H., Go, A. & Miyazawa, T. (1973) Bull. Chem. CCSSCC moiety on its S-S stretching frequency by studying Soc. Jpn. 46, 3407-3411] have attributed these three bands to the Raman spectra of a series of crystalline disulfides whose conformations having none, one, and two trans conformations, structures had been determined by x-ray diffraction tech- respectively, about the C-S bonds. In apparent contradiction to niques (7-10). Unstrained disulfides with GG and G'G' this correlation, Van Wart and Scheraga [Van Wart, H. E. & conformations were found to exhibit S-S stretching bands Scheraga, H. A. (1976) J. Phys. Chem. 80, 1812-1823] found near 508 ± 5 cm-' (7), in agreement with the proposal of that dithioglycolic acid crystals, believed at the time to exist Sugeta et al. (3, 4). However, a commercial sample of only in the trans conformation about both C-S bonds, exhibited dithioglycolic acid, believed from the only crystallographic the S-S stretching band near 510 cm-'. On the basis of this data available at the time to have the TT conformation (R. observation, it was suggested instead that the 510, 525, and 540 Parthasarathy, personal communication), also exhibited the cm 1 bands arose from CCSSCC moieties having none, one, S-S stretch at 508 cm-', in apparent contradiction to the and two A conformations (i.e., those with CC-SS dihedral correlation of Sugeta et al. It was subsequently suggested angles close to 30°), respectively, about the C-S bonds. It has from other experimental and theoretical data (11, 12) that a recently been shown by Nash et al. [Nash, C. P., Olmstead, non-alkane-like rotamer with a low CC-SS dihedral angle, M. M., Weiss-Lopez, B., Musker, W. K., Ramasubbu, M. & termed the A conformation, rather than the trans conforma- Parthasarathy, R. (1985) J. Am. Chem. Soc. 107, 7194-7195] tion might be the "missing" rotamer responsible for the that dithioglycolic acid can crystallize in two different forms, higher frequency S-S stretching bands. one with trans and the other with gauche conformations about Recently, Nash et al. (18) have discovered that dithiogly- both C-S bonds, and that these have S-S stretching bands at 536 colic acid crystallizes in two distinct forms, one with GG and and 510 cm-', respectively. These results are confirmed here the other with TT conformations about the C-S bonds. These and it is shown that our earlier data were collected on the authors have also found that the TT and GG forms exhibit the all-gauche (rather than the all-trans) form. Thus, the correla- S-S stretch at 536 and 510 cm-', respectively. This has raised tion proposed by Sugeta et al. is correct. the question as to which form was examined by us earlier (7). It is shown here that Raman data collected on crystals known When the first laser-excited Raman spectra of proteins were to have the TT conformation indeed exhibit the S-S stretching obtained approximately 15 years ago (1, 2), disulfide stretch- band near 540 cm-', in agreement with the data ofNash et al. ing bands attributable to cystine side chains were prominent- (18) and the correlation proposed by Sugeta et al. (3, 4). ly observed. These bands exhibited frequencies in the range Apparently, as will be shown here, the commercial sample of of 480 to 540 cm-1 and attention was soon directed-toward dithioglycolic acid that we examined earlier must have had understanding how the conformation4: of the CCSSCC frag- the GG conformation. ment of cystine and structurally related aliphatic disulfides influenced the observed frequencies. Sugeta, Go, and 4:The conformation of the CCSSCC unit is described by the CS-SC Miyazawa (3, 4) reported that methyl t-butyl disulfide and and the two CC-SS dihedral angles. Rotation about the S-S bond is di-t-butyl disulfide exhibit single S-S stretching bands near hindered, and unstrained disulfides invariably have CS-SC dihedral 525 and 540 cm-1, respectively. They concluded that the angles with absolute values of 85 ± 200. Because of this hindrance and the C2 symmetry of the CSSC group, all unstrained disulfides presence of a methyl group trans to the distal S atom about exist as enantiomers with CS-SC dihedral angles of +85° and -85°. each C-S bond raises the S-S stretching frequency by 15 With regard to the rotamers formed on rotation about the C-S bonds cm-. In contrast, unstrained primary aliphatic disulfides of the CCSSCC unit, we define the C (for cis), A, G (for gauche), [i.e., those in which the carbon atoms adjacent to the B, and T (for trans) conformations as those having CC-SS dihedral disulfide bond are primary (-CH2-) carbon atoms] exhibit S-S angles of approximately 00, ±30°, ±600, ±900, and 1800, respective- at 510 and 525 cm-1 due to the ly. It should be noted that, for a given screw sense of the disulfide stretching bands both bond, conformations of the CCSSCC unit with equal and opposite coexistence of rotational isomers about the C-S bonds (3-7). values of the CC-SS dihedral angle are nonequivalent and not Assuming that rotation about the C-S bond produced only necessarily isoenergetic. For example, when the CS-SC dihedral gauche (G) and trans (T) rotamers, Sugeta et al. (3, 4) angle is +85°, the A, G, and B rotamers having negative CC-SS proposed a frequency-conformation correlation for primary dihedral angles may have higher energies than those having positive disulfides in which the GG, GT or TG, and TT conformations values because of steric repulsions between CH groups on opposite sides ofthe S-S bond. The higher energy rotamers will be designated as the A', G', and B' rotamers. Thus, the G and G' rotamers have The publication costs of this article were defrayed in part by page charge CC-SS dihedral angles of +600 and -60°, respectively, when the payment. This article must therefore be hereby marked "advertisement" CS-SC dihedral angle is +85°, and CC-SS dihedral angles of -60° in accordance with 18 U.S.C. §1734 solely to indicate this fact. and +600, respectively, when the CS-SC dihedral angle is -85°. 3064 Downloaded by guest on September 27, 2021 Chemistry: Van Wart and Scheraga Proc. Natl. Acad. Sci. USA 83 (1986) 3065 MATERIALS AND METHODS Instead of using a commercial sample, as we did previously (7), we have obtained Raman spectra of crystals of dithioglycolic acid kindly provided by Dr. Parthasarathy of the Roswell Park Memorial Institute. These are from the same batch on which the crystallographic structure determi- nation was carried out and are known to have the TT conformation. Raman spectra were recorded for both finely ground crystalline and solution-phase samples held in capil- lary tubes. A Spectra-Physics model 171-18 argon ion laser was used for excitation and a Spex model 1403 spectrometer was used to obtain the spectra. Data were stored in digital form using a Spex Datamate and the spectra were smoothed by the method of Savitzsky and Golay (13). RESULTS To resolve the question as to which form of dithioglycolic acid had been examined earlier (7), the Raman spectrum of crystals from the batch used for the structural determination ofthe TT form was obtained. The spectrum ofpolycrystalline TT dithioglycolic acid exhibits a clearly defined S-S stretch- ing band at 533 cm-1 (Fig. 1, spectrum A), in reasonable agreement with the frequency of 540 cm-' predicted by the correlation of Sugeta et al. (3, 4) and the value of 536 cm-' found by Nash et al. (18). A single C-S stretching band is C~~~~~~~~ observed at 650 cm-' and an unidentified band is observed at 560 cm-'. On dissolution in either 1 M HCl or 1 M NaOH, 0)~ ~ ~ 6 there is a strong S-S stretching band at 506 cm-' with a shoulder at about 517 cm-' (Fig. 1, spectrum B). When 533 dithioglycolic acid is recovered from HCl solution by rapid rotary evaporation, the resulting polycrystalline material 506 exhibits S-S stretching bands at 506 and 533 cm-', C-S stretching bands at 649 and 661 cm-', and also bands at 560 and 578 cm-' (Fig. 1, spectrum C). Finally, if the crystals of the TT form of dithioglycolic acid are dissolved in methanol and recovered by rotary evaporation, the material obtained exhibits single S-S and C-S stretching bands at 506 cm-' and 662 cm-', respectively, and a band of unknown origin at 579 cm-' (Fig. 1, spectrum D). Several conclusions can be drawn from the data in Fig.
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