Disulfide Bond in Diethyl Disulfide: A Rotational Spectroscopic Study

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Disulfide Bond in Diethyl Disulfide: A Rotational Spectroscopic Study Jiaqi Zhang, Xiaolong Li, Qian Gou, and Gang Feng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03029 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

Disulfide Bond in Diethyl Disulfide: A Rotational Spectroscopic Study

Jiaqi Zhang,† Xiaolong Li,† Qian Gou, †,‡ Gang Feng * †,‡ †

Department of Chemistry, School of Chemistry and Chemical Engineering,

Chongqing University, Daxuecheng South Rd. 55, 401331, Chongqing, China. ‡

Collaborative Innovation Center for Brain Science, Chongqing University, No.174

Shazhengjie, Shapingba, Chongqing, 400044, China.

AUTHOR INFORMATION Corresponding Author * Gang Feng, Email: [email protected]

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ABSTRACT Diethyl disulfide was investigated by pulsed jet Fourier transform microwave spectroscopy. The spectroscopic study was complemented by ab initio calculations. The first two most stable conformers predicted at MP2/6-311++G(d,p) level of theory were observed in the supersonic expansion. Two (GGG) and two

34

13

C and one

34

S isotopologues for the most stable conformer

S isotopes for the second most stable form (GGG') were also assigned in

natural abundance respectively, allowing a thorough investigation of the molecular structure and conformation. The relative population ratio of the two conformers in the supersonic expansion was estimated to be about 4 : 1.

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INTRODUCTION Disulfide bonds formed between the sulfur atoms of the cysteine residues are important structural elements in determining the tertiary structure of proteins and peptides. They are responsible for the bioactivity of many proteins and peptides.1-2 In this context, dialkyl disulfides serve as model compounds for probing the conformational behavior of the R-C-S-S-C-R' skeleton. The conformations of dialkyl disulfide are mainly determined by three dihedral angles τ(R-C-S-S), τ(C-S-S-C) and τ(S-S-C-R'). Each dihedral angle can be gauche corresponding to the torsion angles of about 60° (G) and -60° (G') or trans corresponding to torsion angle of 180° (T). Experimental3-9 and theoretical10-13 efforts have been devoted to investigate the conformational preferences and structures of dialkyl disulfides. Vibrational spectroscopy,3-4 electron diffraction,5 and rotational spectroscopy3-4, 9 have been applied to investigate the structures and conformations of dimethyl disulfide, respectively. Experimentally, only the G orientation of C-S-S-C group could be found for dimethyl disulfide, methyl tert-butyl disulfide, and di-tert-butyl disulfide.9 For ethyl methyl disulfide, the conformer with G oriented C-S-S-C and C-C-S-S structure is about 0.9 kcal mol-1 lower in energy than the conformer with G oriented C-S-S-C and T oriented C-C-S-S structures.5 These investigations have demonstrated that the C-S-S-C group in the most stable structure of disulfides prefers G orientation with a dihedral angle nearly 90°. Multiple configurations arise for diethyl disulfide due to the possible orientations of the methyl groups with respect to the S-S bond as revealed by theoretical calculations.12, 13 In the solid state, the configuration with all C-C-S-S, C-S-S-C and S-S-C-C G oriented (GGG) has 3



been detected by Raman spectroscopy. 9 The conformer with T oriented C-C-S-S and S-S-C-C group and G of C-S-S-C (TGT) has been also suggested to exist in the liquid state.9 Precise experimental information on the conformational preference and molecular structures in the gas phase supplied by rotational spectroscopy can lead to a better understanding of disulfide bond. Herein, the rotational spectra of diethyl disulfide were investigated by pulsed-jet Fourier transform microwave (FTMW) spectroscopy combined with ab initio calculations. The conformational equilibria and geometries in this molecule are discussed in detail.

EXPERIMENTAL SECTION The rotational spectrum was measured by using a coaxially oriented beam-resonator arrangement (COBRA)14-15 type supersonic-jet FTMW spectrometer16 at Chongqing University.17 The spectrometer covers 2-20 GHz frequency region and is operated with the FTMW++ set of program. The instrumental resolution is better than 6 kHz and the accuracy of the frequency measurements is better than 3 kHz. Diethyl disulfide (98%) was obtained commercially and used without further purification. Helium was used as the carrier gas. In general, helium with a backing pressure of 2 bar was passed over a reservoir filled with diethyl disulfide and heated up to 65℃ and then expanded through a solenoid value (Parker-General Value, Series 9, nozzle diameter 0.5 mm) into the Fabry Perot cavity for generating the jet. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition appears as a doublet due to the Doppler effect. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. 4


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RESULTS AND DISCUSSION THEORETICAL CALCULATIONS Since previously published calculations14-15 provided no spectroscopic parameters (e.g., rotational constants) needed for the rotational spectroscopic investigation, additional calculations were performed. The dihedral angles τ(C-C-S-S), τ(C-S-S-C) and τ(S-S-C-C) dominate the conformational behavior of diethyl disulfide, each of which can be G, G', or T oriented. All the possible geometries were full optimized by applying the Second-order Møller–Plesset perturbation theory (MP2) with the 6-311++G(d,p) basis set, yielding six conformers within 10 kJmol-1. Harmonic frequency analysis at the same level of theory proved them to be real minima and provided the zero-point vibrational energies (E0). The geometries and relative energies (∆E, ∆E0 in kJmol-1) of the six conformers are shown in Figure 1. The rotational constants and dipole moment components of these conformers are reported in Table 1. As shown in Figure 1, the most stable conformer has the C-C-S-S-C-C skeleton all G oriented (GGG) with the dihedral angles τ(C-C-S-S), τ(C-S-S-C) and τ(S-S-C-C) of 70°, 86° and 70°, respectively. The second most stable conformer lies 2.0 kJmol-1 higher in energy than the global minimum which has GGG' form with respective dihedral angles of 69°, 98° and -66°. GGT form has the respective dihedral angels of 69°, 85° and 180° which is 2.3 kJmol-1 higher in energy than GGG. For G'GT form, the dihedral angles are calculated to be -64°, 96° and 180°, respectively. TGT form has T oriented C-C-S-S and S-S-C-C with dihedral angles of ~ 180° and τ(C-S-S-C) of 84°. Conformer with G' oriented C-C-S-S and S-S-C-C lies in the highest energy (8.8 kJmol-1 ), in which the dihedral angles are calculated to be -71°, 112° and

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-71° respectively. All the calculations were performed using Gaussian09 program package.18

ROTATIONAL SPECTRA According to the ab initio results, the search for rotational transitions was initially attempted to the c–type transitions of conformer GGG. The first identified lines were assigned to the 321 ← 211 and 322 ← 212 transitions. Then the measurements were extended to other c– type R branch transitions with Jupper = 1-10 and some Q-branch lines. No a– or b–type lines were observed which is in line with the theoretical values of zero of the dipole moment components along the a and b axes. Once the transitions of conformer GGG were removed from the spectrum, a number of less intense transitions were easily assigned to conformer GGG'. In total, 35 b-type transitions with Jupper = 1 - 9 and Ka = 0 - 5 as well as 11 a-type R- branch transitions were measured. No c-type lines were observed, which might be due to the small value of the c dipole moment component of this conformer (~ 0.1 D). A survey scan for conformer GGG' is shown in Figure 2, where also lines belong to the

13

C and

34

S isotopologues of conformer GGG appear. The

atomic numbering, principle axes of the inertia of conformers GGG and GGG' are present in Figure 3. The measured lines were fitted to the Watson’s S-reduced semirigid rotor Hamiltonian with the Ir representation19 using Pickett’s SPFIT program.20 The obtained spectroscopic constants are reported in Table 2 for conformers GGG and GGG'. In addition, the rotational spectra of mono isotopic substituted species were also measured in natural abundance. For conformer GGG, two 13C and one 34S isotopologues were assigned. The C2 symmetry doubles the line intensities. For conformer GGG', only two 6


34

S

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isotopologues were measured because of the weakness of the signals. Because only a smaller number of lines could be measured, the values of the centrifugal distortion constants were fixed at the corresponding values of the parent species, respectively. The obtained results are also included in Table 2. Searching for the rotational spectrum of other conformers were also performed, in particular for that of conformer GGT which is slightly higher in energy than conformer GGG'. However, no rotational transitions belong to these conformers could be identified. This is mostly due to conformational relaxation or low population in the supersonic expansion. In order to shed light on the possible conformational relaxation, a potential energy curve connecting conformers of GGG, GGG' and GGT were calculated at MP2/6-311++G(d,p) level. As shown in Figure 4, the barrier for interconverting conformer GGT to GGG is ~ 7 kJ mol-1, approaching the value of 2kT (~ 5.6 kJ mol-1 at 338 K in our case), indicating conformational relaxation could take place in the pulsed jet.21 However, the barrier for interconverting conformer GGG' to GGG is about 15 kJ mol-1, much higher than the value allowing for such kind of conformational relaxation. These are why conformer GGG' were detected in the jet rather than conformer GGT. The frequencies of all the measured lines are given in Supporting Information.

CONFORMATIONAL INFORMATION The conformational assignments of conformers GGG and GGG' were straightforward by comparing the experimental rotational constants (Table 2) to those of the theoretical ones (Table 1). The relative intensity measurements of several close-lying c-type and b-type transitions allowed estimating the relative population of the two observed conformers in the 7



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supersonic expansion plume. As outlined previously, although the supersonic expansion cannot reach the thermodynamic equilibrium, the relative population of the conformers could be estimated by:22 N1/N2 = (I1µ2γ2ν22ω2)/(I2µ1γ1ν12ω1),

(1)

where I, µ, γ, and ν denote the peak intensity, value of dipole moment component, line strength, transition frequency, and ω the conformational degeneracy (2 for both conformers), respectively. The relative population of the conformers GGG and GGG' is estimated to be NGGG/NGGG' ≈ 4/1. This ratio of population corresponds to a free energy difference (∆G) of 3.4(6) kJmol-1, which is consistent with the free energy difference (2.6 kJmol-1) predicted at the MP2/6-311++G(d, p) level of theory.

MOLECULAR STRUCTURE The Kraitchman method23 was used to calculate the substitution coordinates of the heavy atoms. For conformer GGG, the full substitution coordinates were obtained by taking the C2 symmetry into account, allowing full structure determination of the molecular skeleton. The obtained coordinates were used to calculate the bond lengths, valence angles and dihedral angles of the skeleton. The rs structure of GGG was reported in Table 3 and compared to the equilibrium structure (re) from the MP2/6-311++G (d,p) calculations. For conformer GGG', the substituted coordinates of two sulfur atoms were calculated which results in an S-S bond length of 2.029(3) Å, almost identical to that of conformer GGG (2.026(3) Å). The rs coordinates are in accordance with the ab initio values. In addition, the r0 structure26 for both of the observed conformers was calculated by adjusting the bond lengths, valence angles and dihedral angles to minimize the differences 8


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between the ab initio rotational constants and the experimental ones. The fitted parameters complemented with the ab initio geometries of conformers GGG and GGG' are reported in Table 3. The structural parameters concerning the S-S bond for several disulfide compounds are summarized as comparison in Table 4. The dimethyl substitution induces shortening of the S-S bond length of 0.03 Å whereas the S-S bond length almost unaltered in going from dimethyl substitution to that of diethyl substitution. The valence angles α(S-S-R) of CH3SSCH3 and CH3CH2SSCH2CH3 are about 5° larger than that of HSSH while their dihedral angles are slightly smaller than HSSH. The fluorination of the two hydrogen atoms result in a very short S-S bond length (1.888 Å) and large valence angels α(S-S-Cl) (~ 108°). The S-S bond length of ClSSCl is about 0.06 Å longer than that of FSSF whereas the valence angles α(S-S-Cl) is comparable to the valence angles α(S-S-F). These geometric parameters of ClSSCl are close to those of CH3OSSOCH3 determined by electron diffraction.8

CONCLUSIONS Rotational spectra of diethyl disulfide were measured and assigned by pulsed-jet FTMW spectroscopy. Two conformers were detected in the supersonic expansion. Experimental results and ab initio calculations proved that the global minimum adopts the GGG orientation of the C-C-S-S-C-C skeleton while the second stable conformation is GGG'. The frame structure of the two conformers were determined thoroughly. The potential energy curve connecting conformers GGG, GGG' and GGT calculated at MP2/6-311++G(d,p) level suggests the possible relaxation of the GGT conformer to GGG.

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ASSOCIATED CONTENT Supporting Information Experimental transition frequencies (TABLEs S1-S2); MP2/6-311++G(d,p) calculated geometries of the six conformers (TABLE S3). This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by the Foundation of 100 Young Chongqing University, the Fundamental

Research

Funds

for

the

Central

Universities

(Projects

No.

106112017CDJQJ228807 and 10611CDJXZ238826), National Natural Science Foundation of China (21703021) and Fundamental and Frontier Research Fund of Chongqing (Grant No. cstc2017jcyjA1503).

REFERENCES 1. Gori, A.; Gagni, P.; Rinaldi, S. Disulfide Bond Mimetics: Strategies and Challenges. Chem.-Eur. J. 2017, 23, 14987-14995. 2. Hogg, P. J. Disulfide Bonds as Switches for Protein Function. Trends. Biochem. Sci. 2003, 28, 210-214. 3. Meyer, M. Infrared, Raman, Microwave and Ab Initio Study of Dimethyl Disulfide Structure and Force-Field. J. Mol. Struct. 1992, 273, 99-121. 4. Winnewisser, G.; Yamada, K. M. T. Millimeter, Submillimeter and Infrared-Spectra of Disulphane (HSSH) and Its Isotopic-Species. Vib. Spectrosc. 1991, 1, 263-272. 5. Yokozeki, A.; Bauer, S. H. Structures of Dimethyl Disulfide and Methyl Ethyl Disulfide, Determined by Gas-Phase Electron-Diffraction - Vibrational Analysis for Mean-Square Amplitudes. J. Phys. Chem. 1976, 80, 618-625.

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6. Davis, R. W.; Firth, S. The Microwave-Spectrum of the Chain Isomer of Disulfur Difluoride - FS-SF. J. Mol. Spectrosc. 1991, 145, 225-235. 7. Hartwig, H.; Kretschmer, U.; Dreizler, H. The S-33 Nuclear-Quadrupole Hyperfine-Structure in the Rotational Spectrum of S-32, S-33 Dimethyl Disulfide. Z. Naturfors. Sect. A-J. Phys. Sci. 1995, 50, 131-136. 8. Steudel, R.; Schmidt, H.; Baumeister, E.; Oberhammer, H.; Koritsanszky, T. Sulfur Compounds. 178. Gas-Phase Structure and Vibrational-Spectra of Dimethoxydisulfane, (CH3O)2S2. J. Phys. Chem. 1995, 99, 8987-8993. 9. Sugeta, H.; Go, A.; Miyazawa, T. Vibrational Spectra and Molecular Conformations of Dialkyl Disulfides. B. Chem. Soc. Jpn. 1973, 46, 3407-3411. 10. Zhao, W.; Bandekar, J.; Krimm, S. Vibrational Studies of the Disulfide Group in Proteins. J. Mol. Struct. 1990, 238, 43-54. 11. Gorbitz, C. H. Conformational Properties of Disulfide Bridges. 1. C-S Rotational Potential in Ethyl Hydrodisulphide. J. Phys. Org. Chem. 1993, 6, 615-620. 12. Gorbitz, C. H. Conformational Properties of Disulfide Bridges. 2. Rotational Potentials of Diethyl Disulfide. J. Phys. Org. Chem. 1994, 7, 259-267. 13. Ackermann, K. R.; Koster, J.; Schlucker, S. Conformations and Vibrational Properties of Disulfide Bridges: Potential Energy Distribution in the Model System Diethyl Disulfide. Chem. Phys. 2009, 355, 81-84. 14. Grabow, J.-U.; Stahl, W. A Pulsed Molecular Beam Microwave Fourier Transform Spectrometer with Parallel Molecular Beam and Resonator Axes. Z. Naturforsch. 1990, 45a, 1043-1044. 15. Grabow, J.-U.; Stahl, W.; Dreizler, H. A Multioctave Coaxially Oriented Beam-resonator Arrangement Fourier-transform Microwave Spectrometer. Rev. Sci. Instrum. 1996, 67, 4072-4084. 16. Balle, T. J. Fabry–Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33. 17. Grabow, J.-U.; Gou, Q.; Feng, G. 72nd International Symposium on Molecular Spectroscopy, TH03, Champaign-Urbana, 2017. 18. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, revision A.01; 2009, Gaussian, Inc.: Wallingford, CT, 2009. 19. Watson, J. K. G. In Vibrational Spectra and Structure, Durig, J. R ed.; Elsevier: New York/Amsterdam, 1977; Vol. 6, 1-89. 20. Pickett, H. M. The Fitting and Prediction of Vibration-Rotation Spectra with Spin Interactions. J. Mol. Spectrosc. 1991, 148, 371-377. 21. Ruoff, R. S.; Klots, T. D.; Emilsson, T.; Gutowsky, H. S. Relaxation of Conformers and 11



Isomers in Seeded Supersonic Jets of Inert Gases. J. Chem. Phys. 1990, 93, 3142-3150. 22. Caminati, W.; Grabow, J.-U. Microwave Spectroscopy: Molecular Systems. In Frontiers of Molecular Spectroscopy; Laane, J., Ed.; Elsevier: Amsterdam, The Netherlands, 2009; Chapter 15, pp 455-552. 23. Kraitchman, J. Determination of Molecular Structure from Microwave Spectroscopic Data. Am. J. Phys. 1953, 21, 17-24. 24. Kisiel, Z. Least-squares Mass-dependence Molecular Structures for Selected Weakly Bound Intermolecular Clusters. J. Mol. Spectrosc. 2003, 218, 58-67. 25. Marsden, C. J.; Brown, R. D.; Godfrey, P. D. Microwave Spectrum and Molecular Structure of Disulphur Dichloride, S2Cl2. J. Chem. Soc. Chem. Commun. 1979, 0, 399-401.

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Figure Captions:

FIG. 1. Geometries and relative stability (∆E, ∆E0 in kJmol-1) of the six conformers of diethyl disulfide calculated at MP2/6-311++G(d,p) level of theory.

FIG. 2. Survey scan of the conformer GGG' where transitions belong to the isotopologues of conformer GGG also appear.

34

S and

13

C

FIG. 3. Molecular sketch, atom numbering and the principal axes of inertia of conformers GGG and GGG'.

FIG. 4. Potential energy curve relating to the interconversion of different conformations of diethyl disulfide calculated at the MP2/6-311++G(d,p) level.

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TABLE 1. Spectroscopic parameters of the six conformers of diethyl disulfide calculated at the MP2/6-311++G(d, p) level. A /MHz B /MHz C /MHz µa /D µb /D µc /D

GGG

GGG'

GGT

G'GT

TGT

G'GG'

4227 1271 1194 0.0 0.0 2.3

3358 1424 1309 0.5 2.2 0.1

4197 1189 1104 0.3 1.6 1.8

3688 1352 1121 0.0 2.3 0.4

5322 1046 962 0.0 2.5 0.0

2901 1684 1276 0.0 2.1 0.0

TABLE 2. Experimental spectroscopic parameters of the conformers GGG and GGG'. GGG'

GGG Normal

a

13 b

C3

13

C5

34

S1

Normal a

34

S3

34

S4

A /MHz

4294.4281(9)

4274.0912(8)

4266.5377(9)

4225.0068(6)

3354.0487(6)

3307.3170(5)

3307.0247(5)

B /MHz

1255.5735(4)

1231.7971(4)

1243.7079(4)

1251.8981(3)

1420.9921(3)

1411.7538(3)

1414.3998(3)

C /MHz 1182.0645(3)

1160.223(1)

1172.434(1)

1176.4358(7)

1295.9088(3)

1290.6197(2)

1288.2446(2)

Nc

33

12

13

13

46

21

21

d

0.6

1.1

2.5

2.1

2.2

3.4

4.7

σ /kHz a

For the parent species, the quartic centrifugal distortion constants have been determined to be DJ = 0.496(3), DK = 0.0262(1), DJK = -5.68(2), d1 = 0.009(1), d2 = 0.0032(9) kHz respectively for GGG and DJ = 0.607(2), DK = 7.47(2), DJK = -2.33(1), d1 = -0.077(2), d2 = 0.0081(7) kHz for GGG'. These parameters have been fixed in the fits for the 13C and 34S isotopologues. b Errors in parenthesis are expressed in units of the last digit. c Number of lines in the fit. d Root-mean square error of the fit.

14


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TABLE 3. Theoretical (re, MP2/6-311++G(d, p)) and experimental (rs and r0) structural parameters of conformers GGG and GGG'. re

rs

r0

2.026(3)a 1.827(3) 1.517(3)

2.030(1) 1.823(3) 1.518(4)

102.6(1) 113.9(2)

102.9(2) 113.9(3)

67.1(2) 87.6(1)

67.1(3) 87.4(1)

2.029(3)

2.029(1) 1.825(1) 1.828(1)

GGG Bond lengths/Å SS 2.070 SC 1.816 CC 1.522 Valence angles/° SSC 100.7 SCC 113.8 Dihedral angles/° SSCC 69.7 CSSC 86.2 GGG' Bond lengths/Å S4S3 2.070 S3C2 1.818 C5S4 1.816 Valence angles/° C1C2S3 113.4 C2S3S4 101.7 S3S4C5 101.4 S4C5C6 114.7 Dihedral angles/° C6C5S4S3 -65.9 S4S3C2C1 69.0 C5S4S3C2 97.7 a

104.1(1) 103.3(1) -65.8(2) 65.5(2) 96.7(1)

Errors in parenthesis are expressed in units of the last digit.

15



TABLE 4. Comparison of structural parameter of the disulfide bond.a Compounds

r(S-S) /Å

α(S-S-R) /°

τ(R-S-S-R) /°

Ref.

HSSH CH3SSCH3 CH3CH2SSCH2CH3 FSSF ClSSCl

2.0564 2.027(1) b 2.026(3) 1.888(10) 1.9504(12) 1.960(3)

97.88 102.80(8) 102.6(1) 108.3(5) 107.66(5) 108.2(3)

90.34 84.52(5) 87.6(1) 87.9 85.24(10) 91(4)

3 4 This work 6 25 8

CH3OSSOCH3 c a

rs structure from microwave spectroscopy. Errors in parenthesis are expressed in units of the last digit. c Determined by electron diffraction b

16


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FIG 1

GGG 0, 0

GGG' 1.6, 2.0

GGT 2.3, 2.3

G'GT 3.8, 4.2

TGT 4.9, 4.8

G'GG' 8.8, 8.8

17



FIG 2 541←532

GGG′

322←211

S 221←111

845←836

440←431 &441←432

GGG

744←735

13925

13935

13945

13955

C-3 220←110

542←533

13965

13

643←634

13

642←633

C-5 220←110

34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

13975

13985

Frequency/MHz

18


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FIG 3

GGG

GGG'

19



FIG. 4 20

12

7.0

8

15.1

-1

16 ∆E /kJmol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 0

GGT

GGG ' GGG -80

-40

0

40 80 120 τ (CCSS) /°

160

200

20


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TOC Graphic 43x25mm (300 x 300 DPI)


Disulfide Bond in Diethyl Disulfide: A Rotational Spectroscopic Study

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