Vibrational spectroscopic studies of molecules with biochemical interest: The cysteine zwitterion

János Mink, László Hajba, Judith Mihály, Csaba Németh, Marcell Pálmai, Magnus Sandström

Research output: Contribution to journalArticle

8 Citations (Scopus)

Abstract

The L-cysteine zwitterions in the orthorhombic crystal structure and in aqueous solution, including the deuterated isotopologues HSCD 2CH(NH3+)COO-, DSCH 2CH(ND3+)COO-, and DSCD 2CH(ND3+)COO-, have been studied by mid-infrared, far-infrared, and Raman spectroscopy. Density functional theory (DFT) calculations were performed for an equilibrium molecular geometry of the cysteine zwitterion to obtain vibrational frequencies of fundamental modes, infrared (IR) and Raman intensities, and the depolarization ratio of the Raman bands and combined with normal coordinate force field analyses. The force field obtained for dissolved (in H2O and D2O) cysteine, based on the 4 × 36 experimental fundamental modes of the four isotopologues, was successfully transferred to the two conformers in the solid state. The experimentally observed multiple bands (generally doublets) of L-cysteine and its deuterated isotopologues in the solid state were interpreted based on the coexistence of two conformers in the unit cell. The calculated frequencies were used for full assignments of the fundamental IR and Raman vibrational transitions, including an attempt to interpret all low-frequency vibrations (below 400 cm-1) of the zwitterion also in the solid state. In particular, the hydrogen bonding effects on conformation, bond lengths, and force constants were studied, including those of the distorted NH 3+ amino group. The-S-H and-S-D stretching vibrations were found to be local modes, not sensitive to deuterium substitution of the-CH 2 and-NH3+ groups in the molecule or to the H(D)-S-C-C torsional angle. The two major-S-H or-S-D stretching bands observed in the solid state correspond to different S-H/D bond lengths and resulted in the force constants KSH = 3.618 N·cm-1 and 3.657 N·cm1 for the SH S and SH O hydrogen-bonded interactions. A remarkable result was that the S(H) O interaction was weaker than the S(H) S interaction in the solid state and even weaker in aqueous solution, K SH = 3.715 N·cm1, possibly due to intramolecular interactions between the thiol and amino groups. A general correlation between the S-H/D bond length and vibrational frequency was developed, allowing the bond length to be estimated for sulfhydryl groups in, for example, proteins. The C-S stretching modes were fitted with different C-S stretching force constants, KCS = 3.213 and 2.713 N·cm1, consistent with the different CS bond lengths for the two solid-state conformers.

Original languageEnglish
Pages (from-to)415-483
Number of pages69
JournalApplied Spectroscopy Reviews
Volume47
Issue number6
DOIs
Publication statusPublished - Aug 1 2012

Keywords

  • Cysteine
  • DFT
  • Raman
  • conformation
  • force constants
  • force field
  • hydrogen bond
  • infrared
  • sulfhydryl groups
  • vibrational spectra
  • zwitterion

ASJC Scopus subject areas

  • Instrumentation
  • Spectroscopy

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