Resonance Raman Optical Activity Shows Unusual Structural Sensitivity for Systems in Resonance with Multiple Excited States: Vitamin B₁₂ Case

Abstract

In this work, cobalamins with different upper axial substituents and a cobalamin derivative with a ring modification were studied using chiroptical spectroscopies, in particular resonance Raman optical activity (RROA), to shed light on the influence of structural modifications on RROA spectra in these strongly chiral systems in resonance with multiple excited states at 532 nm excitation. We have demonstrated that for these unique systems RROA possesses augmented structural specificity, surpassing resonance Raman spectroscopy and enabling at the same time measurement of cobalamins at fairy low concentrations of ∼10 −5 mol dm −3. The enhanced structural specificity of RROA is a result of bisignate spectra due to resonance via more than one electronic state. The observation of increased structural capability of RROA for cobalamins opens a new perspective for studying chiral properties of other biological systems incorporating d-metal ions.

Resonance Raman Optical Activity Spectroscopy in Probing Structural Changes Invisible to Circular Dichroism Spectroscopy: A Study on Truncated Vitamin B12 Derivatives

“…However, in the case of the third analog, (CN)(H 2 O)Cby(OMe) 7 ( Figure 1 ), one can find a few modest differences in the Raman profile, especially below 900 cm −1 ( Figure 4 ), that are related mostly to the replacement of the axial ligand CN by H 2 O. In general, consistent with the previous reports [ 32 , 41 ], RR spectra of cobalamins equipped with different axial ligands were strikingly similar, but at the same time, their electronic spectra (UV-Vis and ECD) differed significantly.…”

“…Another equally important spectral feature, close to the ν LA , was the short-axis polarized corrin ring motion: ν SA , located between 1543 and 1549 cm −1 and correlated with the C=C and C=N stretching along the shorter (Co-C10) corrin ring axis, and two other corrin macrocycle C=C stretching modes located above 1560 cm −1 [ 30 ]. The spectral region between 1300 and 1400 cm −1 can be generally assigned to the CH 3 and CH 2 bending vibrations, while the 1100 to 1300 cm −1 region to the CH 2 twisting and wagging, CH bending, as well as the corrin ligand C-C and C-N stretching motions [ 41 ]. Moreover, the low wavenumber region below 550 cm −1 was more sensitive to the replacement of an axial ligand attached to the cobalt ion, and it was dominated by the Co-C stretching vibrations located around 500 cm −1 , that was more intense for an alkyl substituted Co, as well as Co-C≡N bending or C≡N twisting modes, present for the analogs with the CN − axial ligand ( Table 1 ) [ 30 , 41 ].…”

“…The first striking feature of the RR spectrum of (CN)(H 2 O)Cby(OMe) 7 was a low-intensity band, centered at 1111 cm −1 , that was less resolved for the other analogs and was present on their spectra as a shoulder ( Figure 3 ). According to our recent study of cobalamins [ 41 ], that band is most likely involved in the corrin ring C-N stretching motions, but taking into consideration an additional seven -OMe groups in the (CN)(H 2 O)Cby(OMe) 7 structure, one can say that the spectral feature may also be assigned to the O-CH 3 vibrational stretching mode of CO 2 Me groups. Other spectral changes were observed between 900 and 500 cm −1 ( Figure 4 ).…”

“…Furthermore, the band located at ~637 cm −1 for (CN)Cbl, (CN) 2 Cbi and (CN) 2 Cbi-P downshifted to 620 cm −1 for (CN)(H 2 O)Cby(OMe) 7 and the intensity of the 584 cm −1 band increased in the order (CN)Cbl < (CN) 2 Cbi, (CN) 2 Cbi-P < (CN)(H 2 O)Cby(OMe) 7 . Last but not least, a shift was observed for the low-frequency band associated with the Co-C [ 30 ] or generally Co-L [ 41 ] stretching vibrations, from 518 and 484 cm −1 for (CN)Cbl or 519 cm −1 for (CN) 2 Cbi and (CN) 2 Cbi-P to 528 cm −1 for (CN)(H 2 O)Cby(OMe) 7 , which was due to the change of the axial ligand from Dmb or CN − to CN − or H 2 O.…”

“…We have previously shown that for variously modified cobalamin species, resonance Raman optical activity (RROA) spectroscopy is a method of enhanced structural sensitivity compared to other techniques, including resonance Raman (RR) spectroscopy, and at the same time preserves improved sensitivity as related to the ROA effect itself [ 41 ]. According to the single electronic state (SES) limit theory, the RROA spectrum is monosignate with a sign opposite to the ECD signal of the single resonant electronic state.…”