Visible and near‐ultraviolet resonance Raman spectra of photolabile vitamin B₁₂ derivatives with a rapid‐flow technique

Abstract

Resonance Raman spectra, obtained with 5145,4880 and 3638 excitation, are reported for aquo-, methyl-, 5'-deoxyadenosyl-(coenzyme B,,), thiosolfate and cysteinyl cobdamin, of methyl cobmamide, and of Co(XI) cobalamin (Blzr). Laser-induced photodecomposition was held to acceptable levels with a rapid-%ow technique. A rich assortment of corrin ring vibrations are observed above 600 an-', but the frequencies are constant (within *3cm-') for all the derivatives. Some Raman bands show differential enhancement by the visible and near-ultraviolet electronic transitions, reflecting their different polarizntions. B,, shows large relative intensity changes in bands at 1500 and 1600 cm-', witb respect to the Co(1II) derivatives. Despite an absorption spectrum resembling that of B,,, methyl cobmamide shows Raman intensity behavior typical of the other Co(II1) derivatives. INTRODUCIlONThere is much current interest in the chemistry and biochemistry of vitamin B12 and its derivatives.' All of these contain a cobalt atom coordinated to four nitrogen atoms of the corrin ring. One axial coordinate position is occupied by benzimidazole, bound covalently to the ring, although this can be removed chemically, e.g. in the cobinamides.2 The other axial position can be occupied by any of a number of ligands (sixth ligand). Coenzyme B12 contains a carbon-bound 5'-deoxyadenosyl ligand,3 and complexes with other carbon ligands can also be prepared.The corrins have strong absorption bands in the visible and near-u.v. regions, which are due to T-T* electronic transitions. As with the porphyrins, excitation in these bands produces strong resonance enhancement of Raman bands associated with corrin ring vibrations. Several resonance Raman (RR) studies of B12 derivatives have been rep~rted.~" These have shown very similar RR spectra for cobalamins, the Co(II1) B12 derivatives, regardless of the nature of the sixth ligand. Somewhat different spectra are observed for the reduced forms, B12r, which contains Co(I1) and BlZs, which contains Co(1).Wozniak and Spiro reported,6 however, that methyl cobinamide and 'base off methyl cobalamin, in which the bond between benzimidazole and cobalt is dissociated by protonation, both have spectra identical with B12r, rather than other Co(II1) derivatives. They suggested that the loss of benzimidazole might allow the corrin ring to relax to a conformation close to that of B12r. In retrospect, an alternate explanation is that the observed spectra were in fact due to Co(I1) species t Abbreviations: B,, = Vitamin B,, [cr-(5,6-dimethylbenzimidazolyl)cobamide]; BIZ, = Co(I1) Cobalamin; Coenzyme-B,, = 5'-deoxyadenosyl-B,,; Methyl-cobin = Methyl-cobinamide. @ Heyden & Son Ltd, 1977 generated by photolysis in the laser beam. Photolysis of B12 derivatives is particularly facile when they contain ~a r b o n ,~-~ and to a lesser extent sulfur, ligands, and under anaerobic conditions, the product is B12r. Detection of this artifact is made difficult by the similarity of the B12r and methyl cobinamide (Me-Cobin) or base-off ...

Resonance Raman spectroscopic study of the interaction between Co(II)rrinoids and the ATP:corrinoid adenosyltransferase PduO from Lactobacillus reuteri

“…The fact that a vibrational mode showing strong enhancement upon laser excitation into the “α-band” is sensitive to isotopic labeling at C10 implies that significant coupling occurs between corrin SA-stretching motion and an LA-polarized electronic transition. Although it had generally been assumed that the corrin-centered vibrational modes mainly involving SA-stretching motion should only couple to SA-polarized electronic transitions, 45,46 our recent DFT-based rR intensity calculations revealed that these modes can in fact also couple to LA-polarized electronic transitions. 48 …”

“…On the basis of these findings, the ~1500 and ~1550 cm −1 features were assigned as vibrational modes that primarily entail stretching motion of the conjugated C–C and C–N bonds oriented along the LA (the LA-stretching mode) and the SA (the SA-stretching mode), respectively. 45,46 In support of these assignments, the ~1550 cm −1 feature was shown to undergo a significant down-shift upon H/D exchange at C10, while the ~1500 cm −1 feature was largely unaffected by this isotopic substitution. 45 However, on the basis of a density functional theory (DFT)-assisted normal mode analysis for methylcobalamin (MeCbl), 47 Kozlowski and coworkers suggested that the frequency of the SA-stretching mode is in fact lower than that of the LA-stretching mode.…”

“…Originally, the two most strongly enhanced features in the rR spectra of Co(III)rrinoids were assigned on the basis of their rR enhancement patterns. 42–46 The lower-energy feature at ~1500 cm −1 was found to be predominantly enhanced upon laser excitation into the so-called α-band, which arises from the lowest-energy corrin π→π* transition that is polarized along the C5⋯C15 axis (the long axis, LA in Fig. 1), while the higher-energy feature at ~1550 cm −1 was preferentially enhanced upon laser excitation into the so-called γ-band, which arises from a corrin π→π* transition polarized along the Co⋯C10 axis (the short axis, SA in Fig.…”

Resonance Raman spectroscopy in biochemistry and biology

“…As for porphyrins many corrin ring modes are observed above 600 cm" 1 . However, the striking conclusion beginning to emerge from this work (Salama & Spiro, 1977) is that, for band positions, all derivatives give very similar RR spectra. Thus a change in axial ligand or cobalt oxidation state appears not to change the conformation of the corrin in the ground state.…”

DFT Analysis of Co−Alkyl and Co−Adenosyl Vibrational Modes in B12-Cofactors

“…12 Co-alkyl stretching vibrations have been detected by infrared and Raman spectroscopy, [13][14][15][16] and give an indirect indication of the Co-C bond strength. 17,18 Resonance Raman (RR) spectroscopy can provide access to B 12 -containing proteins, [19][20][21] since laser tuning can be used to selectively excite cobalamin vibrations.…”