Photolysis of methylcobalamin. Nature of the reactive excited state

Ascorbic acid assisted photodegradation of methylcobalamin using corrective irrelevant absorption spectrophotometric assay: A kinetic study

Photodegradation of drug substances leads to the formation of known and unknown degradation products. These unknown degradation products interfere and give erroneous results because of absorption on analytical wavelengths. This interference could be eliminated using the correction of irrelevant absorbancies. This study is based on the application of linear and non-linear correction of irrelevant absorption for the determination of methylcobalamin (MC) and hydroxocobalamin in the photolytic degradation assisted by ascorbic acid (AH2). MC follows first-order degradation kinetics and the rate of degradation (kobs) ranges from 1.99-2.34 × 10-2, min-1 at pH 2.0-12.0. The second-order rate constants (k2) for the photochemical interaction of MC and AH2 are in the range of 17.9-60.3 × 10-2 M-1, min-1 (acidic region) and 10.3-24.6 × 10-2 M-1, min-1 (alkaline region). The k2-pH profile was found to be bell-shaped and the maximum rate of degradation in the presence of AH2 is at pH 5.0 (60.3 × 10-2 M-1, min-1) due to the protonation of MC. However, in alkaline pH, the rate of photodegradation decreases due to the ionization form of AH2 which is AH- species.

Structural basis for unusually long wavelength charge transfer transitions in complexes [MCl(ECH(2)CH(2)NMe(2))(PR(3))] (E = Te, Se; M = Pt, Pd): experimental results and TD-DFT calculations

A series of new complexes, the blue compounds [PdCl(TeCH(2)CH(2)NMe(2))(PR(3))] (PR(3) = PEt(3), PPr(n)(3), PBu(n)(3), PMe(2)Ph, PMePh(2), PPh(3), PTol(3)) and the red [PtCl(TeCH(2)CH(2)NMe(2))(PR(3))] (PR(3) = PMe(2)Ph, PMePh(2)), were synthesized and studied spectroscopically ((1)H and (31)P NMR, UV/vis) and by cyclic voltammetry. The structures of [PdCl(TeCH(2)CH(2)NMe(2))(PPr(n)(3))] (2b) [PdCl(TeCH(2)CH(2)NMe(2))(PMePh(2))] (2e), [PtCl(TeCH(2)CH(2)NMe(2))(PMePh(2))] (2i), and the related [PtCl(SeCH(2)CH(2)NMe(2))(PEt(3))] (3) were determined crystallographically, revealing a typical pattern of trans-positioned neutral N and P donor atoms in an approximately square planar setting. The molecules 2b, 2e, and 2i were calculated by TD-DFT methodology to understand the origin of the weak (epsilon approximately 200 M(-1) cm(-1)) long-wavelength bands at about 600 nm for Pd/Te complexes such as 2b or 2e, at ca. 460 nm for Pt/Te systems such as 2i, and at about 405 nm for Pt/Se analogues such as 3. These transitions are identified as charge transfer transitions from the selenolato or tellurolato centers to unoccupied orbitals involving mainly the phosphine coligands for the Pt(II) compounds and more delocalized MOs for the Pd(II) analogues. Calculations and electrochemical data were used to rationalize the effects of metal and chalcogen variation.