Photochemistry of Biologically Important Transition Metal Complexes. I. Cyanocobalamin and Related Corrin Complexes of Rhodium(III)

Abstract

Bei den in dieser Zeitschrift wiedergegebenen Gebrauchsnamen, Handelsnamen, Warenbezeichnungen und dgl handelt es sich häufig um gesetzlich geschützte eingetragene Warenzeichen, auch wenn sie nicht als solche mit ® gekennzeichnet sind. Photochemistry of Biologically Important Transition Metal Complexes. I. Cyanocobalamin and Related Corrin Complexes of Rhodium(III)A. Vogler, R. Hirschmann, H. Otto, and H. KunkelyInstitut für Chemie, Universität Regensburg, 8400 Regensburg, GermanyAbsorptionsspektren, sichtbar und ultraviolett / Emissionsspektren / Komplexverbindungen / PhotochemieThe quantum yield of the well known photoaquation of cyanocobalamin (vitamin B 12 ) was determined (<t> ~ 10" 4 ) and was found to be independent of the irradiating wavelength. In addition cyanocobalamin did not show the characteristic corrin phosphorescence. In agreement with an earlier suggestion it is concluded that the excitation energy initially absorbed by the corrin ligand is rapidly transferred to a reactive excited LF state which lies below the lowest corrin triplet. This conclusion is supported by the behavior of corrin complexes of Rh(III). Contrary to cyanocobalamin the complex dicyanorhodium(III)-corrin did not undergo a photoaquation but has been shown to emit a strong corrin phosphorescence. These observations are consistent with the assumption that the lowest excited LF state lies now well above the lowest corrin triplet. If both axial cyanide ligands which have a strong field are replaced by the weak-field ligand chloride the energy of the lowest excited LF state should drop considerably. The behavior of dichlororhodium(III)-corrin indicates that the lowest corrin triplet lies only slightly below the lowest excited LF state which is responsible for the photoreactivity. The reactive LF state may be populated from the lowest corrin triplet by thermal activation. At 77 K dichlororhodium(III)-corrin showed a strong corrin phosphorescence. At room temperature this emission was essentially quenched and the complex underwent a photoaquation.Die Quantenausbeute der Photoaquotisierung von Cyanocobalamin (Vitamin B 12 ) wurde bestimmt (<£ ~ 10" 4 ). Die Quantenausbeute war unabhängig von der Wellenlänge des Anregungslichtes. Außerdem zeigte Cyanocobalamin nicht die charakteristische Corrinphosphoreszenz. In Übereinstimmung mit einem früheren Vorschlag wird aus diesen Ergebnissen geschlossen, daß die Anregungsenergie, die zunächst vom Corrinliganden absorbiert wird, rasch auf einen reaktiven angeregten LF-Zustand übertragen wird, der unter dem niedrigsten Corrintriplet liegt. Diese Schlußfolgerung wird durch das Verhalten von Corrinkomplexen von Rh(III) unterstützt. Im Gegensatz zu Cyanocobalamin unterlag Dicyanorhodium(III)-corrin keiner Photoaquotisierung, aber emittiert, wie schon früher gezeigt wurde, eine starke

Effect of Ascorbic Acid on the Degradation of Cyanocobalamin and Hydroxocobalamin in Aqueous Solution: A Kinetic Study

“…The CN group attached to Co 3+ in the tetrapyrrolic corrin macrocyclic ring in B 12 is replaced by an OH group in B 12b . B 12 undergoes photolysis to B 12b in aqueous solution which is degraded to irreversible oxidation products (6)(7)(8)(9)(10)(11). Similar reactions occur on the chemical degradation of B 12 and B 12b in the presence of ascorbic acid (AH 2 ) (12)(13)(14) and may be expressed as follows: The effect of thiamine (15,16), nicotinamide (17,18), and riboflavin (19)(20)(21)(22) on the photolysis of B 12 and B 12b has been investigated.…”

Photolysis of methylcobalamin. Nature of the reactive excited state

“…Cyanocobalamin seems to be an exception. Evidence was obtained that the photoaquation of cyanocobalamin is initiated by excited ligand field states [7,8]. Generally, the identification of reactive excited states of cobalamins is hampered by the fact that their absorption spectra are dominated by the intense rr* intraligand bands of the corrin ligand.…”

Photochemical Redox Reaction of Aqueous Benzylpentacyanocobaltate(III)

“…It has been recently shown that the photoaquation of cyanocobalamin originates from an excited LF state 26 . It seems that generally the photochemistry of cobalamins is closely related to that of other transition metal complexes and particularly to the photochemical reactions of pentacyano complexes of Co(III) 26 .…”

“…It seems that generally the photochemistry of cobalamins is closely related to that of other transition metal complexes and particularly to the photochemical reactions of pentacyano complexes of Co(III) 26 .…”