E. Oliveros et al., SINGLET OXYGEN ((1)DELTA(G)) SENSITIZATION AND QUENCHING BY VITAMIN-B-12 DERIVATIVES, Journal of photochemistry and photobiology.B, Biology, 29(1), 1995, pp. 37-44
Two Co(III) corrins, cobester and pyrocobester, and a Co(II) corrinate
, derived from vitamin B-12, have been investigated for their efficien
cies of singlet oxygen (O-2((1) Delta(g))) production and quenching in
polar and non-polar media. The quantum yields of singlet oxygen produ
ction have been determined by measurements of the singlet oxygen lumin
escence in the near-infrared region upon continuous excitation of the
sensitizer. The results show that, among the three derivatives investi
gated, only pyrocobester produces singlet oxygen in significant amount
s, with a quantum yield (Phi(Delta)) of 0.2(+/-0.02) in benzene and 0.
3( +/-0.03) in methanol. Combining luminescence experiments and kineti
c measurements of the vitamin B-12 derivatives disappearance, the rate
constants of singlet oxygen physical quenching and of the chemical re
action of these compounds with singlet oxygen have been determined, in
polar and non-polar media. The results show that the three vitamin Bi
t derivatives are more efficient physical quenchers than chemical acce
pters of singlet oxygen. Among them, the Co(II) corrinate is a particu
larly efficient physical quencher with a rate constant greater than 10
(9) l mol(-1) s(-1) in methanol, most probably implying a charge trans
fer between the Co(II) metal center and singlet oxygen. The chemical r
eactivity, higher for pyrocobester, depends on the structure of the co
rrin ligand, as well as on the degree of oxidation of the metal.