Inactivation of viruses in blood plasma can be achieved by photodynamic procedures using
methylene blue (MB) or other photoactive
dyes. Singlet molecular
oxygen (1O2) probably contributes to the virucidal effects of
photosensitization. We report the inactivation of herpes simplex virus type 1 (HSV-1) and suid herpes virus type 1 (SHV-1) by chemically generated
singlet oxygen, produced by thermal decomposition of the endoperoxide of 3,3'-(1,4-naphthylidene)dipropionate (
NDPO2). We demonstrate that viruses can be inactivated by 1O2 generated by chemiexcitation in a reaction in the dark, even in the presence of human plasma. Virus inactivation in
phosphate-buffered saline (PBS) was enhanced when water was replaced by
deuterium oxide (D2O) and diminished when human plasma or quenchers (
imidazole or
histidine) were added. The
singlet oxygen quenching activities of plasma,
imidazole and
histidine correlated with their inhibitory effects on virus inactivation. The production of 1O2 was assessed by an
indicator reaction: the bleaching of
p-nitrosodimethylaniline (RNO) with
imidazole as 1O2 acceptor. Virus inactivation and
singlet oxygen generation of
NDPO2 were compared with those of MB/light-mediated
photosensitization. Based on similar amounts of 1O2 generated by either procedure, virus inactivation by MB/light was more effective. Virus inactivation by MB/light was not affected by type I quenchers (e.g.
mannitol), but was inhibited by human plasma or
singlet oxygen quenchers. Furthermore, in D2O-based PBS, virus inactivation was more effective than that in H2O. These observations confirm that
singlet oxygen is involved in virus inactivation by MB/light. Taken together, the results demonstrate that
singlet oxygen produced by either procedure is virucidal. The enhanced effect of the photochemical procedure suggests that, in addition to type II, type I reactions and/or the binding affinity of the
dye for the virus contribute to virus killing by MB/light.