Photosensitizer initiated attacks on DNA under dry conditions and their inhibition: a DNA archiving issue

Long-term aging of dry DNA is thought to be due to the attack of diverse cascades of reactive species with probably, no one single initiator of the cascades explaining all circumstances. Photosensitizer-initiated reactions from methylene blue and riboflavin were used to generate two model systems of reactive species around dry DNA in order to understand such systems and how to block them. Damage was assessed using plasmid DNA as a substrate with an in-situ microgel electrophoretic technique. Photodynamic methylene blue damage to DNA was very oxygen dependent but not that of riboflavin. This indicates that indirect type II pathways, probably via singlet oxygen were important for methylene blue but not for riboflavin. In both the absence and presence of oxygen, the DNA protection offered by dry caffeine and urate to both photodynamic agents indicated that most DNA attack was via electrophilic species. Overall, protection of dry archived DNA from spontaneously reactive species such as free radicals appears to be a real issue and, as expected, the predominant species in air appear to involve oxygen but not exclusively or necessarily so.