Formation of the cross-link 1-[N3-deoxycytidyl),2-[N1-deoxyguanosinyl]ethane in DNA treated with N,N'-bis(2-chloroethyl)-N-nitrosourea.

The cross-linked dinucleoside, 1-[N3-deoxycytidyl],2-[N1-deoxyguanosinyl]ethane, has been isolated from DNA which has been exposed to N,N'-bis(2-chloroethyl)-N-nitrosourea. It is probable that this structure is responsible for the interstrand cross-linking observed previously by physical methods. The modification is unique in that it cross-links DNA through two base-pairing positions and probably arises through the transfer of a chloroethyl group to one of the bases followed by a second reaction of this group with the other strand of DNA. Initial attack could be at the N3 position of deoxycytidine, the N1 position of deoxyguanosine, or possibly the O6 position of deoxyguanosine. Attack at the O6 position of deoxyguanosine would require an internal cyclization with the N1 position of deoxyguanosine before secondary reaction with the N3 position of deoxycytidine but would explain resistance to N,N'-bis(2-chloroethyl)-N-nitrosourea in cells capable of removing substituents on the O6 position of guanine.