Spin-labelling of DNA with hydrazine mustard spin label (HMSL)

1. The hydrazine mustard spin label (HMSL), recently synthesized in our laboratory (Raikova, 1977) was used for spin-labelling of DNA. 2. It alkylates both double- and single-stranded DNAs. 3. The reaction of HMSL with DNA was studied with respect to the kinetics of alkylation, dependence on salt concentration and base specificity. 4. It was found that HMSL is a base-specific reagent, alkylating preferentially guanine. According to their ability to bind HMSL, the four deoxyribonucleotides are ordered in the following way: G greater than A greater than C greater than T. 5. The EPR spectra obtained strongly depended on the secondary structure of the spin-labelled DNA: unlike the immobilized spectra of the double-stranded DNAs (2AZZ = 44.8G), the EPR spectra of single-stranded DNAs were non-immobilized (2AZZ = 32.8 G). 6. When sheared double-stranded DNA was spin-labelled, the parameters of the EPR spectrum depended also on the GC content of DNA.