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Three 2´-deoxynucleosides containing semi-flexible spin labels, namely (T)A, (U)A and (U)C, were prepared and incorporated into deoxyoligonucleotides using the phosphoramidite method. All three nucleosides contain 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) connected to the exocyclic amino group; (T)A directly and (U)A as well as (U)C through a urea linkage. (T)A and (U)C showed a minor destabilization of a DNA duplex, as registered by a small decrease in the melting temperature, while (U)A destabilized the duplex by more than 10 °C. Circular dichroism (CD) measurements indicated that all three labels were accommodated in B-DNA duplex. The mobility of the spin label (T)A varied with different base-pairing partners in duplex DNA, with the (T)A•T pair being the least mobile. Furthermore, (T)A showed decreased mobility under acidic conditions for the sequences (T)A•C and (T)A•G, to the extent that the EPR spectrum of the latter became nearly superimposable to that of (T)A•T. The reduced mobility of the (T)A•C and (T)A•G mismatches at pH 5 is consistent with the formation of (T)AH(+)•C and (T)AH(+)•G, in which protonation of N1 of A allows the formation of an additional hydrogen bond to N3 of C and N7 of G, respectively, with G in a syn-conformation. The urea-based spin labels (U)A and (U)C were more mobile than (T)A, but still showed a minor variation in their EPR spectra when paired with A, G, C or T in a DNA duplex. (U)A and (U)C had similar mobility order for the different base pairs, with the lowest mobility when paired with C and the highest when paired with T.

TEMPO-derived spin labels linked to the nucleobases adenine and cytosine for probing local structural perturbations in DNA by EPR spectroscopy