Behavior of the imino protons of the lambda OR3 17mer studied by high resolution NMR

Imino proton resonances of lambda OR3 17mer were observed with time-shared Redfield pulse method by using a JEOL 500 MHz NMR instrument. They show gradual broadening and disappearance with the elevation of temperature indicating the stepwise melting of the duplex. By the selective irradiation at each peak position nuclear Overhauser effects were observed between the imino and adenine C2H protons and between imino protons themselves. Combining these data, fifteen imino proton resonances could be assigned to each base pair except two terminal AT base pairs. Based on the assignment it can be said that AT rich regions near the terminal melt first, followed by the melting of the inside GC rich core. The two AT base pairs in the middle of the GC core are resistant to heat. Spin lattice relaxation times were also observed and the results are consistent with the melting profile.