I have a question why is the resonance assignment of DNA nucleotide starts with assignment of thymine and cytosine aromatic protons in case of non-exchangeable proton assignment ?
Thymine and cytosine have coupled protons that give a cross-peaks in the COSY experiment (for cytosine C6H and C5H and for thymine, methyl group and C6H). This kind of coupled protons do not exist in guanine and adenine. These cross-peaks are convenient starting point in the assigment prcedure.
Thanking You for your reply.
How is the imino and amino proton assigned in exchangeable proton assignment?
And after all, if all the protons are assigned , how is actually the proton distances calculated to predict a structure?