The pKa values of primary and secondary phosphoryl groups of nucleotides are

The primary and secondary phosphoryl groups of nucleotides have pKa values of about 1.0 and 6.2, respectively. The pKa values of the secondary phosphoryl groups are such that they can serve either as proton donors or as proton acceptors at pH values approximately two or more units above or below neutrality. Reference: Harpers illustrated biochemistry 30th edition