# Exact average molecular weight of a dsDNA basepair

I am trying to calculate the exact weight of a given dsDNA.

On the Internet and the literature, different values for the av. molecular weight of one basepair are given :

I tried to calculate the average weight using following molecular weights:

molecule . weight
2'-Deoxycytidin- 5'-monophosphate dCMP 307,197 g/mol
2'-Deoxyguanosine 5'-monophosphate dGMP 347.2212 g/mol
2'-Deoxythymidine-5'-monophosphate dTMP 321.2005 g/mol
water H2O 18,01528 g/mol

adding the matching nucleoside monophosphates together and subtracting one water for each peptide-bond condensation (one on the sense; one on the antisense side); I get following formula for the average dsDNA basepair weight: $$avg.BP.weight = \frac {(dAMP+dTMP-\ce{2H2O})+(dGMP+dCMP-\ce{2H2O})} {2}$$

resulting in : $$\frac {(331,22+321.2005-36,03056)+(347.2212+307,197 -36,03056)} {2} = 617.3892 \frac{g}{mol}$$

which is very close but still 0.57081 short off the "best" literature value.

What am I missing?; Im trying to get this calculation as exact as possible.

Maybe i didn't consider some hydrogen or protons in bonds?

The effect of this changes with the length of your sequence, here is a graph showing it. Blue line is your calculation and red line is the one from literature.