After acylation the formula will be R−(NHCOCH3)x. Hence, the change in the molecular mass is calculated as below
For1oneNH2→14+2=16unitsWhenItturnsintoNH−COCH3→14+1+12+16+12+3=58Changeinmass⇒58−16=42units.SoforeveryNH2,therewillbeanincreaseof42gafteracylation.⇒Thetotalincreaseinmass=390−180=210⇒No.ofaminogroups=42210=5Hencenumberofaminogroupsare5.
(12+16+12+3−1)x=390−180
or42x=210;orx=5