I am wondering that it is the right interpretation to interpret $E=mc^2$ with potential energy. What I mean is this: When I studied nuclear fusion, there was missing mass. The hydrogen's nuclear fusion happens when four hydrogen nucleus fuse into one helium nucleus.
The hydrogen is one proton and the helium is two protons and two neutrons. Neutron is a little heavier than the proton. But there is something strange. Where is the missing mass? As a result, the helium should be heavier than the sum of four protons, but it is not.
So I interpreted this situation as using $E=mc^2$. The potential energy has negative energy, so as the perspective of $m=E/c^2$, it is possible to have negative mass.
But I can't be sure of this interpretation. How can I explain the missing mass?
Plus: Is it possible to interpret binding energy as using $E=mc^2$?
P.s. I'm a Korean student, so I am not used to writing in English. Please give me a comment, if you are find it hard to understand.