# Is there any experimental evidence to distinguish the total gravitational force exerted by the two proton and a helium nucleus?

I know that the sum of the mass of the two proton is greater than the mass of a helium nucleus (two proton). As the mass different is transformed into energy by the Einstein mass energy-equivalence. If the gravity is related with mass, then the gravitational force given by the helium nucleus will be less then the sum of the gravitational force given by the two proton separately. And if it is so, then what is the actual source of gravity?

Don't forget that gravitational forces are about $10^{14}$ times weaker than electromagnetic forces, so you'd need to be able to account for electromagentic forces with better than 14 significant figures of precision before you could hope to have any chance of directly measuring the gravitational forces.