The chemical composition of proteins was discovered by

In the 1950's, Linus Pauling became known as the founder of molecular biology due to his discovery of the spiral structure of proteins (Taton, 1964). Pauling's discoveries contributed to Watson and Crick's breakthrough of the DNA double helix.

Explanation:

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.