We Suggest a method for determination of the mean excitation energies of several biomolecules, such as amino acids, using a Bragg-like sum rule developed for molecular fragments or functional groups. Because the fragment composition of many bio-organic molecules is very similar, we find that many of them, including the amino acids, have similar mean excitation energies (similar to 70 eV, which is interestingly close to the mean excitation energy of the major component of it cell: namely, water). Differences in amino acid mean excitation energies arise from variation of the side chains (-R); addition of -CH2- groups decreases the mean excitation energy.