Although 13C has a natural abundance of only 1.1% and gives inherently much weaker nuclear magnetic resonance signals than does does 1H, it has been found that 13C nuclear magnetic resonance spectra of aqueous solutions of naturally occurring proteins can be observed. A spectrum of hen's egg white lysozyme, an enzyme of molecular weight approximately 14 300, containing 129 amino acid residues, has been partially analyzed by comparison with a computer-simulated spectrum.
Paul C. Lauterbur, "13C Nuclear Magnetic Resonance Spectra of Proteins," Appl. Spectrosc. 24, 450-452 (1970)