Changes in free energy and the reaction quotient should be payed more attention. Stable isotope labeled peptides have been widely applied in the nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS). The combination of SIL peptides with NMR spectroscopy allow for the incorporation of NMR active nuclei which can reduce the complexity of spectra, and then help researchers to obtain novel correlations between atoms for more structural information. (reference: http://www.creative-peptides.com/services/stable-isotope-labeled-peptides.html)