Another strategy is to access the s-orbital contributions to the spin density from measured hyperfine coupling constants (Fermi contact term) . To that end, Electron Paramagnetic Resonance (EPR) and related methods such as Electron-Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM), and Electron-Nuclear-Nuclear Triple Resonance (commonly referred to as TRIPLE) can be applied to single photosynthetic pigments as well as to primary electron donors .