Photosynthetic reaction centres (RCs), such as the RC from purple bacteria or photosystems I and II of plants, have their cofactors arranged in a nearly C2 symmetry. Despite this symmetrical arrangement, the electron pathway in RCs of purple bacteria and photosystem II is entirely unidirectional occurring along a single branch. In photosystem I, however, both electron transfer branches are equally active. It appears that the understanding of the very basic principles of the directionality of light-induced electron transfer is lacking. Here we propose to solve this question by combining laser-flash photochemically induced dynamic nuclear polarization (photo-CIDNP) magic angle spinning (MAS) NMR, providing electronic structure information of the highest occupied molecular orbital (HOMO) and the donor triplet at atomic resolution. This experimental data in combination with theoretical calculations will allow reconstructing the lowest unoccupied molecular orbital (LUMO) from which the electron is transferred. Our approach is based on the assumptions that (i) the donor triplet can be approximated by single electron occupation of both HOMO and LUMO, (ii) the structure of the LUMO is responsible for the directionality of the light-induced electron transfer. Hence, we aim at reconstruction of the LUMO of the electron donor as key for understanding directionality. This reconstruction may also stimulate research on artificial photosynthesis, which is currently facing the problem to direct charge separation into macroscopically useful units.