The Ramachandran plot is a fundamental tool in the analysis of protein structures. Of the 4 basic types of Ramachandran plots, the interactions that determine the generic and proline Ramachandran plots are well understood. The interactions of the glycine and pre-proline Ramachandran plots are not.
Ho, B. K., & Brasseur, R. (2005). The Ramachandran plots of glycine and pre-proline. BMC Structural Biology, 5. https://doi.org/10.1186/1472-6807-5-14