An Arg-rich putative prebiotic protein is as stable as its Lys-rich variant

► The peptide RIA7: acARAAAAIRAIAAIIRAGGYam folds to a 4-helix X-bundle protein. ► RIA7-like proteins might have formed on the prebiotic Earth. ► RIA7 combines with RNA, forming complexes which are often insoluble. ► The conformational stabilities of RIA7 and its Lys-rich variant, KIA7, are the same. ► When solvent exposed, Arg and Lys make identical contributions to protein stability.