Table S1.

Average bond-breaking and bond-forming distances at the TS

PNPs	C1′–N9, Å	C1′–Op, Å
Light	2.59 ± 0.20	2.20 ± 0.13
Heavy	2.46 ± 0.22	2.54 ± 0.18
Light F159Y	3.15 ± 0.25	2.42 ± 0.17
Heavy F159Y	2.58 ± 0.23	2.22 ± 0.20