method sample d13C d15N molar_C_to_N Conventional PLK1 -12.9 16.7 2.8 Conventional PLK2 -12.8 16.8 2.8 Conventional PLK3 -12.9 16.8 2.8 Conventional PLK4 -12.8 16.6 2.7 Conventional PLK5 -12.8 16.7 2.7 Conventional PLK6 -12.9 16.5 2.7 Conventional PLK7 -12.9 16.6 2.8 Conventional PLK8 -13.0 16.6 2.8 Conventional PLK9 -13.1 16.5 2.8 Conventional PLK10 nd 16.6 2.9