PN and ²⁹SiO column density ratios relative to C³⁴S. The two detections of PN in NGC 253 (GMCs 4 and 6) are indicated with red diamonds, while the non-detections towards the remaining GMCs are indicated with grey diamonds. We also added data from other sources in our Galaxy Centre (Rivilla et al. 2018 and references therein): in green we plot the L1157-B1 shock data and the blue dots represent data from shock dominated Galactic Centre clouds.