Peptide binding to PADI4 was also characterised by surface plasmon resonance (SPR) (Figure ). By fitting the data using a two-state reaction model, in which the rate constant for the reverse second step (k-2) was set to zero, Ki and kinact values could be determined (Table ). In most cases, the kinact values are similar whilst the KI values from the COLDERs are generally an order of magnitude higher. Despite this, the rank order of peptides by kinact/Ki is similar. Differences are only observed between the three most potent peptides, which is where we anticipate the most error in our fitting for both methods. To further understand the contribution of 2 to PADI4 binding and inhibition, variants of cP4_4