Table 1. Reactivity of single-cysteine mutants with BrAcEGMe (5 eq.) in PBS pH 7.4 for 1 h at 4 °C, and a single DARPin repeat highlighting the side-chains of residues mutated.

	Mutation	Reaction with BrAcEGMe a (%)
	D72C	100
	G70C	18
	N69C	0
	K68C	16
	V65C	5
	E64C	17
	E61C	57
	L60C	4
	H59C	29

^aDetermined by relative peak areas in mass spectrometry.