We report site-directed protein spin labelling via Suzuki–Miyaura coupling of a nitroxide boronic acid label with the genetically encoded amino acid 4-iodo-L-phenylalanine. The resulting spin label bears a rigid biphenyl linkage with lower flexibility than spin label R1. It is suitable to obtain defined electron paramagnetic resonance distance distributions and to report protein–membrane interactions and conformational transitions of α-synuclein.