Enabling the controlled assembly of antibody conjugates with a loading of two modules without antibody engineering† †Electronic supplementary information (ESI) available: 1H and 13C NMR spectra for all small molecules, ELISA, SDS-PAGE gels and UV-vis analysis (where applicable) for all bioconjugates. See DOI: 10.1039/c6sc03655d Click here for additional data file.

All reagents were purchased from Aldrich, AlfaAesar, Sino Biologial Inc or Lumiprobe and were used as received. Where described below Pet. refers to petroleum ether (40–60 °C). All reactions were monitored by thin-layer chromatography (TLC) on pre-coated SIL G/UV254 silica gel plates (254 m) purchased from VWR. Flash column chromatography was carried out with Kiesegel 60M 0.04/0.063 mm (200–400 mesh) silica gel. 1 H and 13 C NMR spectra were recorded at ambient temperature on a Bruker Avance 300 instrument operating at a frequency of 300 MHz for 1 H and 75 MHz for 13 C, a Bruker Avance 500 instrument operating at a frequency of 500 MHz for 1 H and 125 MHz for 13 C, and a Bruker Avance 600 instrument operating at a frequency of 600 MHz for 1 H and 150 MHz for 13 C in CDCl3 or CD3OD (as indicated below). The chemical shifts () for 1 H and 13 C are quoted relative to residual signals of the solvent on the ppm scale. 1 H NMR peaks are reported as singlet (s), doublet (d), triplet (t), quartet (q), doublet of doublets (dd), triplet of doublets (td), doublet of triplets (dt), triplet of triplets (tt) m (multiplet) and br (broad). Coupling constants (J values) are reported in Hertz (Hz) and are H-H coupling constants unless otherwise stated. Signal multiplicities in 13 C NMR were determined using the distortionless enhancement by phase transfer (DEPT) spectral editing technique. Infrared spectra were obtained on a Perkin Elmer Spectrum 100 FTIR Spectrometer operating in ATR mode with frequencies given in reciprocal centimetres (cm -1 ). Melting points were measured with a Gallenkamp apparatus and are uncorrected. All bioconjugation reactions were carried out in triplicate.