Compound 2

Compound 1 (97%) and compound 3 (>97%) were purchased from Aladdin. Compound 2 was purchased from Sigma-Aldrich. Compound 4 (96%) was purchased from Alfa Aesar. Compound 5 (97%) was purchased from BIDE. Compound 6 (98%) was purchased from Macklin. 1-Amino-2-methylanthraquinone (>90%) was purchased from Shanghai Xian Ding Biotechnology Co. Ltd. (Shanghai, China). Compound 1 was recrystallized twice from hot acetone. Compound 4 was recrystallized twice from a mixture of hot acetone and acetonitrile. 1-Amino-2-methylanthraquinone was recrystallized twice from hot ethanol until it is pure according to the ¹H NMR spectrum. Other solvents and chemicals are commercially purchased and used as obtained without further purification. BIH was prepared based on a method from the literature^{69 (link)}.

GLP-1 (7-36) amide, oxytomodulin, glucagon, exendin-4 (exenatide) and exendin-3 were synthesized by Alta Biosciences (University of Birmingham, Birmingham, UK) and prepared as 1 mM stocks in water. Liraglutide was supplied by George Eliot Hospital NHS Trust (Nuneaton, UK). Small-molecule agonists, compound 2 and BETP were obtained from Sigma-Aldrich (St Louis, MO, USA) and prepared as 100 mM stocks in DMSO. Yeast nitrogen base and yeast extract were purchased from Difco (Franklin Lakes, NJ, USA). Flurorescein-Di-β-D-glucopyranoside (FDGlu) was purchased from Invitrogen (Paisley, UK). All other reagents were purchased from Sigma-Aldrich.