I wonder how to label a peptide and a small molecule in order to perform BRET? Would it be an issue that the relative large tag molecule such as RLuc or YFP might affect the behavior of a short peptide or a small molecule?
You can tag the peptide or a small molecule with Alexa 488 fluorophore, similar wavelength as YFP and can do BRET with Rlu tagged proteins. It is a challenge to label the peptide with Rlu or YFP. I hope this can help you.