Specific binding of a foreign core ligand to a PAS domain, wherein the PAS domain is predetermined, prefolded in its native state, and comprises a hydrophobic core that has no NMR-apparent a priori formed ligand cavity, is determined using NMR spectra of the PAS domain in the presence and absence of a foreign ligand bound within the hydrophobic core. A functional surface binding specificity of a PAS domain is changed by (a) introducing into the hydrophobic core of the PAS domain a foreign ligand of the PAS domain; and (b) detecting a change in the functional surface binding specificity of the PAS domain.