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Surface plasmon resonance (SPR)-based biosensors are widely used instrumentsfor characterizing molecular interactions. In theory the SPR signal dependsonly on mass changes for interacting molecules of same chemical nature. Whetherconformational changes of interacting molecules also contribute to the SPRsignal is still a subject of lively debates. Works have been published claimingthat conformational changes were detected but all factors contributing to theSPR signal were not carefully considered, in addition to often using no orimproper controls. In the present work we used a very well-characterizedoligonucleotide, the thrombin-binding DNA aptamer (TBA), which upon binding ofpotassium ions folds into a two G-tetrad antiparallel G-quadruplex structure.All terms contributing to the maximal expected SPR response, Rmax, inparticular the refractive index increment, RII, of both partners and thefraction of immobilized TBA target available, ca, were experimentally assessed.The resulting Rmax was then compared to the maximal experimental SPR responsefor potassium ions binding to TBA using appropriate controls. Regardless howthe RIIs were measured, by SPR or refractometry, and how much TBA available forinteracting with potassium ions was considered, the theoretical and theexperimental SPR responses never matched, the former being always lower thanthe latter. Using a straightforward experimental model system and by thoroughlytaking into account all contributing factors we therefore conclude thatconformational changes can indeed contribute to the measured SPR signal.

Do conformational changes contribute to the surface plasmon resonance signal?