Comparing the FTIR difference spectra of the rhodopsin --> metarhodopsin II transition in membranes and in dodecylmaltoside detergent, characteristic variations are observed between 1715 and 1750 cm(-1). By repeating the measurements with the rhodopsin mutant D83N/E122Q, the spectral variation between the samples in membranes versus detergent could be assigned to a difference band at 1743(+)/1724(-) cm(-1), which does not exhibit a deuteration-induced downshift. We provide evidence that this band is probably caused by the C=O stretch of only one ester group of one lipid molecule. This group interacts with the dark state of rhodopsin, whereas in metarhodopsin II, the lipid molecule behaves as if it were in the bulk lipid phase.