The behavior of membrane proteins in monolayers at the gas–water interface: comparison between photosystem II, rhodopsin and bacteriorhodopsin

It has been postulated, without supporting evidence, for decades that proteins are denatured once spread in monolayers at the gas–water interface. In the present study, the effect of different experimental conditions on the structure of three membrane proteins has been investigated by polarization modulated infrared reflection absorption spectroscopy in situ in monolayers at the gas–water interface. We have found that photosystem II core complex (PS II CC) is less sensitive to denaturation than rhodopsin. In fact, denaturation of rhodopsin could only be prevented when spreading was performed at 4°C. In contrast, bacteriorhodopsin was found to remain native when monolayer spreading was performed in conditions that were found to denature both PS II CC and rhodopsin. This behavior may be explained by the two-dimensional crystalline structure of bacteriorhodopsin. In conclusion, conditions can be found where the native structure of membrane proteins is maintained after their spreading in monolayers at the gas–water interface.