MICROSECOND ATOMIC-FORCE SENSING OF PROTEIN CONFORMATIONAL DYNAMICS -IMPLICATIONS FOR THE PRIMARY LIGHT-INDUCED EVENTS IN BACTERIORHODOPSIN

In this paper a new atomic force sensing technique is presented for dy namically probing conformational changes in proteins, The method is ap plied to the light-induced changes in the membrane-bound proton pump b acteriorhodopsin (bR), The microsecond time-resolution of the method, as presently implemented, covers many of the intermediates of the bR p hotocycle which is well characterized by spectroscopical methods, In a ddition to the native pigment, we have studied bR proteins substituted with chemically modified retinal chromophores. These synthetic chromo phores were designed to restrict their ability to isomerize, while mai ntaining the basic characteristic of a large light-induced charge redi stribution in the vertically excited Franck-Condon state. An analysis of the atomic force sensing signals lead us to conclude that protein c onformational changes in bR can be initiated as a result of a light-tr iggered redistribution of electronic charge in the retinal chromophore , even when isomerization cannot take place. Although the coupling mec hanism of such changes to the light-induced proton pump is still not e stablished, our data question the current working hypothesis which att ributes all primary events in retinal proteins to an initial trans dou ble left right arrow cis isomerization.