STM OF LANGMUIR MONOLAYER FILMS OF PURPLE MEMBRANES FROM HALOBACTERIA

Monolayer Langmuir films of purple membranes of Halobacteria deposited on the surface of highly ordered pyrolytic graphite were investigated , using scanning tunneling microscopy (STM) under normal conditions in the absence of any conducting coatings and/or replicas. The procedure of sample preparation and the methods of their study, using STM are r eported. As is shown, the technique of STM makes it possible to obtain stable contrast images of the membrane surface, which has a regular q uasicrystalline structure of bacteriorhodopsin (BR) trimers with the p arameter in a range of 61.9-63.1 angstrom. The optimum conditions to o btain contrast images are found to be, namely, the modulation regime ( particularly, during the measurements of space distribution of the tun nel current first derivative by bias voltage at a constant tunnel curr ent), the initial bias voltages about 150+300 mV, the negative potenti al at a probing tip and a rather deep modulation U(m) = 100 mV at a fr equency of 2.4 kHz. As is shown, the topographic mode of measurements does not give contrast images of the membrane surface structure. The s canning at low bias voltages 10-15 mV is demonstrated to result in dis ruption of the samples studied, whereas at high voltages 1-1.5 V a con siderable noise level, an unstable tunnel current and a loss in the ST M contrast images are observed. The problem of anomalous high conducti vity, space resolution, the nature of the noise character in STM exper iments with biological objects are discussed. Possible electron conduc tivity and that closely related to the protein hydration state, where charge transfer occurs through the network of collective surface hydro gen bonds coordinated by polar hydration centers are anal sed.