On protein monolayers

Abstract

In recent years examination by means of the X-rays, expecially in the hands of Meyer and Mark, and of Astbury, and Bernal, has gone far to elucidate the structure of natural proteins existing in the filamentous form. Other important natural proteins occur as membranes, which, in many cases at least, are apparently so thin that one is led to suspect that they might be true films or monolayers. It appeared desirable to attempt an examination of the properties of very thin artificially prepared protein films. Before any attempt can be made to elucidate the structure of such layers and to compute the thickness of a film from experimental data on the weight per square centimetre and the bulk density, it is necessary to ensure both the uniformity and continuity of the film under observation. One might suppose that a true uniform protein monolayer would exist with a thickness as low as 4 Å., which is the lesser dimension as determined by means of the X-rays of the cross section of a simple polypeptide chain. Further, in the compression of films of long chain hydrocarbon molecules with polar heads, transitions of state in the two dimensions are observable; vaporous, liquid and solid films being readily identified. It is improbable that such will be obtained with protein films, for with the extremely long polypeptide chain extended on the surface a vapour liquid transition would be difficult to detect. Moreover, the solid form, if such is formed, on account of the hydrophilic character of the chain, would be expected to possess properties entirely different from the solid form of the non polar hydrocarbon chain containing molecules where we may consider that a two-dimensional microcrystalline structure results. Two methods of quantitative examination present themselves: the force area method devised by Langmuir and applied extensively by N. K. Adam, and the method of surface potentials developed by Schulman and Rideal. Information on the nature and change in the packing or configuration of the surface layer is obtainable by the former, and the uniformity of the film as well as the change in configuration, involving an alteration in the magnitude of the vertical component of the electric moment of the polar groups can be investigated by the latter method.