Pulsed laser deposition of the lysozyme protein: an unexpected “Inverse MAPLE” process

Films of organic materials are commonly deposited by laser assisted methods, such as MAPLE (matrix-assisted pulsed laser evaporation), where a few percent of the film material in the target is protected by a light-absorbing volatile matrix. Another possibility is to irradiate the dry organic material directly for film production, as in PLD (pulsed laser deposition), where the film molecules may undergo strong fragmentation. In this presentation we report an alternative surprising mechanism for film deposition of the protein lysozyme in vacuum, when a small amount of residual water drives the ejection and deposition of lysozyme. This can be called an “inverse MAPLE” process, since the ratio of “matrix” to film material in the target is 10:90, which is inverse of the typical MAPLE process where the film material is dissolved in the matrix down to several wt.%.
Lysozyme is a well-known protein which is used in food processing and is also an important constituent of human secretions such as sweat and saliva. It has a well-defined mass (14307 u) and can easily be detected by mass spectrometric methods such as MALDI (Matrix-assisted laser desorption ionization) in contrast to many other organic materials. Also, the thermal properties of lysozyme, including the heat-induced decomposition behavior are comparatively well-known.
The ablation of lysozyme from a dry pressed target in vacuum was measured by weight loss in nanosecond laser ablation at 355 with a fluence of 0.5 to 6 J/cm2. Films with a significant number of intact lysozyme molecules have been produced by direct laser irradiation of a pressed target and the number of intact molecules shows a maximum at around 2.5 J/cm2. Apparently, there is a certain range of laser fluences when the transfer of intact lysozyme to the film substrate is possible.
The experimental results are explained with the help of molecular-level computer simulations. The simulations show that pure lysozyme cannot ablate without complete fragmentation. However, small pockets of trapped water provide the necessary expansion of the target and the ejection of intact lysozyme molecules above a certain fluence threshold, below which no lysozyme molecules are ejected. For high fluences all molecules are ejected as fragments. For a reasonable concentration of water (10%) the fluence dependence similar to that obtained experimentally is observed in the simulations.