Structure of the ring in drop coating deposited proteins and its implication for Raman spectroscopy of biomolecules

Drop coating deposition Raman spectroscopy represents a new technique that enables nondestructive measurements of solutions with concentration of biomolecules down to 1 mu M. It has been demonstrated that the solution structure is preserved even after drying and Raman spectra taken from the glass-like dried deposit and a solution are virtually identical. Here, we report for the first time measurements of the structure of a drop coating ring. Proteins deposited at the outer part of the ring perimeter are affected by desiccation and the spectra differ significantly from those taken in solution. Reproducible measurements of biomolecules by means of drop coating deposition Raman spectroscopy must therefore be obtained from central or slightly inward-located parts of the coating ring. The structure as well as mechanisms participating in the formation of the coating rings is explained on the basis of recently published physical theories of droplets desiccation. Formation of the final shape of the ring is analogous to processes that give rise to desiccated droplets, whereas the coating ring behaves as an independent ring droplet in final stages of desiccation of an ancestral droplet. Its structure is dominated by a dip or plateau in the upper part. Oscillation of the ancestral droplet contact line is probably responsible for complete desiccation of the proteins at the outer perimeter of the coating ring. It seems plausible the arrangement of a glassy skin at the coating ring surface caused by the accumulation of the biomolecules near this region plays an important role in preservation of solution-like spectral shape. (C) 2006 Elsevier B.V. All rights reserved.