Infrared investigation on the conformation of proteins deposited on polyethylene films

Aqueous protein solutions deposited and dried on thin polyethylene sheets were analyzed by Fourier transform infrared spectroscopy. This convenient sampling method produced reliable estimates of protein secondary structure on relatively small quantities of protein. The total amount of protein deposited and examined by infrared spectroscopy ranged from 200 to 80 micrograms. To estimate secondary structure, principal component regression and partial least squares (PLS) analyses were applied to the infrared spectra from 12 different deposited proteins. Principal component regression with five principal components provided the fractions of helix, beta-sheet, turn, and other or random structure present in the proteins with standard deviations of 6.3, 7.3, 7.0, and 6.3%, respectively, compared to a reference data set of X-ray structures. Similar results were achieved through PLS analysis. Factor analysis provided reliable estimates of helix and beta-sheet structure with prediction errors similar to those obtained by other infrared methods. Analysis of various types of turn structure grouped together was unsuccessful.