The hydration of Concanavalin A studied by infrared spectroscopy

The influence of a stepwise hydration on the secondary structure of Concanavalin A, a 273 amino acid residues long lectin protein, was monitored by infrared spectroscopy. An analysis of Amide I and Amide III bands, assignment of model bands and determination of the populations of secondary structure elements using computed integral intensity of particular amino bands, was used to determine the proportion of beta-sheet in protein films recorded under various steps of hydration and solution in water. In dry protein film 53% of amino acid residues are in beta-sheet conformation. The hydration increases a population of beta-sheet to 57% determined in fully hydrated film which finally reached 61% in water solution. On the basis of characteristic differential spectra calculated from the various hydration levels, we established that in the initial stage of hydration water molecules bind through hydrogen bonds directly to the main and side chains of protein. Hydration of side chains mainly occupies COO- and COOH groups. The increase of beta-sheet population induced by hydration rearranged the water molecules attached to COOH side chain groups. A parallel analysis of Amide III bands shows that the Amid HI region provides more complete information regarding the protein structure. Contemporary analysis of this region is very supportive, because it offers additional structural parameters which significantly contribute to reliability of secondary structure analysis by applying Amide I mode. Moreover, besides the comparable information about the population of secondary structure elements, the analysis of the Amide III area provides also the distribution of conformations of amino acids which are found in unstructured parts of protein. (C) 2017 Elsevier B.V. All rights reserved.