Comparison of extractants and applicability of MALDI–TOF-MS in the analysis of soil proteinaceous material from different types of soil

In order to understand better the chemistry of soil proteinaceous material, there is a need for an effective extraction and purification method for different types of soil. The aim of this study was to test the effectiveness of different extractants and of phenol extraction for the extraction and purification of native soil proteinaceous material and the added protein (bovine serum albumin, BSA) from different types of soil, and to test the applicability of matrix-assisted laser desorption/ionization–time-of-flight-mass spectrometry (MALDI–TOF-MS) for the analysis of soil proteinaceous material. Extraction of three types of boreal soil was carried out with water or different buffers. Purification of the extracted proteinaceous material was carried out with phenol extraction, the concentration being measured using Bradford’s method, and the analysis performed with MALDI–TOF-MS. The concentration of extracted proteinaceous material was dependent on the soil and the extractant, and the amount of N in the extracted material was ca. 3–90% of soil total N, being highest for sandy soil and lowest for highly organic forest floor and fine-textured clay soil. Non-purified extracts contained high amounts of non-proteinaceous (e.g. humic) material, especially the highly organic soil; this material may cause artifacts with Bradford’s method. The recovery of BSA was dependent on soil and extractant, suggesting a different sorption mechanism for different types of soil. MALDI–TOF-MS spectra suggested that the molecular weight of the extracted compounds was mainly <10 kDa, but compounds in the range ca. 15–50 kDa were also detected. However, individual compounds could not be identified.