Fungal pigments inhibit the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of darkly pigmented fungi

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI–TOF MS) has been used to discriminate moniliaceous fungal species; however, darkly pigmented fungi yield poor fingerprint mass spectra that contain few peaks of low relative abundance. In this study, the effect of dark fungal pigments on the observed MALDI mass spectra was investigated. Peptide and protein samples containing varying concentrations of synthetic melanin or fungal pigments extracted from Aspergillus niger were analyzed by MALDI–TOF and MALDI–qTOF (quadrupole TOF) MS. Signal suppression was observed in samples containing greater than 250 ng/μl pigment. Microscopic examination of the MALDI sample deposit was usually heterogeneous, with regions of high pigment concentration appearing as black. Acquisition of MALDI mass spectra from these darkly pigmented regions of the sample deposit yielded poor or no M+H]⁺ ion signal. In contrast, nonpigmented regions within the sample deposit and hyphal negative control extracts of A. niger were not inhibited. This study demonstrated that dark fungal pigments inhibited the desorption/ionization process during MALDI–MS; however, these fungi may be successfully analyzed by MALDI–TOF MS when culture methods that suppress pigment expression are used. The addition of tricyclazole to the fungal growth media blocks fungal melanin synthesis and results in less melanized fungi that may be analyzed by MALDI–TOF MS.