Alkylated dihydroxybenzoic acid as a MALDI matrix additive for hydrophobic peptide analysis.

Hydrophobic peptides are generally difficult to detect using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) because the majority of MALDI matrixes are hydrophilic and therefore have a low affinity for hydrophobic peptides. Here, we report on a novel matrix additive, o-alkylated dihydroxybenzoic acid (ADHB), which is a 2,5-dihydroxybenzoic acid (DHB) derivative incorporating a hydrophobic alkyl chain on a hydroxyl group to improve its affinity for hydrophobic peptides, thereby improving MALDI-MS sensitivity. The addition of ADHB to the conventional matrix α-cyano-4-hydroxycinnamic acid (CHCA) improved the sensitivity of hydrophobic peptides 10- to 100-fold. The sequence coverage of phosphorylase b digest was increased using ADHB. MS imaging indicated that hydrophobic peptides were enriched in the rim of a matrix/analyte dried spot when ADHB was used. In conclusion, the addition of ADHB to the standard matrix led to improved sensitivity of hydrophobic peptides by MALDI-MS.