A Caffeic Acid Matrix Improves Detection and Imaging of Proteins with High Molecular Weight Close to 200,000 Da in Tissues by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging.

To our knowledge, this was the first study in which caffeic acid (CA) was successfully evaluated as a matrix to enhance the detection and imaging of endogenous proteins in three biological tissue sections (, a rat brain and and germinating soybean seeds) by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Our results show several properties of CA, including strong ultraviolet absorption, a super-wide MS detection mass range close to 200,000 Da, micrometer-sized matrix crystals, uniform matrix deposition, and high ionization efficiency. More high-molecular-weight (HMW) protein ion signals (/ > 30,000) could be clearly detected in biological tissues with the use of CA, compared to two commonly used MALDI matrices, , sinapinic acid (SA) and ferulic acid (FA). Notably, CA shows excellent performance for HMW protein detection from biological tissues in the mass range / > 80,000, compared to the use of SA and FA. Furthermore, the use of a CA matrix also significantly enhanced the imaging of proteins on the surface of selected biological tissue sections. Three HMW protein ion signals (/ 50,419, / 65,874, and / 191,872) from a rat brain, two sweet proteins (mabinlin-2 and mabinlin-4) from a seed, and three HMW protein ion signals (/ 94,838, / 134,204, and / 198,738) from a germinating soybean seed were successfully imaged for the first time. Our study proves that CA has the potential to become a standard organic acid matrix for enhanced tissue imaging of HMW proteins by MALDI-MSI in both animal and plant tissues.