Effect of esterification on MALDI–MS detection sensitivity for amino acids

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI–MS) spectra for methyl esters H–X–OMe of 11 amino acid X were measured at various analyte/matrix mixing ratios using α-cyano-4-hydroxycinnamic acid (CHCA) matrix. For each amino acid, the effect of esterification on MALDI signals was examined by comparing the signal intensity ratio (H–X–OMe)H+/(CHCA)H+ for the ester H–X–OMe with the comparable ratio XH+/(CHCA)H+ for the corresponding amino acid X. For all 11 amino acids, the ratios for the esters are in line with a thermal equilibrium model [M. Tsuge, K. Hoshina, Investigation of protonation efficiency for amino acids in matrix-assisted laser desorption/ionization, Bull. Chem. Soc. Jpn. 83 (2010), 1188–1192.]. For 9 of the amino acids – Ala, Arg, Gly, Ile, Leu, Phe, Ser, Trp, and Val – the ratio for the amino acid agrees with the ratio for the ester, within the standard error. For the other 2 amino acids – His and Lys – the ratio for the amino acid is exceptionally smaller than the ratio for the ester, indicating that the effect of esterification is significant. This compensation by esterification suggests that for His/CHCA and Lys/CHCA systems, the coexistence of a carboxyl group and a basic side chain are responsible for suppression in the formation of HisH+ and LysH+, possibly by reducing the amount of desorbed analyte and/or decreasing the effective gas-phase basicity (GB) in the MALDI plume.

Figure optionsDownload high-quality image (180 K)Download as PowerPoint slideResearch highlights▶ MALDI–MS spectra for methyl esters H–X–OMe of 11 amino acid X were measured using CHCA. ▶ The effect of esterification is significant for His and Lys. ▶ The coexistence of a carboxyl group and a basic amino chain are responsible for suppression in the formation of HisH+ and LysH+.