Investigation of Protonation Efficiency for Amino Acids in Matrix-Assisted Laser Desorption/Ionization

Abstract

Abstract MALDI spectra for 20 natural amino acids were measured at various mixing ratios of amino acid (A) and matrix (M: CHCA or DHB) to clarify the main protonation process as well as MALDI plume conditions, which affect the detection sensitivity of analytes in MALDI. Signal intensities of protonated amino acids (AH+) relative to protonated matrix (MH+), obtained as AH+/MH+, revealed a clear correlation between AH+/MH+ and the gas-phase basicity (GB) of the respective amino acids except for three basic amino acids, Lys, His, and Arg. This relationship between AH+/MH+ and GB was interpreted in terms of a protonation model in which AH+ is produced through a reversible proton-transfer reaction of MH+ + A M + AH+ reaching thermal equilibrium in the MALDI plume. The GB of matrix molecules and MALDI plume temperatures independently determined from analysis of the present data (851 kJ mol−1 and 1510 K for CHCA and 830 kJ mol−1 and 1330 K for DHB) agreed well with values in the literature, indicating the validity of the present interpretation. The ratios AH+/MH+ for Lys, His, and Arg had lower values than those expected from the GB by a factor of 10−1–10−2. This discrepancy could have been caused by proton acceptance by amino groups in amino acids, which are caused by an intramolecular hydrogen bond, determine the magnitude of the GB for basic amino acids.