Mechanism of lithiation of amino acids in aqueous solutions: A laser desorption/ionization-time-of- flight mass spectrometry and theoretical study

Abstract

Mono- and di-lithiation of small amino acids (AA) including alanine, proline, histidine and arginine were studied by laser desorption/ionization-time of flight (TOF) mass spectrometry. The mass spectra showed that two adduct cations of amino acids [AA + Li]+ and [AA+2Li-H]+ are formed. The relative intensities of the [AA + Li]+ and [AA+2Li-H]+ peaks in mass spectrum depend on the nature of side chain of the amino acids and the pH of the solution containing AA and lithium salt. In basic solution, the [AA+2Li-H]+ peak was more intense than that of [AA + Li]+ and a reverse trend was observed in the acidic solution. Furthermore, it was found that the di-lithiation of arginine and histidine takes place more favorably than alanine and proline which was attributed to presence of strong basic sites in the side chains of the formers. The theoretical calculations showed that attachment of the first Li+ cation to amino acids enhances their acidity so that [AA + Li]+ is more easily releases it proton compared to AA. Hence, Li+ attachment, basicity of the side chain of AA, and solution pH influence the formation of [AA+2Li-H]+.