A new method to measure small peptides amended in seawater using high performance liquid chromatography coupled with mass spectrometry

Quantifying peptide decomposition rate is crucial in understanding marine carbon and nitrogen cycling, because proteins and peptides constitute a major fraction of labile organic matter. However, analytical techniques of detecting small peptides in nanomolar levels in seawater are limited. A new method was developed to measure low concentrations of small peptides amended in seawater, using high performance liquid chromatography coupled with mass spectrometry (HPLC–MS). This technique reduces the detection limit of small peptides by two orders of magnitude relative to the common ultraviolet (UV) detection. A 6-way valve was added before the MS and the valve was programmed to guide the salt peak to waste before the peptide peak was introduced to the MS. Therefore, peptides amended in seawater were injected directly to the HPLC–MS without desalting pretreatment. This new method can detect as low as 0.23 pmol of tetrapeptide alanine–valine–phenylalanine–alanine (AVFA), a peptide fragment of Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO), with less than 5% precision (relative standard deviation). This method was applied successfully to determine decomposition rates of two small peptides, AVFA and serine–tryptophan–glycine–alanine (SWGA), in coastal oceans, and first data of peptide hydrolysis rates using small plain peptides at lower than micromolar concentrations were obtained. Hydrolysis of AVFA and its fluorescent analog was also compared at such low concentration levels. This analytical method broadens our capability to examine the biogeochemical behavior of small peptides, including their hydrolysis, decomposition, and other possible transformation processes in aquatic environments.