We introduce a stable isotope labeling approach for glycopeptides that allows a specific glycosylation site in a protein to be quantitatively evaluated using mass spectrometry. Succinic anhydride is used to specifically label primary amino groups of the peptide portion of the glycopeptides. The heavy form (D₄¹³C₄) provides an 8 Da mass increment over the light natural form (H₄¹²C₄), allowing simultaneous analysis and direct comparison of two glycopeptide profiles in a single MS scan. We have optimized a protocol for an in-solution trypsin digestion, a one-pot labeling procedure, and a post-labeling solid-phase extraction to obtain purified and labeled glycopeptides. We provide the first demonstration of this approach by comparing IgG1 Fc glycopeptides from polyclonal IgG samples with respect to their galactosylation and sialylation patterns using MALDI MS and LC-ESI-MS.

We demonstrate a new approach for the site-specific identification and characterization of protein N-glycosylation. It is based on a nano-liquid chromatography microarray- matrix assisted laser desorption/ionization-MS platform, which employs droplet microfluidics for onplate nanoliter reactions. A chromatographic separation of a proteolytic digest is deposited at a high frequency on the microarray. In this way, a short separation run is archived into thousands of nanoliter reaction cavities, and chromatographic peaks are spread over multiple array spots. After fractionation, each other spot is treated with PNGaseF to generate two correlated traces within one run, one with treated spots where glycans are enzymatically released from the peptides, and one containing the intact glycopeptides. Mining for distinct glycosites is performed by searching for the predicted deglycosylated peptides in the treated trace. An identified peptide then leads directly to the position of the "intact" glycopeptide clusters, which are located in the adjacent spots. Furthermore, the deglycosylated peptide can be sequenced efficiently in a simple collision-induced dissociation-MS experiment. We applied the microarray approach to a detailed site-specific glycosylation analysis of human serum IgM. By scanning the treated spots with low-resolution matrix assisted laser desorption/ionization-time-offlight- MS, we observed all five deglycosylated peptides, including the one originating from the secretory chain. A detailed glycopeptide characterization was then accomplished on the adjacent, untreated spots with high mass resolution and high mass accuracy using a matrix assisted laser desorption ionization-Fourier transform-MS. We present the first detailed and comprehensive mass spectrometric analysis on the glycopeptide level for human polyclonal IgM with high mass accuracy. Besides complex type glycans on Asn 395, 332, 171, and on the J chain, we observed oligomannosidic glycans on Asn 563, Asn 402 and minor amounts of oligomannosidic glycans on the glycosite Asn 171. Furthermore, hybrid type glycans were found on Asn 402, Asn 171 and in traces Asn 332.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a fast analysis tool employed for the detection of a broad range of analytes. However, MALDI-MS has a reputation of not being suitable for quantitative analysis. Inhomogeneous analyte/matrix co-crystallization, spot-to-spot inhomogeneity, as well as a typically low number of replicates are the main contributing factors. Here, we present a novel MALDI sample target for quantitative MALDI-MS applications, which addresses the limitations mentioned above. The platform is based on the recently developed microarray for mass spectrometry (MAMS) technology and contains parallel lanes of hydrophilic reservoirs. Samples are not pipetted manually but deposited by dragging one or several sample droplets with a metal sliding device along these lanes. Sample is rapidly and automatically aliquoted into the sample spots due to the interplay of hydrophilic/hydrophobic interactions. With a few microliters of sample, it is possible to aliquot up to 40 replicates within seconds, each aliquot containing just 10 nL. The analyte droplet dries immediately and homogeneously, and consumption of the whole spot during MALDI-MS analysis is typically accomplished within few seconds. We evaluated these sample targets with respect to their suitability for use with different samples and matrices. Furthermore, we tested their application for generating calibration curves of standard peptides with α-cyano-4-hdydroxycinnamic acid as a matrix. For angiotensin II and [Glu1]-fibrinopeptide B we achieved coefficients of determination (r2) greater than 0.99 without the use of internal standards.