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Decorin is a small leucine-rich repeat proteoglycan linked to a single chondroitin or dermatan sulfate (CS/DS) glycosaminoglycan (GAG) chain depending on tissue expressed. Determination of molecular characteristics of CS/DS, is essential for understanding decorin functions. CS/DS analysis includes the identification of glycoforms obtained after detachment from coreprotein followed by collecting information upon epimerization, sulfate content and its distribution. For this purpose, one of the most efficient methods is electrospray ionization (ESI) tandem mass spectrometry (MS/MS). Here we present an approach for structural investigation of human skin decorin CS disaccharides based on combining a specific enzyme digestion with negative ion mode ESI MS-screening followed by fragmentation analysis by collision-induced dissociation (CID) MS/MS. The obtained results showed clearly that human decorin encompasses oversulfated CS motifs in which the site of oversulfation is at glucuronic acid. Moreover, it was possible to demonstrate that the presence of regular and/or irregular GAG domains, which play a critical biological role, can be directly documented by ion sequencing in tandem MS experiments. [ABSTRACT FROM AUTHOR]