A cyclic disulfide heptadecapeptide (TIP17ox; 2) derived from the lectin-like 17-amino acid domain of human tumor necrosis factor-alpha [TNF-alpha (100-116)] was synthesised and demonstrated to bind specifically to N,N-diacetylchitobiose, a disaccharide present in many glycan structures of glycoproteins. Although the TIP domain forms a loop structure in the native TNF-alpha protein, we show in this study by high-resolution ESI-FTICR mass spectrometry that a homologous linear heptadecapeptide (TIP17rd; 1) binds with comparable affinity to chitobiose, suggesting that cyclisation is not essential for carbohydrate binding. ESI-FTICR-MS was used as an efficient tool for the direct molecular characterisation of TIP peptide-carbohydrate complexes. The specific binding of the TNF-TIP domain to chitobiose and other carbohydrate motifs in glycoproteins may explain the high proteolytic stability of these peptides in biological fluids. A considerably higher proteolytic stability in human plasma was found by mass spectrometric analysis for the cyclic TIP peptide 2, compared to the linear peptide 1. Furthermore, affinity-proteomics studies using immobilised cyclic TIP peptide 2 provided the identification of specific interacting glycoproteins in plasma.