The glycosaminoglyeans heparin and heparan sulfate (HS) bind to fibroblast growth factor FGF1 and promote its dimerization, a proposed prerequisite for binding to a cellular receptor and triggering mitogenic signals. The problem of minimal structural requirements for heparin/HS sequences to bind FGF1 was approached by surface plasmon resonance (SPR), NMR spectroscopy, and MALDI mass spectrometry studies using the three synthetic tetrasaccharides GlcNSO(3)60R-ldoA2SO(3)-GleNSO(3)60R'-IdoA2SO(3)OPr (AA, R = R' = SO3; BA, R = H, R' = SO3; BB, R = R' = H; Pr, propyl). AA and BA significantly interact with the protein, whereas BB is practically inactive. The NNR spectra show that, whereas the interaction of AA primarily involves the GlcNSO(3)6SO(3)IdoA2SO(3) disaccharide moiety at its nonreducing end, residues at both the nonreducing (NR) and reducing side (R) appear to be involved in the weaker complex of BA. Furthermore, MALDI experiments show that, in addition to 1:1 protein:tetrasaccharide complexes, AA and BA are able to form 2:1 complexes, indicating that heparin/ HS-induced dimerization of FGF1 requires only one 6-OSO3 group per tetrasaccharide. (C) 2002 Elsevier Science (USA).