15 páginas, 7 figuras, 1 tabla.The binding site and backbone dynamics of a bioactive complex formed by the acidic fibroblast growth factor (FGF-1) and a specifically designed heparin hexasaccharide has been investigated by HSQC and relaxation NMR methods. The comparison of the relaxation data for the free and bound states has allowed showing that the complex is monomeric, and still induces mutagenesis, and that the protein backbone presents reduced motion in different timescale in its bound state, except in certain points that are involved in the interaction with the fibroblast growth factor receptor (FGFR).This work was supported by the Direccio´ n General de Investigación Científica y Técnica (Grants BQU2000-1501-C02-01, BQU2002-0374, and
BQU2003-03550-C03-01).Peer reviewe