C-13-NMR RELAXATION STUDY OF HEPARIN-DISACCHARIDE INTERACTIONS WITH TRIPEPTIDES GRG AND GKG

Heparin is a polydisperse sulphated copolymer consisting mostly of 1 - -> 4 linked glucosamine and uronic acid residues, i.e. 2-de oxy-2-sulp hamido-D-glucopyranose 6-sulphate and L-idopyranosyluronic acid 2-sulp hate. C-13 NMR has been used to study the interactions of heparinase-d erived and purified heparin disaccharide with N- and C-terminally-bloc ked tripeptides GRG and GKG. Titration of the disaccharide with peptid e indicates that GRG binds the disaccharide more strongly than does GK G, with interactions in either case being stronger at uronate ring pos itions. In the presence of GRG, a carboxylate pK(a) depression suggest s electrostatic interactions between the arginine guanidinium group an d the uronate carboxylate group. C-13 relaxation data have been acquir ed for all disaccharide and peptide carbons in the presence and absenc e of GRG and GKG. C-13 relaxation rates for the disaccharide are signi ficantly faster in the presence of peptide, especially with GRG. Analy sis of these relaxation data has been done in terms of molecular diffu sion constants, D-perpendicular to and D-parallel to and an angle alph a between D-parallel to and a molecular frame defined by the moment of inertia tensor calculated for an internally rigid disaccharide. Disac charide conformational space in these calculations has been sampled fo r both uronate half-chair forms (H-2(1) and H-1(2)) and over a range o f glycosidic bond angles defined by motional order parameters and inte r-residue nuclear Overhauser effects (+/-30 degrees from the average). In the absence of peptide, the ratio D-perpendicular to/D-parallel to falls between 0.4 and 0.7; therefore molecular diffusion occurs prefe rentially about D-parallel to, which runs through both disaccharide ri ngs. In the presence of peptide, D-perpendicular to/D-parallel to is d ecreased, indicating that GRG is oriented along D-parallel to and prox imal to the uronic acid ring A model for this is shown.