High-resolution NMR and molecular modeling studies on complex carbohydrates: characterization of O-.beta.-D-Gal-(1.fwdarw.3)-O-.beta.-D-Gal-(1.fwdarw.4)-O-.beta.-D-Xyl-(1.fwdarw.0)-L-Ser, a carbohydrate-protein linkage region fragment from connective tissue proteoglycans