Optical characteristics of carboxyl group in relation to the circular dichroic properties and dissociation constants of glycosaminoglycans.

Circular dichroism studies of glycosaminoglycans including chemically transformed heparins at various pH values reveal that carboxyl chromophore plays an important role in the dichroic behavior of the polymers. With decreasing pH, iduronic acid-containing glycosaminoglycans show increased negative ellipticity near 220 nm whereas the polymers containing glucuronic acid display enhanced negative dichroism near 230 nm and decreased negative dichroism around 210 nm. The pH-dependent optical properties have been utilized to determine the pKa values of uronic acid moieties. The acid strengths of the iduronic acid-containing glycosaminoglycans are inherently smaller than those of corresponding glucuronic acid-containing polymers. Glycosaminoglycans in which the amino sugars are linked with iduronic acid display a very weak n leads to pi* amide transition, or none. The rotational strength at 210 nm of these polymers is largely due to iduronic acid moieties. The CD variations above 200 nm with change in pH do not indicate any major conformational transition of the molecules but the difference between dermatan sulfate and heparin can be attributed to difference either in iduronic acid conformation or in intersaccharide linkages.