The nature of water -macromolecule interactions in aqueous Many of these studies have used biopolymers such as agarose model polymers has been investigated using quantitative measureand albumin as prototype systems, which reproduce many of ments of magnetization transfer. Cross-linked polymer gels comthe features of relaxation found in tissue. Unfortunately these posed of 94% water, 3% N,N-methylene-bis-acrylamide, and 3% molecules are inherently complex and interpretations of the functional monomer (acrylamide, methacrylamide, acrylic acid, data are not always straightforward. They are also limited in methacrylic acid, 2-hydroxyethyl-acrylate, or 2-hydroxyethyl-methterms of what variations of composition can be achieved.
acrylate) were studied. Water -macromolecule interactions were
Although water relaxation in solutions of diamagnetic proteins modified by varying the pH and specific functional group on the is relatively well documented, there is a growing realization monomer. The magnitudes of the interactions were quantified by that tissue relaxation cannot be considered a simple sum of the measuring the rate of proton nuclear spin magnetization exchange effects of isolated macromolecules (7). Instead the dominant between the polymer matrix and the water. This rate was highly sensitive to the presence of carboxyl side groups on the macromole-relaxation effect comes from supramolecular organization, cule. However, the dependence of the rate on pH was not consistent such as distinguishes solutions of albumin from their heatwith simple acid/base-catalyzed chemical exchange, and instead, treated and cross-linked counterparts ( 3). The mechanism by the data suggest that multiequilibria proton exchange, a wide distriwhich this magnetization transfer occurs, and its dependence bution in surface group pK values, and/or a macromolecular strucon the rigidity and composition of the matrix, is not well tural dependence on pH may play a significant role in magnetization understood. Two competing views have emphasized either the transfer in polymer systems. These model polymer gels afford useful role of proton chemical exchange (8, 9) or that of interfacial insights into the relevance of chemical composition and chemical solvent protons (''bound water'') (3) as conduits for spin dynamics on relaxation in tissues. แญง 1998 Academic Press exchange. We have therefore sought to study simple polymer