The apparent biocompatibility of many synthetic and natural aqueous gel materials has encouraged their study and testing for a wide variety of biomedical device applications. Many of the physical and in particular the interfacial properties of such gels are highly dependent on the organization of water within and on the surface of the hydrogel. Water is an important component of such gels, varying from about 30 to nearly 100 wt-%, yet the role of water in the gels has been virtually ignored. This paper briefly reviews the nature of water structure in pure bulk water, in solutions, and a t interfaces. Polywater or anomalous water is also briefly reviewed. Evidence is presented that the water in many hydrogel systems can exist in at least three different, structurally distinct forms. A hypothesis is presented which can be used to evaluate and study the nature of water in bulk hydrogels. Consideration is also given to the role of organized water a t the hydrogel surface on the interfacial properties of such systems.

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To finalize a scheme intended to calculate directly approximate charge distributions for proteins, parameters for a modified PEOE method are obtained for both ionic and aromatic functional groups. For ionic groups, the electrostatic potentials of several ionic molecules (computed with the 6-3 1G** basis set) are used as constraints; for aromatic groups, the experimental gas-phase dipole and quadrupole moments of several aromatic molecules are used as constraints. The electrostatic potentials calculated with the modified PEOE point charges of ionic molecules are quite close to the ab initio electrostatic potential surfaces, and the dipole and quadrupole moments calculated with the modified PEOE charges of aromatic molecules agree well with experimental data. The method enables the charge distribution, described as a set of atom-centered monopoles, to be calculated directly. This avoids assumptions concerning transferability of charges from small model compounds and describes in an approximate fashion the redistribution of charge upon assembly of amino acid residues into polypeptide molecules. The computational requirements for the procedure are negligible. (12) Toluene: Rudolph, H. D.; Dreizler, H.; Jaeschke, A.; Wendling, P. 2. Naturforsch. 1967, 220, 940. o-Xylene: Rudolph, H. D.; Walzer, K.; Krutzik, I. J. Mol. Spectrom. 1973, 47, 314. Phenol: Pedersen, T.; Larsen, N. W.; Nygaard, L. J. Mol. S r r c t . 1969.4, 59. Aniline: Lister, D. G.; Tyler, J. K.; Hog, J. N.; Larsen, N. W. J. Mol. Struct. 1974, 23, 253. Pyridine: Sorensen, G . 0.; Mahler, L.; RastrupAndersen, N. J. Mol. Struct. 1974, 20, 119. 2-Methylpyridine: Dreizler, H.; Rudolph, H. D.; Miider, H. 2. Na-turJorsch. 1970, 25a, 25. 4-Methylpyridine: Rudolph, H. D.; Dreizler, H.; Seiler, H. Z . Naturforsch. 1967, 22a, 1738. Pyrrole: Nygaard, L.; Nielsen, J. T.; Kirchheiner, J.; Maltesen, G.; RastrupAndersen, J.; Ssrensen, G. 0. J. Mol. Slruct. 1%9, 3, 491. N-methylpyrrole: Arnold, W.; Dreizler, H.; Rudolph, H. D.

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