We take great pleasure in presenting Vol. IV of Biological Magnetic Resonance, a series that continues to give us pride. In this volume, we are pleased to have our first chapter on the applications of ESR to problems in medicine, written by Butterfield. Armitage and Otvos describe their extensive Cd-l13 NMR study in a chapter that should delight the spectroscopists as well as the biochemists, since the systems investigated have not yet been modeled. Kaptein presents an eloquent exposition of the principles and applications to biological systems of the photo-CIDNP technique, to which he has made pioneering contributions. Perkins tells everything one always wanted to know about the applications of ring current calculations in structural studies of biological macromolecules. Our philosophy has been, and continues to be, to present topics of current interest by authors who are active in their field, while maintaining the inter national flavor of the series. Ideally, the coverage of each topic should approach that found both in a textbook and in a reference book, rather than being a mere literature review. We are grateful to the authors for their cooperation in this respect. We continue to solicit the comments and suggestions of our readers and our colleagues, and thank those who have already responded, including the reviewers in the periodicals. Lawrence J. Berliner Jacques Reuben ix Contents Chapter 1 Spin Labeling in Disease D. Allan Butterfield 1. Introduction ...................................... . 2. Membrane Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 2 . . . . . . . 2.1. General Principles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. The Erythrocyte Membrane. . . . . . . . . . . . . . . . . . . 4 . . . . .