New textbooks at all levels of chemistry appear with great regularity. So me fields such as basic biochemistry, organic reaction mechanisms, weIl represented by many excellent and chemical thermodynamics are texts, and new or revised editions are published sufficiently often to keep up with progress in research. However, some areas of chemistry, especially many of those taught at the graduate level, suffer from areal lack of up-to-date textbooks. The most serious needs occur in fields that are rapidly changing. Textbooks in these subjects usually have to be written by scientists actually involved in the research that is advancing the field. It is not often easy to persuade such individuals to set time aside to help spread the knowledge they have accumulated. Our goal, in this series, is to pinpoint areas of chemistry where recent progress has outpaced what is covered in any available textbooks, and then seek out and per suade experts in these fields to produce relatively concise but instructive introductions to their fields. These should serve the needs of one semester or one quarter graduate courses in chemistry and biochem­ istry. In some cases the availability of texts in active research areas should help stimulate the creation of new courses. Charles R. Cantor vii Preface to the Third Edition It was over 100 years ago that Emil Fischer postulated his ingenious "lock-and-key" principle, whieh was subsequently applied to the devel­ opment of a modern theory of enzyme catalysis.