X-Ray Structure of a Self-Assembled Gelating Fiber

摘要

Almost seventy years ago, chemist Dorothy Jordan noted that "gels are easier to recognize than to define". We will attempt a definition nonetheless: Gels are semi-rigid colloidal systems rich in liquid. Protoplasm ranks as the most widespread exampleof this peculiar state of matter. A gel is formed by a molecular network, often fibrous in nature, that entrains the dispersing medium within its capillary spaces. Most organic gelators are polymeric (for example, gelatin, poly(acrylic acid), and hyaluronic acid), but small molecules (for example, cholesteryl 4-(2-anthryloxy)butanoate, 5-hexadecyl-2,4,6-triaminopyrimidine, N-octylgluconamide, and ar-borols) can induce gelation as well. We have long been interested in a little-known but remarkable gelator, dibenzoyl-L-cystine (DBC). Although most gels require 1-10% gelator, DBC in water rigidifies at concentrations as low as 2 mM (0.1%). While attempting to learn how such miniscule quantities of DBC manage to self-assemble into a macroscopic network, weperformed an X-ray analysis of a gel "fiber". Structural data at the atomic level is virtually unavailable for gel networks, and it is only through good fortune that these data became accessible. But before giving the details, let us summarize the properties of acylcystines in water.