Vol. 61, No. 4, 1974 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS NONACTIN AND ITS ALKALI COMPLEXES--A RAMAN SPEC TROSCOPIC STUDY Irvin M. Asher, George D. J. Phillies and H. Eugene Stanley Harvard-MIT Program in Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA 02139 Received November 1,1974 Summary: The first laser Raman spectra of the ionophorous antibiotic non- aetin and its corrplexes with Na +, K ÷, Rb +, Cs +, and NH 4 + are reported. Changes in the Raman spectra indicate that the crystalline conformation of nonactin differs appreciably from those which it assumes in solution. In the alkali ion complexes, the frequency of the ester carbonyl stretch varies with the unhydrated radius R, approxirr~tely as 1/R. The frequency of this mode in the nonactin-NH4 + complex is significantly higher than would be expected for an alkali ion of the same radius. Other spectral changes attendant on complex- ation are discussed. Several maerocyclic antibiotics are known to facilitate selective ion trans- port in biological and model systems 1. Because of their utility as models of selectivity in biological membranes, these antibiotics have been intensively investigated by a variety of techniques. The actins are of particular interest because their internal cavities are sufficiently flexible to accommodate ions of different radii 2' 3. Complexation is nonetheless selective; for example, Li + is not appreciably complexed by nonactin, while the Na +, K +, Rb +, and Cs + complexes have formation constants (in methanol) of 240, 3800, 3400, and 900 liters mole -1 respectively 4. Uncorrplexed nonactin, shown in Fig. la, contains alternating ester link- 5 ages and tetrahydrofuran rings . The analogs monactin, dinactin, trinactin, and tetranactin have increasing numbers of the asterisked methyl groups (Fig. la) replaced with ethyl groups. The origin of the biological activity of nonactin and its ability to extract salts of monovalent cations and lipid soluble anions (e. g., picrate) into organic solvents is suggested by the X-ray crys- tallographic structure of the nonactin-K + complex 6 (Fig. lb). The ester and "Work supported in part by the Research Corporation, the National Science Foundation, the National Heart and Lung Institute (HL 14322-03; R.W. Mann, Principal Investigator), and a National Institutes of Health Biomedical Sciences Support Grant (NIH-5-505-RR07047-08) to MIT. Copyright © 1974 by Academic Press, Inc. All rights o] reproduction in any form reserved. 1356