Six syntheses of gramicidin A have been carried out, each with 90% lC enrichment of a single carbonyl carbon these being the formyl, Val-1I Trp-9, and

By means of 23Na NMR, two ion binding sites were observed in phospholipid-packaged gramicidin channels and the four associated rate constants were approximated. Limits also were placed on a fifth rate constant for an intrachannel ion translocation. By using Eyring rate theory to introduce voltage dependence, these rate constants were used in steady-state-current equations for calculation of gramicidin single-channel currents for two-and three-site models. Calculated single-channel currents are compared with previously published experimental single-channel currents obtained by electrical measurements on Na+ transport across gramicidindoped planar lipid bilayers. The calculated results for the twoand three-site models compare favorably with the experimental results. Accordingly, it is demonstrated that NMR-derived rate constants can be coupled with Eyring rate theory to calculate currents through a transmembrane channel and to do so within levels of variation that compare with the differences obtained on planar lipid bilayers formed with different lipids., effects conductance of cations across planar lipid bilayer membranes (2) by means of discrete increments (3-5). The conductance steps are due to the formation of a continuous transmembrane channel (5, 6), with a single transmembrane channel being capable of conducting 107 ions/sec at 1 M salt, 100 mV transmembrane potential, and 25°C (7,8). This is a single-channel conductance of the order of 10 pS.As proposed in 1971 (9, 10) and now widely accepted (11, 12), the transmembrane channel is formed from the formyl-toformyl hydrogen-bonded dimerization of two helical monomers containing 6.3 residues per turn and a 4 A diameter channel (Fig. 1). The length of the channel is about 26 A (see figure 6 of ref. 13 It has recently been demonstrated that functional gramicidin channels can be packaged in lipid micelles (17,18). This allows for spectroscopic characterization during ion titrations without the development of complicating electrochemical gradients that would make difficult the analysis of cation interactions with channels incorporated into lipid vesicles. Interestingly, the energy of activation for sodium exchange with the micellar channel, at approximately equimolar sodium and channel concentrations, is essentially the same as the overall energy of activation for transport through the channel (19). Accordingly, any other translocation rate for movements within the channel are taken to be faster than the off-rate from the tight-binding site.The first rate analysis of ion transport through the grarnicidin channel was carried out by Lauger (20 to give an x-axis intercept of -K1' when Na concentration is sufficiently greater than the site concentration (24). Fig. 2 shows such a plot for GA with Na concentration ranging Abbreviation: GA, gramicidin A. 2028The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate thi...

Ion-induced chemical shifts in the carbonyl carbon resonances of synthesized ad verified (1-13C)D-Val8 gramicidin A and (1-13C)D-Leu14 gramicidin A are utilized in combination with the previously determined location of the ion binding sites of the gramicidin A channel (using the carbonyls of L-residues) to determine that the helix sense of the gramicidin A channel) is left-handed. Having resolved the handedness issue, the location of the ion binding sites (which are fundamental to understanding the mechanism of ion transport) are further delineated with the results indicating two sites separated by just over 20 A. Furthermore, the demonstration that the divalent barium ion interacts at the binding site while not being transported through the channel is used to argue that the mechanism of monovalent vs. divalent cation selectivity is due to the positive image force contribution to the central barrier.

Circular dichroism and absorption data are presented for low molecular weight poly-L-serine. Ellipticity extrema are observed in 80% trifluoroethanol at 222 and 197 µ with molar ellipticities of -0.99 X 104 and 5.2 X 104, respectively. The data are resolved into a set of component Gaussian functions. Resolved circular dichroism curves are centered at 222 and 197 µ with rotational strengths of -5.6 X 10"40 and 32 X 10"4°, respectively. Resolved absorption curves were obtained at 214, 197, and 184 µ with dipole strengths of 0.9 X 10~39, 2.85 X 10-39, and 6.8 X 10_3S, respectively. The major absorption was in the 184-µ band and appeared to have little or no corresponding rotational strength. The long-wavelength CD band, likely an -* transition, appeared to be at longer wavelengths and narrower than the corresponding absorption curve, in accordance with predictions of Moffitt and Moscowitz for magnetically allowed transitions. The point is made that ,3-type CD patterns are variable as to magnitude and position of the extrema when comparing different polypeptides as well as when studying solvent and temperature effects on the same polypeptide. Comparing the molar ellipticity of the long-wavelength CD band of poly-L-serine to that of L-5-hydroxymethylpyrrolid-2-one and considering the structures involved lead to the conclusion that the hydroxymethyl side chains provide significant perturbations contributing to the rotational strength of the long-wavelength transition. The poly-L-serine data, due to the absence of side-chain absorptions in the wavelength range studied, are presented as a set of reference properties characterizing a ß conformation. X-Raydiffraction studies have demonstrated the occurrence of two types of ordered protein structures, helical and pleated sheet. An a helix has been

Succinyl derivatives of gramicidin were tested for their ability to form channels in planar artificial lipid bilayers. Both N-succinyldeformylgramicidin methyl ester and charged O-succinylgramicidin formed channels, but the channels had markedly different sizes and lifetimes. This implies that gramicidin forms channels by end-to-end association. However, the doubly charged N,O-bissuccinyldeformylgramicidin was inactive, which suggests that only end-to-end association of gramicidin may result in channel formation.