Polycrystalline samples of iron-containing ferrichrome A, a cyclic hexapeptide obtained from the fungus Ustillago sphaerogena, have been investigated by paramagnetic resonance. Spectra were obtained at several temperatures between 300° and 1°K; a prominent line of 400-Oe width located at g=4.3 was observed at all temperatures, while at 1°K additional resonances at g values of 9.6, 1.3, and 1.0 were observed. The spectra are interpreted by assuming a spin Hamiltonian containing crystal-field terms large compared with the Zeeman splittings; the crystal-field situation is intermediate between the case of axial symmetry, with H=D[Sz2−(1/3)S(S+1)]+gβS·H and a model proposed by Castner, Newell, Holton, and Slichter to explain certain iron resonances occurring at g=4.3, with H=E(Sx2−Sy2)+gβS·H. We have computed g values, energy eigenvalues, and eigenfunctions to be expected for the region between these two extremes, and the results should be useful in interpreting similar spectra due to iron situated in strong crystal fields of low symmetry.

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Wiedemann and Zinn 1 have recently reported unusual Mossbauer spectra for Er 3+ in ErFe0 3 , at sample temperatures below the N£el temperature (4.3°K) 2 of the erbium sublattice (the ferromagnetic iron-lattice Curie temperature is 3 640°K). The Mossbauer transition employed was the 2 + -0 + , 80.6-keV gamma transition of Er 166 . We show below that these spectra may be readily interpreted in terms of spin-relaxation effects. The ErFeO s system, therefore, appears to be the first reported example of such behavior in the Mossbauer-effect (ME) spectra from a magnetically ordered system.The spin-relaxation interpretation differs from that of Wiedemann and Zinn, who suggested that there are several magnetically inequivalent Er 3+ sites in ErFe0 3 and that the effective magnetic fields acting on the erbium nuclei in the various sites have different temperature dependences, thus leading to the observed spectra. 1 However, crystal structure, 4 mag-(1965) [translation: Opt. Spectry. (USSR) Jj), 252 (1965)].10 The phase-coherent detection, made possible by the coherence between the light modulation and the applied rf field, makes the effective bandwidth much smaller, and hence the detection much easier than in the detection of incoherent light beats as performed by A. T. . netic structure, 2 and recent ME spectra 5 of Dy 3+ in isomorphous DyFe0 3 argue for at most two magnetically inequivalent Er 3+ sites. The magnetic and Mossbauer data are, in fact, well represented by a single magnetic rare-earth site. With this assumption relaxation spectra were computed and excellent agreement between experiment and theory was obtained.In order to discuss the rare-earth hyperfine fields, it is first necessary to decide on a model for the pertinent electronic properties of the magnetic erbium ions. In the temperature range of interest, the erbium ions are exposed to an exchange field made up of two contributions: (1) an effectively constant exchange field from the iron atoms, and (2) at T^ T c (Er 3+ ), an induced molecular field in the erbium sublattice arising from exchange between the rareearth ions. Near the transition temperature, the constant iron field dominates the erbium exchange field. The induced field in the erbium sublattice which gives rise to collective Recent low-temperature Er 166 Mossbauer data, reported by Wiedemann and Zinn in a study of ErFe0 3 , are interpreted in terms of spin-relaxation effects. This appears to be the first time relaxation effects have been observed in the Mossbauer spectra from a magnetically ordered system. 949

The adhesion forces between various surfaces were measured using the "surface forces apparatus" technique. This technique allows for the thickness of surface layers and the adhesion force between them to be directly measured in controlled vapor or liquid environments. Three types of biological surfaces were prepared by depositing various lipid-protein monolayers (with thicknesses ranging from 1 to 4 nm) on the inert, molecularly smooth mica surface: (i) hydrophobic lipid monolayers; (ii) amphiphilic polyelectrolyte surfaces of adsorbed polylysine; and (iii) deposited bacterial S-layer proteins. The adhesion, swelling, and wetting properties of these surfaces was measured as a function of relative humidity and time. Initial adhesion is due mainly to the van der Waals forces arising from nonpolar (hydrophobic) contacts. Following adhesive contact, significant molecular rearrangements can occur which alter their hydrophobic-hydrophilic balance and increase their adhesion with time. Increased adhesion is generally enhanced by (i) increased relative humidity (or degree of hydration); (ii) increased contact time; and (iii) increased rates of separation. The results are likely to be applicable to the adhesion of many other biosurfaces, and show that the hydrophobicity of a lipid or protein surface is not an intrinsic property of that surface but depends on its environment (e.g., on whether it is in aqueous solution or exposed to the atmosphere), and on the relative humidity of the atmosphere. It also depends on whether the surface is in adhesive contact with another surface and-when considering dynamic (nonequilibrium) conditions-on the time and previous history of its interaction with that surface.