X-ray Scattering Studies of Maquette Peptide Monolayers. 1. Reflectivity and Grazing Incidence Diffraction at the Air/Water Interface

Abstract

We present isotherm and X-ray reflectivity (XR) measurements from Langmuir monolayers of a de novo synthetic di-α-helical peptide, consisting of two identical 31-residue, mostly α-helical peptide units joined by a disulfide bond at their amino-termini. Fitting the XR data to slab models shows that the dihelices lie in the plane of the interface at low pressures. The monolayers were insufficiently stable for study at high pressures, but Langmuir films based on a derivative of the peptide alkylated at its amino termini permitted investigations over a larger range of pressures. We observed an orientational transition, in which the α-helices begin by lying in the plane of the interface at low surface pressures and orient themselves approximately normal to the interface at high pressures. We draw the same conclusions from the XR data when we analyze it using box refinement, an iterative, model-independent method for recovering structure from XR data. Mixtures of these palmitoylated peptides with a fatty acid (palmitic acid) or a phospholipid (DLPE) behaved similarly. None of the systems produced peaks in the grazing incidence diffraction signal indicative of long-range ordering of the upright α-helices. Off-specular in-plane scattering measurements based on the difference signal between the peptide/DLPE mixture and pure DLPE suggest that the peptide achieves only liquidlike order within the plane. We discuss the implications and prospects for future work on designed peptide monolayers incorporating prosthetic groups that could be used to study electron transfer in proteins and provide a basis for biomolecular electronics applications.

Specular neutron reflectivity and the structure of artificial protein maquettes vectorially oriented at interfaces

“…BBC16 monolayers were already characterized by chemical [12] and x-ray reflectivity methods [7,13], making them good systems for further structural investigation by neutron reflectivity. Key aspects of the peptide design and x-ray reflectivity results are summarized in Figs.…”

“…The synthetic unit [CD 3 ͑CD 2 ͒ 14 -CGGGE IWKLH EEFLK KFEEL LKLHE ERLKK L-CONH 2 ] is homodimerized to form BBC16 (Fig. 1) [7,12]. In a series of singly labeled peptides, a Leucine residue at position 9, 14, 21, or 28 in the ␣-helix's sequence was replaced with Leucine with a perdeuterated ͑D 10 ͒ sidechain.…”

“…DLPE (dilauroyl phosphatidylethanolamine) with perdeuterated ͑D 46 ͒ lauroyl chains (D-DLPE) was purchased by custom order from Avanti Polarlipids (Alabaster, AL) as a 1 mg/mL chloroform solution and used as received. Mixed monolayers in a 2:1 ratio of DLPE:BBC16 were spread as described previously [7], but with only a single stage deposition. The resulting isotherms were similar to those published [7].…”

“…Mixed monolayers in a 2:1 ratio of DLPE:BBC16 were spread as described previously [7], but with only a single stage deposition. The resulting isotherms were similar to those published [7].…”

“…For example, previously designed four-helix bundle motifs [1], with apposed histidine residues at selected positions within the bundle for the bishistidyl coordination of metallo-porphyrin based prosthetic groups, can be used to generate interesting electron transfer [2][3][4] and nonlinear optical properties [5,6] at the molecular level. These can be vectorially oriented at a macroscopic interface also through their design [7]. However, one must necessarily ascertain whether the molecular structures as designed are indeed manifest at the interface in order to ensure that the intended molecular functionality can result in the desired material properties.…”