A series of helical structures for gramicidin A, with alternating L and D residues, are characterized as to number of residues per turn, atoms in hydrogenbonded rings, and dihedral angles. Because of alternating peptide C-O directions, these helices are capable of forming head-to-head hydrogen-bonded dimers with the capacity of functioning as transmembrane channels. The dimers are characterized as to channel length, pore size, and expected ion selectivity.In a test of the proposed head-to-head association for channel formation, the malonyl dimer [NN'-(dideform yl gramicidin A)-malonamideJ was synthesized. The chemical and conformational integrity of the product was verified by nuclear magnetic resonance; in lipid bilayer studies, the dimer was found to be a potent mediator of ion conductance with the predicted concentration dependence.Thus, the results on malonyl gramicidin A prove headto-head association in formation of the transmembrane channel, and the results are consistent with the specific geometrical configuration involved in head-to-head dimerization of lr(L,D) helices. At this stage, the action of gramicidin A on membranes with lipid-layer thicknesses of 30 A or less can best be understood in terms of the 7r(LD) helix with 6.3 residues per turn.On the basis of the Pauling-Corey-Donohue postulates for polypeptide structure (1-4), the conformational energy diagrams for backbone dihedral angles of polypeptides (5, 6), and the characteristics of gramicidin A-mediated ion conductance across lipid bilayers (7, 8,*), coupled with the significant fact that the amino acids in this peptide are in alternating I-D sequence (9, 10), a left-handed 2r(LD) helix has been proposed for the gramicidin A transmembrane channel (11), which has the unusual property of being capable of head-to-head dimerization. The head-to-head hydrogen-bonded association of two helices is possible because the peptide C-O bonds alternate in direction, with orientations primarily parallel and antiparallel to the helix axis. In this paper we generalize the possibilities of head-to-head association of left-handed gramicidin A helices of the TK(LD) type and demonstrate, by examining the lipid bilayer activity and NMR spectrum of a derivative obtained by chemically coupling the a-amino moieties of two deformyl gramicidin A molecules (i.e., by forming the malonamide dimer), that the transmembrane channel is formed by head-to-head association.THE SET OF 7(L.D) HELICES The sequence of gramicidin A, HCO-iVal-Gly-L-Ala-Trp-D-Leu-i-Trp-NHCH2CH20H, with its alternating L and D residues, makes possible a set of helices that differ from all previously described helices in that the peptide C-O bond vectors alternate with components parallel and antiparallel to the axis of the helix. If we define the direction of the helix axis vector from the amino to the carboxyl terminus, the C-O moieties of the L residues in lr(LD) helices of gramicidin A have components parallel to this vector, whereas for the D residues, they are antiparallel to it. In the a-...

We report that a lipoprotein-based nanoplatform generated by conjugating tumor-homing molecules to the protein components of naturally occurring lipoproteins reroutes them from their normal lipoprotein receptors to other selected cancer-associated receptors. Multiple copies of these targeting moieties may be attached to the same nanoparticle, or a variety of different targeting moieties can be attached. Such a diverse set of tumor-homing molecules could be used to create a variety of conjugated lipoproteins as multifunctional, biocompatible nanoplatforms with a broad application to both cancer imaging and treatment. The same principle can be applied to imaging and treatment of other diseases and for monitoring specific tissues. To validate this concept, we prepared a low-density lipoprotein (LDL)-based folate receptor (FR)-targeted agent by conjugating folic acid to the Lys residues of the apolipoprotein B (apoB)-100 protein. To demonstrate the ability of the lipoprotein-based nanoplatform to deliver surfaceloaded and core-loaded payloads, the particles were labeled either with the optical reporter 1,1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine that was intercalated in the phospholipid monolayer or with the lipophilic photodynamic therapy agent, tetra-tbutyl-silicon phthalocyanine bisoleate, that was reconstituted into the lipid core. Cellular localization of the labeled LDL was monitored by confocal microscopy and flow cytometry in FR-overexpressing KB cells, in FR-nonexpressing CHO and HT-1080 cells, and in LDL receptor-overexpressing HepG 2 cells. These studies demonstrate that the folic acid conjugation to the Lys side-chain amino groups blocks binding to the normal LDL receptor and reroutes the resulting conjugate to cancer cells through their FRs.folate receptor ͉ low-density lipoprotein receptor ͉ biocompatible ͉ optical imaging ͉ photodynamic therapy

To prepare near-infrared fluorescence imaging and photodynamic therapy agents targeted at glucose transporters, pyropheophorbide 2-deoxyglucosamide (Pyro-2DG) was synthesized and evaluated in a 9L glioma rat model. Fluorescence imaging studies demonstrate that Pyro-2DG is selectively accumulated in the tumor. Upon its photoactivation, we demonstrate that this agent efficiently causes selective mitochondrial damage to the region of a tumor that was photoirradiated after administration of this agent, but does not affect tissues photoirradiated in the absence of the agent or tissues treated with the agent that are not photoirradiated. Preliminary confocal microscopy studies suggest that Pyro-2DG is delivered and trapped in tumor cells via the GLUT/hexokinase pathway and therefore is useful both as a tumor-targeted NIR fluorescence imaging probe and as a PDT agent for the destruction of cancer.