Self-assembled peptide amphiphile nanofibers have been investigated for their potential use as in vivo scaffolds for tissue engineering and drug delivery applications. We report here the synthesis of magnetic resonance (MR) active peptide amphiphile molecules that self-assemble into spherical and fiber-like nanostructures, enhancing T(1) relaxation time. This new class of MR contrast agents can potentially be used to combine high-resolution three-dimensional MR fate mapping of tissue-engineered scaffolds with targeting of specific cellular receptors.

Inspired by biology, contemporary chemistry is challenged with the synthesis of architecturally defined functional structures that are much larger than ordinary molecules. [1] One emerging strategy is the use of noncovalent interactions in polymolecular assemblies to craft the shapes, dimensions, and functions of nanostructures. The scientific goal is to gain access to unknown static and dynamic functions with supramolecular systems which could be designed with the same ease that small molecules are now synthesized. Supramolecular pores, [2] tubes, [3] cylinders, [4] helices, [5] and mushroomlike noncentrosymmetric clusters [6] have been among the targets of recent work in this area. We report here a torsional strain mechanism to tune the pitch of micrometer-long helical assemblies in the range of tens to hundreds of nanometers.Helices in biology, such as double-helical DNA, protein coiled coils, and twisted b sheets, [7] have inspired an extensive amount of work on synthetic helical nanostructures.[5] Artificial structures containing peptide b sheets have been of particular interest [8,9] since they only require short amino acid sequences to form and also because of their relevance in diseases linked to amyloid fibrils. During our recent work on the functionalization of cylindrical nanofibers formed by tripeptide amphiphiles in organic solvents (such as cyclohexyl chloride), [9,10] we found that simple modification of the compounds led to nanostructures with dramatically different morphologies.Compounds 1 and 2 were synthesized with an acetate and a dimethyl acetate end group, respectively, and we used atomic force microscopy (AFM) to examine the morphology of the supramolecular aggregates they form. At a concentration of 1 % (by weight) both compounds dissolve in cyclohexyl chloride at about 80 8C and form translucent selfsupporting gels upon cooling to room temperature. AFM studies of the diluted gels dried on silicon substrates revealed straight cylindrical fibers for compound 1 (Figure 1 a). In contrast, AFM images of the aggregates formed by molecules of compound 2 show helices with a regular pitch (Figure 1 b) of 22(AE2) nm (Figure 1 c), and the orientation of the height contour clearly indicates these helices have a left-handed sense.The significant change in the morphology of the assembly when the acetate end group in 1 was replaced by a dimethyl acetate substituent in 2 suggested that a bulkier substituent at the terminus of the alkyl segment causes twisting of the initially cylindrical assemblies. To gain a mechanistic view of this process, we considered first the driving force for aggregation in these systems. In a low polarity solvent, the amphiphilic molecules studied here assemble as a result of solvophobic interactions as well as the formation of an intermolecular b sheet between the peptide segments.[9] The charged tetraalkylammonium head groups are buried inside the nanofibers as a result of their low affinity for the organic solvent, and the less polar tails are present on the surfaces of t...

Current interest in biomaterials for tissue engineering and drug delivery applications have spurred research into self-assembling peptide amphiphiles (PAs). Nanofiber networks formed from self-assembling PAs can be used as biomaterial scaffolds with the advantage of specificity by the incorporation of peptide-epitopes. Imaging the materials noninvasively will give information as to their fate in vivo. We report here the synthesis and in vitro MR images of self-assembling peptide amphiphile contrast agents (PACAs) that form nanofibers. At 400 MHz using a 0.1 mM Gd(III) conjugate of the PA we observed a T(1) three times that of a control gel. The PA derivative was doped into various epitope bearing PA solutions and upon gelling resulted in a homogeneous biomaterial as imaged by MRI.

The precise structural control is known for self-assembly into closed spherical structures (e.g., micelles), but similar control of open structures is much more challenging. Inspired by natural tobacco mosaic virus, we present the use of a rigid-rod template to control the size of a one-dimensional self-assembly. We believe that this strategy is novel for organic self-assembly and should provide a general approach to controlling size and dimension.

The majority of clinically used contrast agents (CAs) for magnetic resonance imaging have low relaxivities and thus require high concentrations for signal enhancement. Research has turned to multivalent, macromolecular CAs to increase CA efficiency. However, previously developed macromolecular CAs do not provide high relaxivities, have limited biocompatibility, and/or do not have a structure that is readily modifiable to tailor to particular applications. We report a new family of multivalent, biomacromolecular, genetically engineered protein polymer-based CAs; the protein backbone contains evenly spaced lysines that are derivatized with gadolinium (Gd(III)) chelators. The protein's length and repeating amino acid sequence are genetically specified. We reproducibly obtained conjugates with an average of 8 -9 Gd(III) chelators per protein. These multivalent CAs reproducibly provide a high relaxivity of 7.3 mM -1 s -1 per Gd(III) and 62.6 mM -1 s -1 per molecule. Furthermore, they can be incorporated into biomaterial hydrogels via chemical crosslinking of remaining free lysines, and provide a dramatic contrast enhancement. Thus, these protein polymer CAs could be a useful tool for following the evolution of tissue engineering scaffolds.One significant barrier to the development of new generations of biocompatible materials, particularly tissue engineering hydrogels, is an inability to non-invasively evaluate the properties and performance of the biomaterial over time (1-6). Magnetic Resonance Imaging (MRI) is capable of whole animal or human imaging at high spatial and temporal resolution, and is an ideal modality for evaluating tissue engineering scaffolds in vivo (7-11). Exogenous contrast agents (CAs) increase the relaxation rate (1/T 1 ) of water protons and therefore improve image contrast. However, current clinically used CAs have low relaxivities (3-7 mM -1 s -1 ) (12) and thus must be used at high concentrations for useful MRI signal enhancement (12).T 1 CAs provide positive contrast by employing a paramagnetic ion (typically gadolinium, Gd (III)). The efficacy of a contrast agent is dominated by three parameters: q, the number of We have designed, synthesized and characterized a macromolecular T 1 contrast agent based on artificial proteins. The backbone of these multivalent MRI contrast agents is derived from E. coli expression of monodisperse protein polymers. Gd(III) chelators were chemically conjugated to the backbone to create contrast agents that display high relaxivity in solution and when incorporated into a hydrogel. The covalent incorporation of these protein polymer contrast agents into a gel allows the potential for imaging tissue engineering scaffolds. Here, we report the design, synthesis, and characterization of a novel family of multivalent, macromolecular CAs based on genetically engineered proteins with repetitive sequences that form the backbone for subsequent chemical modification with Gd(III) chelators. These "protein polymer" CAs have high relaxivities in aqueous solution. Mo...