In this review we intend to provide a relatively comprehensive summary of the work of supramolecular hydrogelators after 2004 and to put emphasis particularly on the applications of supramolecular hydrogels/hydrogelators as molecular biomaterials. After a brief introduction of methods for generating supramolecular hydrogels, we discuss supramolecular hydrogelators on the basis of their categories, such as small organic molecules, coordination complexes, peptides, nucleobases, and saccharides. Following molecular design, we focus on various potential applications of supramolecular hydrogels as molecular biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramolecular hydrogelators after they form supramolecular assemblies but prior to reaching the critical gelation concentration because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of molecules, including low molecular weight ones. Finally, we provide a perspective on supramolecular hydrogelators. We hope that this review will serve as an updated introduction and reference for researchers who are interested in exploring supramolecular hydrogelators as molecular biomaterials for addressing the societal needs at various frontiers.

As systemically used therapeutics for treatmenting acute or chronic pains or inflammations, non-steroidal anti-inflammatory drugs (NSAID) also associate with the adverse gastrointestinal and renal effects and cardiovascular risks. Thus, it is beneficial to develop topical gels that selectively inhibit cyclooxygenase-2 (COX-2) for the management of local inflammation. In this work, we demonstrate that the covalent conjugation of D-amino acids to naproxen (i.e., an NSAID) not only affords supramolecular hydrogelators for the topical gels, but also unexpectedly and significantly elevates the selectivity towards COX-2 about 20 times at little expense of the activity of naproxen. This work illustrates a previously unexplored approach that employs D-amino acids for the development of functional molecules that have dual or multiple roles and exceptional biostability, which offers a new class of molecular hydrogels of therapeutic agents.

In the rapidly developing areas of nanobiotechnology, magnetic nanoparticles (MNPs) are one type of the most well-established nanomaterials because of their biocompatibility and the potential applications as alternative contrast enhancing agents for magnetic resonance imaging (MRI). While the development of MNPs as alternative contrast agents for MRI application has moved quickly to testing in animal models and clinical trials, other applications of biofunctional MNPs have been explored extensively at the stage of qualitative or conceptual demonstration. In this critical review, we summarize the development of two straightforward applications of biofunctional MNPs--manipulating proteins and manipulating cells--in the last five years or so and hope to provide a relatively comprehensive assessment that may help the future developments. Specifically, we start with the examination of the strategy for the surface functionalization of MNPs because the applications of MNPs essentially depend on the molecular interactions between the functional molecules on the MNPs and the intended biological targets. Then, we discuss the use of MNPs for manipulating proteins since protein interactions are critical for biological functions. Afterwards, we evaluate the development of the use of MNPs to manipulate cells because the response of MNPs to a magnetic field offers a unique way to modulate cellular behavior in a non-contact or "remote" mode (i.e. the magnet exerts force on the cells without direct contact). Finally, we provide a perspective on the future directions and challenges in the development of MNPs for these two applications. By reviewing the examples of the design and applications of biofunctional MNPs, we hope that this article will provide a reference point for the future development of MNPs that address the present challenges and lead to new opportunities in nanomedicine and nanobiotechnology (137 references).

D-peptides, as the enantiomers of the naturally occurring L-peptides, usually resist endogenous proteases and presumably insensitive to most enzymes. But it is unclear whether or how a phosphatase catalyzes the dephosphorylation from D-peptides. In this work, we examine the formation of the nanofibers of D-peptides via enzymatic dephosphorylation. By comparing the enzymatic hydrogelation of L-peptide and D-peptide based hydrogelators, we find that the chirality of the precursors of the hydrogelators affects little on the enzymatic hydrogelation resulted from the removal of phosphate group from a tyrosine phosphate residue. The attachment of a therapeutic agent (e.g., taxol) or a fluorophore (e.g., 4-nitro-2,1,3-benzoxadiazole (NBD)) to the D-peptide based hydrogelators afford a new type of biostable or biocompatible hydrogelators, which may find applications in intratumoral chemotherapy or intracellular imaging, respectively. This work, as the first comprehensive and systematic study of the unexpected enzymatic dephosphorylation of D-peptides, illustrates a useful approach to generate supramolecular hydrogels that have both biostability and other desired functions.

Selective inhibition of cancer cells remains a challenge in chemotherapy. Here we report the molecular and cellular validation of enzyme-instructed self-assembly (EISA) as a multiple step process for selectively killing cancer cells that overexpress alkaline phosphatases (ALPs). We design and synthesize two kinds of d-tetrapeptide containing one or two phosphotyrosine residues and with the N-terminal capped by a naphthyl group. Upon enzymatic dephosphorylation, these d-tetrapeptides turn into self-assembling molecules to form nanofibers in water. Incubating these d-tetrapeptides with several cancer cell lines and one normal cell line, the unphosphorylated d-tetrapeptides are innocuous to all the cell lines, the mono- and diphosphorylated d-tetrapeptides selectively inhibit the cancer cells, but not the normal cell. The monophosphorylated d-tetrapeptides exhibit more potent inhibitory activity than the diphosphorylated d-tetrapeptides do; the cancer cell lines express higher level of ALPs are more susceptible to inhibition by the phosphorylated d-tetrapeptides; the precursors of d-tetrapeptides that possess higher self-assembling abilities exhibit higher inhibitory activities. These results confirm the important role of enzymatic reaction and self-assembly. Using uncompetitive inhibitors of ALPs and fluorescent d-tetrapeptides, we delineate that the enzyme catalyzed dephosphorylation and the self-assembly steps, together, result in the localization of the nanofibers of d-tetrapeptides for killing the cancer cells. We find that the cell death modality likely associates with the cell type and prove the interactions between nanofibers and the death receptors. This work illustrates a paradigm-shifting and biomimetic approach and contributes useful molecular insights for the development of spatiotemporal defined supramolecular processes/assemblies as potential anticancer therapeutics.

Summary Alkaline phosphatase (ALP), an ectoenzyme, plays important roles in biology. But there is no activity probes for imaging ALPs in live cell environment due to the diffusion and cytotoxicity of current probes. Here we report the profiling of the activities of ALPs on live cells by enzyme-instructed self-assembly (EISA) of a D-peptidic derivative that forms fluorescent, non-diffusive nanofibrils. Our study reveals the significantly higher activities of ALP on cancer cells than on stromal cells in their co-culture and shows an inherent and dynamic difference in ALP activities between drug sensitive and resistant cancer cells or between cancer cells with and without hormonal stimulation. Being complementary to genomic profiling of cells, EISA, as a reaction-diffusion controlled process, achieves high spatiotemporal resolution for profiling activities of ALPs of live cells at single cell level. The activity probes of ALP contribute to understanding the reversible phosphorylation/dephosphorylation in the extracellular domains that is an emerging frontier in biomedicine.

This review discusses supramolecular biofunctional materials, a novel class of biomaterials formed by small molecules that are held together via noncovalent interactions. The complexity of biology and relevant biomedical problems not only inspire, but also demand effective molecular design for functional materials. Supramolecular biofunctional materials offer (almost) unlimited possibilities and opportunities to address challenging biomedical problems. Rational molecular design of supramolecular biofunctional materials exploit powerful and versatile noncovalent interactions, which offer many advantages, such as responsiveness, reversibility, tunability, biomimicry, modularity, predictability, and, most importantly, adaptiveness. In this review, besides elaborating on the merits of supramolecular biofunctional materials (mainly in the form of hydrogels and/or nanoscale assemblies) resulting from noncovalent interactions, we also discuss the advantages of small peptides as a prevalent molecular platform to generate a wide range of supramolecular biofunctional materials for the applications in drug delivery, tissue engineering, immunology, cancer therapy, fluorescent imaging, and stem cell regulation. This review aims to provide a brief synopsis of recent achievements at the intersection of supramolecular chemistry and biomedical science in hope of contributing to the multidisciplinary research on supramolecular biofunctional materials for a wide range of applications. We envision that supramolecular biofunctional materials will contribute to the development of new therapies that will ultimately lead to a paradigm shift for developing next generation biomaterials for medicine.

Peptides made of d-amino acids, as the enantiomer of corresponding l-peptides, are able to resist proteolysis. It is, however, unclear or much less explored whether or how d-amino acids affect the cellular response of supramolecular nanofibers formed by enzyme-triggered self-assembly of d-peptides. In this work, we choose a cell compatible molecule, Nap-l-Phe-l-Phe-l-pTyr (LLL-1P), and systematically replace the l-amino acids in this tripeptidic precursor or its hydrogelator by the corresponding d-amino acid(s). The replacement of even one d-amino acid in this tripeptidic precursor increases its proteolytic resistance. The results of static light scattering and TEM images show the formation of nanostructures upon the addition of alkaline phosphatase, even at concentrations below the minimum gelation concentration (mgc). All these isomers are able to form ordered nanostructures and exhibit different morphologies. According to the cell viability assay on these stereochemical isomers, cells exhibit drastically different responses to the enantiomeric precursors, but almost same responses to the enantiomeric hydrogelators. Furthermore, the different cellular responses of LLL-1P and DDD-1P largely originate from the ecto-phosphatases catalyzed self-assembly of DDD-1 on the surface of cells. Therefore, this report not only illustrates a new way for tailoring the properties of supramolecular assemblies, but also provides new insights to answering the fundamental question of how mammalian cells respond to enzymatic formation of nanoscale supramolecular assemblies (e.g., nanofibers) of d-peptides.