The cell-penetrating peptide TAT(48-60) induces a non-lamellar phase in DMPC membranes.

Cell-penetrating peptides (CPPs) are short polycationic sequences that can translocate into cells without disintegrating the plasma membrane. CPPs are useful tools for delivering cargo, but their molecular mechanism of crossing the lipid bilayer remains unclear. Here we study the interaction of the HIV-derived CPP TAT (48-60) with model membranes by solid-state NMR spectroscopy and electron microscopy. The peptide induces a pronounced isotropic (31)P NMR signal in zwitterionic DMPC, but not in anionic DMPG bilayers. Octaarginine and to a lesser extent octalysine have the same effect, in contrast to other cationic amphiphilic membrane-active peptides. The observed non-lamellar lipid morphology is attributed to specific interactions of polycationic peptides with phosphocholine head groups, rather than to electrostatic interactions. Freeze-fracture electron microscopy indicates that TAT(48-60) induces the formation of rodlike, presumably inverted micelles in DMPC, which may represent intermediates during the translocation across eukaryotic membranes.     

Unraveling dendrimer translocation across cell membrane mimics

Poly(amidoamine) (PAMAM) dendrimers are promising candidates in several applications within the medical field. However, it is still to date not fully understood whether they are able to passively translocate across lipid bilayers. Recently, we used fluorescence microscopy to show that PAMAM dendrimers induced changes in the permeability of lipid membranes but the dendrimers themselves could not translocate to be released into the vesicle lumen. Because of the lack of resolution, these experiments could not assess whether the dendrimers were able to translocate but remained attached to the membrane. Using quartz crystal microbalance with dissipation monitoring and neutron reflectivity, a structural investigation was performed to determine how dendrimers interact with zwitterionic and negatively charged lipid bilayers. We hereby show that dendrimers adsorb on top of lipid bilayers without significant dendrimer translocation, regardless of the lipid membrane surface charge. Thus, most likely dendrimers are actively transported through cell membranes by protein-mediated endocytosis in agreement with previous cell studies. Finally, the higher activity of PAMAM dendrimers for phosphoglycerol-containing membranes is in line with their high antimicrobial activity against Gram-negative bacteria.     

Gain-of-function analogues of the pore-forming peptide melittin selected by orthogonal high-throughput screening

We recently developed an orthogonal, high-throughput assay to identify peptides that self-assemble into potent, equilibrium pores in synthetic lipid bilayers. Here, we use this assay as a high-throughput screen to select highly potent pore-forming peptides from a 7776-member rational combinatorial peptide library based on the sequence of the natural pore-forming peptide toxin melittin. In the library we varied ten critical residues in the melittin sequence, chosen to test specific structural hypotheses about the mechanism of pore formation. Using the new high-throughput assay, we screened the library for gain-of-function sequences at a peptide to lipid ratio of 1:1000 where native melittin is not active. More than 99% of the library sequences were also inactive under these conditions. A small number of library members (0.1%) were highly active. From these we identified 14 potent, gain-of-function, pore-forming sequences. These sequences differed from melittin in only 2-6 amino acids out of 26. Some native residues were highly conserved and others were consistently changed. The two factors that were essential for gain-of-function were the preservation of melittin's proline-dependent break in the middle of the helix and the improvement and extension the amphipathic nature of the α-helix. In particular the highly cationic carboxyl-terminal sequence of melittin, is consistently changed in the gain-of-function variants to a sequence that it is capable of participating in an extended amphipathic α-helix. The most potent variants reside in a membrane-spanning orientation, in contrast to the parent melittin, which is predominantly surface bound. This structural information, taken together with the high-throughput tools developed for this work, enable the identification, refinement and optimization of pore-forming peptides for many potential applications.     

Mechanisms of the interaction of alpha-helical transmembrane peptides with phospholipid bilayers

The synthetic peptide acetyl-K(2)-G-L(24)-K(2)-A-amide (P(24)) and its analogs have been successfully utilized as models of the hydrophobic transmembrane alpha-helical segments of integral membrane proteins. The central polyleucine region of these peptides was designed to form a maximally stable, very hydrophobic alpha-helix which will partition strongly into the hydrophobic environment of the lipid bilayer core, while the dilysine caps were designed to anchor the ends of these peptides to the polar surface of the lipid bilayer and to inhibit the lateral aggregation of these peptides. Moreover, the normally positively charged N-terminus and the negatively charged C-terminus have both been blocked in order to provide a symmetrical tetracationic peptide, which will more faithfully mimic the transbilayer region of natural membrane proteins and preclude favorable electrostatic interactions. In fact, P(24) adopts a very stable alpha-helical conformation and transbilayer orientation in lipid model membranes. The results of our recent studies of the interaction of this family of alpha-helical transmembrane peptides with phospholipid bilayers are summarized here.     

Cooperative and reciprocal chiral structure formation of an alanine-based peptide confined at the surface of cationic surfactant membranes

The confinement of anionic oligoalanine peptides at the surface of cationic membranes can cooperatively reinforce peptide/peptide interactions and induce secondary-structure formation, and, reciprocally, induce chirality expression of the membrane at the mesoscopic level, thus leading to the formation of three-dimensional chiral fibrillar networks. Such a strong binding effect of peptides with cationic membranes and the resulting cooperative assembly behaviors are observed with two different types of cationic surfactant, namely, two-head two-tail gemini and one-head two-tail surfactants. The ensemble of assembly properties, such as critical micellar concentration (cmc), Krafft temperature (T(k) ), molecular area at the air/water interface, molecular organization (as studied by FTIR attenuated total reflectance (ATR) measurements and small-angle X-ray scattering), and morphology of the aggregates (as observed by optical and electron microscopy studies), are reported. The results clearly demonstrate that the molecular organization and mesoscopic supramolecular structures are controlled by a subtle balance between the peptide/peptide interactions, ionic interactions between the membranes and peptides, and the interactions the between surfactant molecules, which are governed by hydrophobicity and steric interactions. Investigation into such cooperative organization can shed light on the mechanism of supramolecular chirality expression in membrane systems and allow understanding of the structure of peptides in interactions with lipid bilayers.     

Comparison between RGD-peptide-modified titanium and borosilicate surfaces

The use of synthetic peptides containing adhesive sequences, such as the Arg-Gly-Asp (RGD) motif, represents a promising strategy to control biological interactions at the cell–material interface. These peptides are known to improve the tissue–material contact owing to highly specific binding to cellular membrane receptors known as integrins, thereby promoting the adhesion, migration and proliferation of cells. The peptides were coupled to borosilicate glass and titanium surfaces using silanisation chemistry. A tryptophan residue was incorporated into the amino acid sequences of selected peptides to facilitate the detection of the covalently bound peptides. Successful peptide immobilisation was proven by fluorimetric measurements. The confocal imaging analysis suggests a homogeneous distribution of the immobilised peptide across the biomaterial surface. In vitro cell proliferation assays were employed to compare the adhesion potentials of the well-known RGD-containing peptides GRGDSP, GRADSP and RGDS to the three peptides designed by our group. The results demonstrate that the RGD sequence is not necessarily required to enhance the adhesion of cells to non-biological surfaces. Moreover, it is shown that the number of adhering cells can be increased by changes in the peptide hydrophobicity. Changes in the cytoskeleton are observed depending on the type of RGD-peptide modification.     

Effects of synthetic amphiphilic alpha-helical peptides on the electrochemical and structural properties of supported hybrid bilayers on gold

Amphiphilic alpha-helices were formed from designed synthetic peptides comprising alanine, phenylalanine, and lysine residues. The insertion of the alpha-helical peptides into hybrid bilayers assembled on gold was studied by a variety of methods to assess the resulting structural characteristics, such as electrical resistance and molecular orientation. Self-assembled monolayers (SAMs) of dodecanethiol (DDT); octadecanethiol (ODT); and 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) were formed on gold substrates with and without incorporated peptide. Supported hybrid bilayers and multilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were formed on SAMs by the "paint-freeze" method of bilayer formation. Modeling of electrochemical impedance spectroscopy data using equivalent electrochemical circuits revealed that the addition of peptide decreased dramatically the resistive element of the bilayer films while maintaining the value of the capacitive element, indicating successful incorporation of peptide into a well-formed bilayer. Near-edge X-ray absorption fine structure spectroscopy data provided evidence that the molecules in the SAMs and hybrid multilayers were ordered even in the presence of peptide. The peptide insertion into the SAM was confirmed by observing the pi* resonance peak correlating with phenylalanine and a peak in the nitrogen K-edge regime attributable to the peptide bond.     

The interaction of a peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with DPPC model membranes: a DSC study

Depending on their hydrophobicity, peptides can interact differently with lipid membranes inducing dramatic modifications into their host systems. In the present paper, the interaction of a synthetic peptide with a scrambled hydrophobic/hydrophilic sequence (Pro-Asp-Ala-Asp-Ala-His-Ala-His-Ala-His-Ala-Ala-Ala-His-Gly) (PADH) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membranes has been investigated by differential scanning calorimetry (DSC), adopting three different experimental approaches. In the first, the peptide is forced to be included into the hydrocarbon region of the lipid bilayer, by codissolving it with the lipid giving rise to mixed multilamellar vesicles–peptide systems; in the second, this system is passed through an extruder, thus producing large unilamellar vesicles–peptide systems; in the third, it is allowed to interact with the external surface of the membrane.

The whole of the DSC results obtained have shown that the incorporation of the peptide into the lipid bilayer by means of the first method induces a decrease in the enthalpy of the gel–liquid crystal transition of the membrane and a shift of the transition to the lower temperatures, thus resembling, in spite of its prevalently hydrophilic nature, the behavior of transbilayer hydrophobic peptides. The extrusion of these systems creates unilamellar vesicles free of peptides but of smaller size as evidenced by the decreased cooperativity of the transition. The peptide, added externally to the DPPC model membrane, has no effect on the phase behavior of the bilayer.

These findings suggest that the effect of the interaction of scrambled hydrophobic/hydrophilic peptides into lipid bilayers strongly affects the thermotropic behavior of the host membrane depending on the preparation method of the lipid/peptide systems. The whole of the results obtained in the present paper can be useful in approaching studies of bioactive peptides/lipids systems.     

Fluorescence analysis of the interaction of two peptide sequences of hepatitis GB virus C with liposomes

The physicochemical characterization of the peptide sequences E2 (39-53) and E2 (32-59) corresponding to the structural protein E2 of the GB virus C was done by studying their interaction with model membranes. The peptides showed surface activity concentration dependent when injected beneath a buffered solution. This tendency to accumulate into the air/water interface suggested a potential ability of these peptides to interact with bilayers. For that reason, Small Unilamellar Liposomes (SUVs) of 1,2-dimyiristoyl-sn-Glycero-3-Phosphocholine (DMPC) or 1,2-dimyiristoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (DMPG) were chosen as a mimetic membranes. A series of fluorescence experiments based on tryptophan peptide fluorescence or with fluorescence labeled SUVs, were done to cover different aspects of peptide interaction with bilayers. Steady state fluorescence anisotropy studies with N-(7-nitro-2-1,3-benzoxadiazol-4-yl) dioleoylphosphatidylethanolamine (NBD-PE) or 1-[4-(trimethylammonium) phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) labeled SUVs indicated that only the long peptide was able to change the lipid microenvironment of DMPG vesicles by slightly increasing the rigidity of the bilayer both above and under the lipid main transition temperature. These results were concordant with the slight blue shift of the maximum tryptophan wavelength emission after E2 (32-53) peptide incubation with DMPG vesicles. Our data provide useful information for the design of synthetic immunopeptides that can be incorporated into a liposomal system with a potential to promote a direct delivery of the membrane-incorporated immunogen to the immunocompetent cells, thus increasing the immuno response from the host.     

A Usual G‐Protein‐Coupled Receptor in Unusual Membranes

G‐protein‐coupled receptors (GPCRs) are the largest family of membrane‐bound receptors and constitute about 50 % of all known drug targets. They offer great potential for membrane protein nanotechnologies. We report here a charge‐interaction‐directed reconstitution mechanism that induces spontaneous insertion of bovine rhodopsin, the eukaryotic GPCR, into both lipid‐ and polymer‐based artificial membranes. We reveal a new allosteric mode of rhodopsin activation incurred by the non‐biological membranes: the cationic membrane drives a transition from the inactive MI to the activated MII state in the absence of high [H⁺] or negative spontaneous curvature. We attribute this activation to the attractive charge interaction between the membrane surface and the deprotonated Glu134 residue of the rhodopsin‐conserved ERY sequence motif that helps break the cytoplasmic “ionic lock”. This study unveils a novel design concept of non‐biological membranes to reconstitute and harness GPCR functions in synthetic systems.     

Folding and translocation of the undecamer of poly-L-leucine across the water-hexane interface. A molecular dynamics study.

The undecamer of poly-L-leucine at the water-hexane interface is studied by molecular dynamics simulations. This represents a simple model relevant to folding and insertion of hydrophobic peptides into membranes. The peptide, initially placed in a random coil conformation on the aqueous side of the system, rapidly translocates toward the hexane phase and undergoes interfacial folding into an alpha-helix in the subsequent 36 ns. Folding is nonsequential and highly dynamic. The initially formed helical segment at the N-terminus of the undecamer becomes transiently broken and, subsequently, reforms before the remainder of the peptide folds from the C-terminus. The formation of intramolecular hydrogen bonds during the folding of the peptide is preceded by a dehydration of the participating polar groups, as they become immersed in hexane. Folding proceeds through a short-lived intermediate, a 3(10)-helix, which rapidly interconverts to an alpha-helix. Both helices contribute to the equilibrium ensemble of folded structures. The helical peptide is largely buried in hexane, yet remains adsorbed at the interface. Its preferred orientation is parallel to the interface, although the perpendicular arrangement with the N-terminus immersed in hexane is only slightly less favorable. In contrast, the reversed orientation is highly unfavorable, because it would require dehydration of C-terminus carbonyl groups that do not participate in intramolecular hydrogen bonding. For the same reason, the transfer of the undecamer from the interface to the bulk hexane is also unfavorable. The results suggest that hydrophobic peptides fold in the interfacial region and, simultaneously, translocate into the nonpolar side of the interface. It is further implied that peptide insertion into the membrane is accomplished by rotating from the parallel to the perpendicular orientation, most likely in such a way that the N-terminus penetrates the bilayer.     

A highly charged voltage-sensor helix spontaneously translocates across membranes

Moving freely: A recent model for voltage gating of potassium channels proposed that the four arginine residues of the voltage-sensing S4 helix (left) are in direct contact with the membrane lipids and move into the hydrocarbon core of the membrane during gating. It is demonstrated that the physical properties of the isolated S4 sequence (right) are sufficient to allow it to freely translocate across synthetic membranes.     

Heavy‐Atom Labeled Transmembrane β‐Peptides: Synthesis,CD‐Spectroscopy,and X‐ray Diffraction Studies in Model Lipid Multilayer

Transmembrane β‐peptides are promising candidates for the design of well‐controlled membrane anchors in lipid membranes. Here, we present the synthesis of transmembrane β‐peptides with and without tryptophan anchors, as well as a novel iodine‐labeled d ‐β³‐amino acid. By using one or more of the heavy‐atom labeled amino acids as markers, the orientation of the helical peptide was inferred based on the electron‐density profile determined by X‐ray reflectivity. The β‐peptides were synthesized through manual Fmoc‐based solid‐phase peptide synthesis (SPPS) and reconstituted in unilamellar vesicles forming a right‐handed 3₁₄‐helix secondary structure, as shown by circular dichroism spectroscopy. We then integrated the β‐peptide into solid‐supported membrane stacks and carried out X‐ray reflectivity and grazing incidence small‐angle X‐ray scattering to determine the β‐peptide orientation and its effect on the membrane bilayers. These β‐peptides adopt a well‐ordered transmembrane motif in the solid‐supported model membrane, maintaining the basic structure of the original bilayer with some distinct alterations. Notably, the helical tilt angle, which accommodates the positive hydrophobic mismatch, induces a tilt of the acyl chains. The tilted chains, in turn, lead to a membrane thinning effect.     

Interactions of membrane-active peptides with thick, neutral, nonzwitterionic bilayers

Alamethicin is a well-studied channel-forming peptide that has a prototypical amphipathic helix structure. It permeabilizes both microbial and mammalian cell membranes, causing loss of membrane polarization and leakage of endogenous contents. Antimicrobial peptide-lipid systems have been studied quite extensively and have led to significant advancements in membrane biophysics. These studies have been performed on lipid bilayers that are generally charged or zwitterionic and restricted to a thickness range of 3-5 nm. Bilayers of amphiphilic diblock copolymers are a relatively new class of membranes that can have significantly different physicochemical properties compared with those of lipid membranes. In particular, they can be made uncharged, nonzwitterionic, and much thicker than their lipid counterparts. In an effort to extend studies of membrane-protein interactions to these synthetic membranes, we have characterized the interactions of alamethicin and several other membrane-active peptides with diblock copolymer bilayers. We find that although alamethicin is too small to span the bilayer, the peptide interacts with, and ruptures, thick polymer membranes.     

Phosphate-mediated arginine insertion into lipid membranes and pore formation by a cationic membrane peptide from solid-state NMR

The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the (13)C-(31)P distances for an Arg-rich beta-hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid (31)P: 4.0-6.5 A in anionic POPE/POPG membranes and 6.5-8.0 A in zwitterionic POPC membranes. The shortest distance of 4.0 A, found for a guanidinium Czeta at the beta-turn, suggests N-H...O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a beta-hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane beta-hairpin peptide to have short (13)C-(31)P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local (31)P plane parallel to the beta-strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.     

Transport across artificial membranes–an analytical perspective

Biosensors that make use of transport processes across lipid membranes are very rare even though a stimulus, the binding of a single analyte molecule, can enhance the sensor response manifold if the analyte leads to the transport of more than one ion or molecule across the membrane. Prerequisite for a proper function of such membrane based biosensors is the formation of lipid bilayers attached to a support that allow for the insertion of membrane peptides and proteins in a functional manner. In this review, the current state of the art technologies to obtain lipid membranes on various supports are described. Solid supported membranes on transparent and electrically conducting surfaces, lipid bilayers on micromachined apertures and on porous materials are discussed. The focus lies on the applicability of such membranes for the investigation of transport phenomena across lipid bilayers facilitated by membrane embedded peptides, channel proteins and transporters. Carriers and channel forming peptides, which are easy to handle and rather robust, are used frequently to build up membrane based biosensors. However, channel forming proteins and transporters are more difficult to insert functionally and thus, there are yet only few examples that demonstrate the applicability of such systems as biosensor devices.      

Ion specific surface forces between membrane surfaces

Entities such as ion distributions and forces between lipid membranes depend on effects due to the intervening salt solution that have not been recognized previously. These specific ion or Hofmeister effects influence membrane fusion. A typical illustrative example is this: measurements of forces between double-chained cationic bilayers adsorbed onto molecularly smooth mica surfaces across different 0.6-2 mM salt solutions have revealed a large degree of ion specificity [Pashley et al. J. Phys. Chem. 1986, 90, 1637]. This has been interpreted in terms of very specific anion "binding" to the adsorbed bilayers, as it would too for micelles and other self-assembled systems. However, we show here that inclusion of nonelectrostatic (NES) or ionic dispersion potentials acting between ions and the two surfaces explains such "ion binding". The observed Hofmeister sequence for the calculated pressure without any direct ion binding is given correctly. This demonstrates the importance of a source of ion specificity that has been ignored. It is due to ionic physisorption caused by attractive NES ionic dispersion potentials. There appear to be some far reaching consequences for interpretations of membrane intermolecular interactions in salt solutions.     

The use of fullerene substituted phenylalanine amino acid as a passport for peptides through cell membranes

We report the formation of a fullerene-peptide conjugate via the incorporation of a fullerene substituted phenylalanine derivative, "Bucky amino acid" (Baa), to a cationic peptide, which acts as a passport for intracellular delivery, enabling transport of a range of sequences into HEK-293, HepG2, and neuroblastoma cells where the peptides in the absence of the fullerene amino acid cannot enter the cell. Delivery of the fullerene species to either the cytoplasm or nucleus of the cell is demonstrated. Fullerene peptides based on the nuclear localization sequence (NLS), H-Baa-Lys(FITC)-Lys-Lys-Arg-Lys-Val-OH, can actively cross over the cell membrane and accumulate significantly around the nucleus of HEK-293 and neuroblastoma cells, while H-Baa-Lys(FITC)-Lys8-OH accumulates in the cytoplasm. Cellular studies show that the uptake for the anionic peptide Baa-Lys(FITC)Glu4Gly3Ser-OH is greatly reduced in comparison with the cationic fullerene peptides of the same concentration. The hydrophobic nature of the fullerene assisting peptide transport is suggested by the effect of gamma-cyclodextrin (CD) in lowering the efficacy of transport. These data suggest that the incorporation of a fullerene-based amino acid provides a route for the intracellular delivery of peptides and as a consequence the creation of a new class of cell penetrating peptides.     

Interactions between antimicrobial polynorbornenes and phospholipid vesicles monitored by light scattering and microcalorimetry

Antimicrobial polynorbornenes composed of facially amphiphilic monomers have been previously reported to accurately emulate the antimicrobial activity of natural host-defense peptides (HDPs). The lethal mechanism of most HDPs involves binding to the membrane surface of bacteria leading to compromised phospholipid bilayers. In this paper, the interactions between biomimetic vesicle membranes and these cationic antimicrobial polynorbornenes are reported. Vesicle dye-leakage experiments were consistent with previous biological assays and corroborated a mode of action involving membrane disruption. Dynamic light scattering (DLS) showed that these antimicrobial polymers cause extensive aggregation of vesicles without complete bilayer disintegration as observed with surfactants that efficiently solubilize the membrane. Fluorescence microscopy on vesicles and bacterial cells also showed polymer-induced aggregation of both synthetic vesicles and bacterial cells. Isothermal titration calorimetry (ITC) afforded free energy of binding values (Delta G) and polymer to lipid binding ratios, plus revealed that the interaction is entropically favorable (Delta S>0, Delta H>0). It was observed that the strength of vesicle binding was similar between the active polymers while the binding stoichiometries were dramatically different.     

Evaluation of diverse peptidyl motifs for cellular delivery of semiconductor quantum dots

Cell-penetrating peptides (CPPs) have rapidly become a mainstay technology for facilitating the delivery of a wide variety of nanomaterials to cells and tissues. Currently, the library of CPPs to choose from is still limited, with the HIV TAT-derived motif still being the most used. Among the many materials routinely delivered by CPPs, nanoparticles are of particular interest for a plethora of labeling, imaging, sensing, diagnostic, and therapeutic applications. The development of nanoparticle-based technologies for many of these uses will require access to a much larger number of functional peptide motifs that can both facilitate cellular delivery of different types of nanoparticles to cells and be used interchangeably in the presence of other peptides and proteins on the same surface. Here, we evaluate the utility of four peptidyl motifs for their ability to facilitate delivery of luminescent semiconductor quantum dots (QDs) in a model cell culture system. We find that an LAH4 motif, derived from a membrane-inserting antimicrobial peptide, and a chimeric sequence that combines a sweet arrow peptide with a portion originating from the superoxide dismutase enzyme provide effective cellular delivery of QDs. Interestingly, a derivative of the latter sequence lacking just a methyl group was found to be quite inefficient, suggesting that even small changes can have significant functional outcomes. Delivery was effected using 1 h incubation with cells, and fluorescent counterstaining strongly suggests an endosomal uptake process that requires a critical minimum number or ratio of peptides to be displayed on the QD surface. Concomitant cytoviability testing showed that the QD–peptide conjugates are minimally cytotoxic in the model COS-1 cell line tested. Potential applications of these peptides in the context of cellular delivery of nanoparticles and a variety of other (bio)molecules are discussed.