Insights into nucleotide recognition by cell division protein FtsZ from a mant-GTP competition assay and molecular dynamics

Essential cell division protein FtsZ forms the bacterial cytokinetic ring and is a target for new antibiotics. FtsZ monomers bind GTP and assemble into filaments. Hydrolysis to GDP at the association interface between monomers leads to filament disassembly. We have developed a homogeneous competition assay, employing the fluorescence anisotropy change of mant-GTP upon binding to nucleotide-free FtsZ, which detects compounds binding to the nucleotide site in FtsZ monomers and measures their affinities within the millimolar to 10 nM range. We have employed this method to determine the apparent contributions of the guanine, ribose, and the α-, β-, and γ-phosphates to the free energy change of nucleotide binding. Similar relative contributions have also been estimated through molecular dynamics and binding free energy calculations, employing the crystal structures of FtsZ-nucleotide complexes. We find an energetically dominant contribution of the β-phosphate, comparable to the whole guanosine moiety. GTP and GDP bind with similar observed affinity to FtsZ monomers. Loss of the regulatory γ-phosphate results in a predicted accommodation of GDP which has not been observed in the crystal structures. The binding affinities of a series of C8-substituted GTP analogues, known to inhibit FtsZ but not eukaryotic tubulin assembly, correlate with their inhibitory capacity on FtsZ polymerization. Our methods permit testing of FtsZ inhibitors targeting its nucleotide site, as well as compounds from virtual screening of large synthetic libraries. Our results give insight into the FtsZ-nucleotide interactions, which could be useful in the rational design of new inhibitors, especially GTP phosphate mimetics.     

Conformational analysis of methyl 6-O-[(R)- and (S)-1-carboxyethyl]-α-D-galactopyranoside by MM and Langevin dynamics simulations

The conformational space of methyl 6-O-[(R)- and (S)- 1-carboxyethyl]-α-D-galactopyranoside has been investigated. A grid search employing energy minimization at each grid point over the three major degrees of freedom, namely φ, ψ and ω, identified low energy regions. The R-isomer shows five low energy conformers within ca. 1 kcal mol−1 of the global energy minimum. The S-isomer has two conformers within a few tenths of a kcal mol−1 of the global energy minimum. Langevin dynamics simulations have been have been performed at 300 K for 30 ns of each isomer. The φ dihedral angle has as its major conformer (g−1) for the R-isomer whereas it is the (g+) conformer for the S-isomer. For the ψ dihedral angle the (t) conformer has the highest population for both isomers. The dihedral angle ω has the (g+) conformer most highly populated, both for the R- and S-isomer. The above five and two conformational states for the R- and S-isomers, respectively, make up 90% in each case of the populated states during the Langevin dynamics (LD) simulations. Rate constants for the ω dihedral angle have been calculated based on a number correlation function. Three bond homo- and heteronuclear, i.e. proton and carbon-13, coupling constants have been calculated from the dynamics trajectories for comparison to experimental values. The heteronuclear coupling constant H2′,C6 has been measured for the S-isomer and found to be 3.3 Hz. The J value calculated from the LD simulations, namely 2.6 Hz, is in fair agreement with experiment. A comparison to the X-ray structure of the R-isomer shows that the conformation of the crystalline compound occupies the low energy region most highly populated as a single R-conformer (30%) during the LD simulations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light activation of the isomerization and deprotonation of the protonated Schiff base retinal

We perform an ab initio analysis of the photoisomerization of the protonated Schiff base of retinal (PSB-retinal) from 11-cis to 11-trans rotating the C10-C11=C12-C13 dihedral angle from 0° (cis) to -180° (trans). We find that the retinal molecule shows the lowest rotational barrier (0.22 eV) when its charge state is zero as compared to the barrier for the protonated molecule which is ∼0.89 eV. We conclude that rotation most likely takes place in the excited state of the deprotonated retinal. The addition of a proton creates a much larger barrier implying a switching behavior of retinal that might be useful for several applications in molecular electronics. All conformations of the retinal compound absorb in the green region with small shifts following the dihedral angle rotation; however, the Schiff base of retinal (SB-retinal) at trans-conformation absorbs in the violet region. The rotation of the dihedral angle around the C11=C12 π-bond affects the absorption energy of the retinal and the binding energy of the SB-retinal with the proton at the N-Schiff; the binding energy is slightly lower at the trans-SB-retinal than at other conformations of the retinal.     

Pressure-dependent <Superscript>13</Superscript>C chemical shifts in proteins: origins and applications

Pressure-dependent ¹³C chemical shifts have been measured for aliphatic carbons in barnase and Protein G. Up to 200 MPa (2 kbar), most shift changes are linear, demonstrating pressure-independent compressibilities. CH₃, CH₂ and CH carbon shifts change on average by +0.23, −0.09 and −0.18 ppm, respectively, due to a combination of bond shortening and changes in bond angles, the latter matching one explanation for the γ-gauche effect. In addition, there is a residue-specific component, arising from both local compression and conformational change. To assess the relative magnitudes of these effects, residue-specific shift changes for protein G were converted into structural restraints and used to calculate the change in structure with pressure, using a genetic algorithm to convert shift changes into dihedral angle restraints. The results demonstrate that residual ¹³Cα shifts are dominated by dihedral angle changes and can be used to calculate structural change, whereas ¹³Cβ shifts retain significant dependence on local compression, making them less useful as structural restraints.     

Interactions in pseudorotoxanes based on crown ether-secondary ammonium motifs. A theoretical study

A theoretical analysis of the nature of the interactions in dibenzo[24]crown-8 (DB24C8)-n-dibutylammonium (DBM)—pseudorotaxane complex at the MP2 and DFT levels shows that the main contribution to the binding energy is the electrostatic interaction with moderate (20–25%) correlation stabilization. The total binding energy in the DB24C8-DBM complex represents a sum of the binding energies of two NH–O and one CH–O hydrogen bonds and the latter constitutes about 25% of the total interaction energy, giving the total binding energy of −41.2 kcal mol⁻¹ at the BHandHLYP/6-311++G** level. Deprotonation of the DB24C8-DBM complex reduces the binding energy by some 50 kcal mol⁻¹, giving metastable complexes DB24C8-DBA-1 or DB24C8-DBA-2, which will dissociate to give free crown ether and n-dibutylamine because of the strong exchange repulsion that prevails in neutral complexes. Figure Formation of DB24C8-DBM pseudorotoxane complex     

Enantiomer and conformer recognition of (+) and (−)-disparlure and their analogs by the pheromone binding proteins of the gypsy moth,Lymantria dispar

Adult female gypsy moths produce a sex pheromone (+)-(7R,8S)-2-methyl-7,8-epoxyoctadecane, (+)-disparlure, to attract male gypsy moths. To better understand the recognition of (+)-disparlure by the male’s olfactory system, we synthesized racemic and enantiopure oxa and thia analogs of (+)-disparlure (ee > 98%). Ab initio calculations of the conformeric landscapes around the dihedral angles C_{5–6–7–8} and C_{7–8–9–10} of (+)-disparlure and corresponding dihedral angles of analogs revealed that introduction of the heteroatom changes the conformeric landscape around these important epitopes. The energy difference between HOMO and LUMO decreased after oxygen or sulfur was introduced into the backbone. Consistent with this, an enhancement of binding affinity between sulfur analogs and the pheromone-binding proteins (PBPs) was observed in vitro. Docking of the pheromone and analogs onto models of the two known PBPs of the gypsy moth revealed that the internal binding pocket of PBP1 showed higher selectivity than that of PBP2, consistent with in vitro binding assays. Further energy analysis revealed that enantiomers adopted different conformations with different energies when docked in the internal binding pocket of PBPs, resulting in enantiomer discrimination of PBPs towards disparlure and its analogs.     

Photosynthetic response and recovery of Antarctic marine benthic microalgae exposed to elevated irradiances and temperatures

Exposure to high temperatures affects the photosynthetic processes in marine benthic microalgae by limiting the transport of electrons, thus reducing the ability of the cell to use light. This causes damage to the Photosystem II (PSII) and may lead to photoinhibition. However, the PSII of benthic microalgal communities from Brown Bay, eastern Antarctica, were relatively unaffected by significant changes in temperature. Benthic microalgae exposed to temperatures up to 8°C and an irradiance of 450 μmol photons m⁻² s⁻¹ did not experience any photosynthetic damage or irreversible photoinhibition. The effective quantum yield (∆F/F _m′) at 8°C (0.433 ± 0.042) was higher by comparison to cell incubated at −0.1°C (0.373 ± 0.015) with similar irradiances. Temperatures down to −5°C at a similar irradiance showed a decrease in photosynthesis with decreasing temperature, but no severe photoinhibition as the cells were able to dissipate excess energy via non-photochemical quenching and recover from damage. These responses are consistent with those recorded in past studies on Antarctic benthic microalgae and suggest that short-term temperature change (from −5 to 8°C) will not do irreversible damage to the PSII and will not affect the photosynthesis of the benthic microalgae.     

Cryopreservation at −80°C of Agaricus blazei on rice grains

The preservation of Agaricus blazei is generally done by mycelial subculturing, but this technique may cause genetic degenerations. Despite this, there is not an efficient protocol established to preserve this fungus and cryopreservation could be an alternative. This study aimed to evaluate two freezing protocols for cryopreservation at −80°C of A. blazei strains. Five fungus strains grown on rice grains with husk and were transferred to glycerol (10%) in cryovials. Next, the cryovials were submitted to two freezing temperature protocols: (1) cryopreservation starting at 25°C, then at 8°C for 30 min and kept at −80°C; (2) cryopreservation starting at 25°C, then 8°C for 30 min, −196°C for 15 min and kept at −80°C. After 1 year of cryopreservation, the cryovials were thawed in a water bath at 30°C for 15 min and transferred to malt extract agar medium. It was concluded that the one-year cryopreservation process of A. blazei, grown on rice grains and cryopreserved at −80°C in glycerol 10%, is viable. The slow freezing, from 8 to −80°C, is effective whereas the fast freezing, from 8 to −196°C and then to −80°C, is ineffective. The different genetic characteristics among the strains of this fungus do not interfere in the cryopreservation process.     

Conformational and thermodynamic characterization of the premolten globule state occurring during unfolding of the molten globule state of cytochrome c

It has already been shown that the mutant Leu94Gly of horse cytochrome c exists in a molten globule (MG) state. We have carried out studies of reversible folding and unfolding induced by LiCl of this mutant at pH 6.0 and 25 °C by observing changes in the difference molar absorption coefficient at 402 nm, the mean residue ellipticity at 222 nm, and the difference mean residue ellipticity at 409 nm. This process is a three-state process when measured by these probes. The stable folding intermediate state has been characterized by far- and near-UV circular dichroism, tryptophan fluorescence, 8-anilino-1-naphthalenesulfonic acid binding, and dynamic light scattering measurements, which led us to conclude that the intermediate is a premolten globule (PMG). Analysis of the reversible unfolding transition curves for the stability of different states in terms of the Gibbs free energy change at pH 6.0 and 25 °C led us to conclude that the MG state is more stable than the PMG state by 5.4 ± 0.1 kcal mol⁻¹, whereas the PMG state is more stable than the denatured (D) state by only 1.1 ± 0.1 kcal mol⁻¹. A comparison of the conformational and thermodynamic properties of the LiCl-induced PMG state at pH 6.0 with those of the PMG state induced by NaCl at pH 2.0 suggests that a similar PMG state is obtained under both denaturing conditions. Differential scanning calorimetry measurements suggest that heat induces a reversible two-state transition between MG and D states.     

Rate-limiting guanosine 5'-triphosphate hydrolysis during nucleotide turnover by FtsZ, a prokaryotic tubulin homologue involved in bacterial cell division

FtsZ is a prokaryotic tubulin homologue that polymerizes into a dynamic ring during cell division. GTP binding and hydrolysis provide the energy for FtsZ dynamics. However, the precise role of hydrolysis in polymer assembly and turnover is not understood, limiting our understanding of how FtsZ functions in the cell. Here we investigate GTP hydrolysis during the FtsZ polymerization cycle using several complementary approaches that avoid technical caveats of previous studies. We find that at steady state approximately 80% of FtsZ polymer subunits are bound to GTP. In addition, we use pre-steady-state, single turnover assays to directly measure the rate of hydrolysis. Hydrolysis was found to occur at approximately 8/min and to be a rate-limiting step in GTP turnover; phosphate release rapidly followed. These results clarify previously conflicting results in the literature and suggest that pure FtsZ polymers, unlike microtubules, may not be able to undergo dynamic instability or to store energy in the polymer for force production.     

Single tryptophan mutants of FtsZ: Nucleotide binding/exchange and conformational transitions

Cell division protein FtsZ cooperatively self-assembles into straight filaments when bound to GTP. A set of conformational changes that are linked to FtsZ GTPase activity are involved in the transition from straight to curved filaments that eventually disassemble. In this work, we characterized the fluorescence of single Trp mutants as a reporter of the predicted conformational changes between the GDP- and GTP-states of Escherichia coli FtsZ. Steady-state fluorescence characterization showed the Trp senses different environments and displays low solvent accessibility. Time-resolved fluorescence data indicated that the main conformational changes in FtsZ occur at the interaction surface between the N and C domains, but also minor rearrangements were detected in the bulk of the N domain. Surprisingly, despite its location near the bottom protofilament interface at the C domain, the Trp 275 fluorescence lifetime did not report changes between the GDP and GTP states. The equilibrium unfolding of FtsZ features an intermediate that is stabilized by the nucleotide bound in the N-domain as well as by quaternary protein–protein interactions. In this context, we characterized the unfolding of the Trp mutants using time-resolved fluorescence and phasor plot analysis. A novel picture of the structural transition from the native state in the absence of denaturant, to the solvent-exposed unfolded state is presented. Taken together our results show that conformational changes between the GDP and GTP states of FtsZ, such as those observed in FtsZ unfolding, are restricted to the interaction surface between the N and C domains.     

FtsZ filament structures in different nucleotide states reveal the mechanism of assembly dynamics

Treadmilling protein filaments perform essential cellular functions by growing from one end while shrinking from the other, driven by nucleotide hydrolysis. Bacterial cell division relies on the primitive tubulin homolog FtsZ, a target for antibiotic discovery that assembles into single treadmilling filaments that hydrolyse GTP at an active site formed upon subunit association. We determined high-resolution filament structures of FtsZ from the pathogen Staphylococcus aureus in complex with different nucleotide analogs and cations, including mimetics of the ground and transition states of catalysis. Together with mutational and biochemical analyses, our structures reveal interactions made by the GTP γ-phosphate and Mg²⁺ at the subunit interface, a K⁺ ion stabilizing loop T7 for co-catalysis, new roles of key residues at the active site and a nearby crosstalk area, and rearrangements of a dynamic water shell bridging adjacent subunits upon GTP hydrolysis. We propose a mechanistic model that integrates nucleotide hydrolysis signaling with assembly-associated conformational changes and filament treadmilling. Equivalent assembly mechanisms may apply to more complex tubulin and actin cytomotive filaments that share analogous features with FtsZ.

Bacterial cell division critically relies on the tubulin homolog FtsZ, which assembles into filaments that treadmill, fuelled by GTP hydrolysis. This structural and biochemical study of FtsZ from Staphylocuccus aureus reveals the mechanism of GTP hydrolysis and its connection with filament dynamics.     

Physiological differences in the growth of <Emphasis Type="Italic">Sargassum horneri</Emphasis> between the germling and adult stages

The effects of temperature, irradiance, and daylength on Sargassum horneri growth were examined at the germling and adult stages to discern their physiological differences. Temperature–irradiance (10, 15, 20, 25, 30°C × 20, 40, 80 μmol photons m⁻²s⁻¹) and daylength (8, 12, 16, 24 h) experiments were carried out. The germlings and blades of S. horneri grew over a wide range of temperatures (10–25°C), irradiances (20–80 μmol photons m⁻²s⁻¹), and daylengths (8–24 h). At the optimal growth conditions, the relative growth rates (RGR) of the germlings were 21% day⁻¹ (25°C, 20 μmol photons m⁻²s⁻¹) and 13% day⁻¹ (8 h daylength). In contrast, the RGRs of the blade weights were 4% day⁻¹ (15°C, 20 μmol photons m⁻²s⁻¹) and 5% day⁻¹ (12 h daylength). Negative growth rates were found at 20 μmol photons m⁻²s⁻¹ of 20°C and 25°C treatments after 12 days. This phenomenon coincides with the necrosis of S. horneri blades in field populations. In conclusion, we found physiological differences between S. horneri germlings and adults with respect to daylength and temperature optima. The growth of S. horneri germlings could be enhanced at 25°C, 20 μmol photons m⁻²s⁻¹, and 8 h daylength for construction of Sargassum beds and restoration of barren areas.     

Early development of germlings of <Emphasis Type="Italic">Sargassum thunbergii</Emphasis> (Fucales,Phaeophyta) under laboratory conditions

Morphology and culture studies on germlings of Sargassum thunbergii (Mertens et Roth) Kuntze were carried out under controlled laboratory conditions. Growth characteristics of these germlings grown under different temperatures (from 10 to 25°C), irradiances (from 9 to 88 μmol photons m⁻² s⁻¹), and under blue and white light conditions are described. The development of embryonic germlings follows the classic “8 nuclei 1 egg” type described for Sargassaceae. Fertilized eggs spent 5–6 h developing into multicellular germlings with abundant rhizoids after fertilization. Under conditions of 20°C, 44 μmol photons m⁻² s⁻¹ and photoperiod of 12 h, young germlings with one or two leaflets reached 2–3 mm in length after 8 weeks. Temperature variations (10, 15, 20, 25°C) under 88 μmol photons m⁻² s⁻¹ significantly influenced the growth rate within the first week, although this effect became less obvious after 8 weeks, especially at 15 and 20°C. Variation in germling growth was highly significant under different irradiances (9, 18, 44, 88 μmol photons m⁻² s⁻¹) at 25°C. Low temperature (10°C) reduced germling growth. Growth of germlings cultured under blue light was lower than in white light. Optimal growth of these germlings occurred at 25°C and 44 μmol photons m⁻² s⁻¹.     

Effects of light and temperature on the attachment and development of <Emphasis Type="BoldItalic">Gloiopeltis tenax</Emphasis> and <Emphasis Type="BoldItalic">Gloiopeltis furcata</Emphasis> tetraspores

Two 60-day experiments were conducted to study the influence of photon flux density (PFD) and temperature on the attachment and development of Gloiopeltis tenax and Gloiopeltis furcata tetraspores. In the first experiment, tetraspores of the two Gloiopeltis species were incubated at five temperature ranges (8°C, 12°C, 16°C, 20°C, 24°C) under a constant PFD of 80 μmol photons m⁻² s⁻¹ with a photoperiod of 12:12. In a second experiment, tetraspores were incubated under five PFD gradients (30, 55, 80, 105, 130 μmol photons m⁻² s⁻¹) at a constant temperature of 16°C with a photoperiod of 12:12. Maximum density of attached tetraspores was observed at 16°C for both species. Maximum per cent of spore germinating into disc was recorded at 12–16°C for G. tenax and 8–12°C for G. furcata. Maximum per cent of discs producing erect axes for G. tenax and G. furcata were recorded at 24°C and 20°C, respectively. Light had no significant effect on tetraspore attachment and developing into disc, but it affected the growth, sprouting and survival of its discs. Under 30–55 μmol photons m⁻² s⁻¹, the discs of the two species of Gloiopeltis did not form thallus until the end of the experiment. Optimum PFD range for G. tenax discs was 80–105 μmol photons m⁻² s⁻¹, whilst it was 80–130 μmol photons m⁻² s⁻¹ for G. furcata. Results presented in this study are expected to assist the progress of artificial seeding of Gloiopeltis.     

Netropsin binding in five duplex-dimer DNA constructs as a function of size and distance between binding sites: circular dichroism and absorption spectroscopy

The optical activity induced on binding the drug netrospin (NET) in the minor groove of DNA is studied in five oligonucleotides (OGNs) as a function of (1) the size of the binding site in (5′-(GC)₂AATT(GC)₂-3′)₂ (OGN 1a) versus (5′-(GC)₂AAATTT(GC)₂-3′)₂ (OGN 1b) and (2) the distance between two AATT binding sites in (5′-(GC)₂AATT(GC) _x AATT(GC)₂-3′)₂, with x = 1, 2, or 3 (OGNs 2a, b, c, respectively). NET binding is monitored via the induced circular dichroism (CD) at ~315 nm, where the nucleic acids are optically inactive. The CD titrations, fit to a tight binding model, yield lower limits for the binding constant, K_a, ≥8 × 10⁷ M⁻¹ for OGN 1a and ≥2 × 10⁸ M⁻¹ for OGNs 2a, b, c in 1 mM buffer. In 100 mM buffer, tight binding occurs in all five OGNs with K_a ≥ 8 × 10⁷ M⁻¹ for OGN 1a and ≥1 × 10⁸ M⁻¹ for OGNs 1b and 2a, b, c. In contrast, the elongated AAATTT binding site of OGN 1b results in weak binding of NET in 1 mM buffer, where competing electrostatic interactions with the solvent environment are lower. In the constructs with two binding sites, the increase in flexibility introduced by intervening GC base pairs does not induce co-operative binding, although differences in the number of binding sites, n (2.05–2.65), indicate that there may be differences in the way NET is bound in OGNs 2a, b, c. In addition, the large shifts in the absorption spectra induced in bound versus free NET, and effects on the CD spectral bands at higher energy, are discussed in terms of electrostatic and excitonic interactions.     

Side-chain conformational changes upon Protein-Protein Association

Conformational changes upon protein-protein association are the key element of the binding mechanism. The study presents a systematic large-scale analysis of such conformational changes in the side chains. The results indicate that short and long side chains have different propensities for the conformational changes. Long side chains with three or more dihedral angles are often subject to large conformational transition. Shorter residues with one or two dihedral angles typically undergo local conformational changes not leading to a conformational transition. A relationship between the local readjustments and the equilibrium fluctuations of a side chain around its unbound conformation is suggested. Most of the side chains undergo larger changes in the dihedral angle most distant from the backbone. The frequencies of the core-to-surface interface transitions of six nonpolar residues and Tyr are larger than the frequencies of the opposite surface-to-core transitions. The binding increases both polar and nonpolar interface areas. However, the increase of the nonpolar area is larger for all considered classes of protein complexes, suggesting that the protein association perturbs the unbound interfaces to increase the hydrophobic contribution to the binding free energy. To test modeling approaches to side-chain flexibility in protein docking, conformational changes in the X-ray set were compared with those in the docking decoy sets. The results lead to a better understanding of the conformational changes in proteins and suggest directions for efficient conformational sampling in docking protocols.     

Interleukin-18 gene promoter and serum level in women with ovarian cancer

IL-18, initially defined as a potent inducer of IFN- γ production, is a systemic, multifunctional cytokine with both pro-cancerous and anti-cancer activities. The contribution of the IL-18 promoter polymorphisms at positions −607 (C/A) and −137 (G/C) to cancer development has been reported. We sought to examine IL-18 serum level and its polymorphisms in Iranian women with ovarian cancer. Single nucleotide polymorphisms (SNPs) at positions −607 (C/A) and −137 (G/C) were analyzed by allele-specific polymerase chain reaction in 85 women with ovarian cancer and 158 healthy controls. IL-18 serum level was determined using ELISA method. No significant association was found between the allele, genotype, and haplotype distributions of the SNPs and ovarian cancer. Mean IL-18 serum level was significantly higher in patients than in controls (P = 0.008). Comparing IL-18 serum levels with genotypes at positions −607 and −137 revealed no significant difference. No association was also found between IL-18 levels and the disease stage. In conclusion, our results indicate that IL-18 promoter polymorphisms at positions −607 (C/A) and −137 (G/C) appear not to confer susceptibility to ovarian cancer in Iranian population; however, IL-18 serum level increases in ovarian cancer patients.     

Micelles obtained by aggregation of gemini surfactants containing the CCK8 peptide and a gadolinium complex

Two gemini surfactants, [C18CysL5CCK8]₂ and [C18CysDTPAGlu]₂, containing, respectively, the CCK8 peptide and the DTPAGlu chelating agent or its gadolinium complex have been prepared by linking lipophilic chains through a disulfide bond between two cysteine residues. The two surfactants aggregate in water solution forming pure or mixed micelles, with a critical micellar concentration in the 5 × 10⁻⁶–5 × 10⁻⁵ mol kg⁻¹ range, as measured by fluorescence spectroscopy. As indicated by small-angle neutron scattering, the shape and size of the micelles are influenced by the temperature: increasing temperature leads to progressive reduction of the size of the supramolecular aggregates. Cylindrical structures found at lower temperatures (10–40 °C) evolve into ellipsoidal micelles at 50–80 °C. Furthermore, the surface-exposed CCK8 peptide changes its conformation above a transition temperature of approximately 45 °C, going from a β-sheet to a random-coil structure, as indicated by circular dichroism measurements. The mixed aggregate obtained by coaggregation of the two gemini-based amphiphilic compounds, [C18CysDTPAGlu(Gd)]₂ and [C18CysL5CCK8]₂ in 70:30 molar ratio, represents the first example of a peptide-containing gemini surfactant as a potential target-selective contrast agent in MRI. In fact, it presents a high relaxivity value of the gadolinium complex, 21.5 mM⁻¹ s⁻¹, and the CCK8 bioactive peptide exposed on the external surface is therefore capable of selective targeting of the cholecystokinin receptors. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.