John M. Sanderson

A supramolecular system for quantifying aromatic stacking interactions.

A supramolecular complex for investigating the thermodynamic properties of intermolecular aromatic stacking interactions has been developed. The conformation of the complex is locked in a single well-defined conformation by an array of H-bonding interactions that force two aromatic rings on one end of the complex into a stacked geometry. Chemical double-mutant cycles have been used to measure an anthracene-aniline interaction (+0.6 +/- 0.8 kJ mol(-1)) and a pentafluorophenyl-aniline interaction (-0.4 +/- 0.9 kJ mol(-1)) in this system. Although the interactions are very weak, the pentafluorophenyl interaction is attractive, whereas the anthracene interaction is repulsive: this is consistent with the dominance of pi-electron electrostatic interactions. The nitropyrrole subunits used to control the conformation of these complexes lead to problems of aggregation and multiple conformational equilibria. The implications for the thermodynamic analysis are examined in detail, and the double-mutant-cycle approach is found to be remarkably robust with respect to such effects, since systematic errors in individual experiments are removed in a pair-wise fashion when the cycle is constructed.

Surface features of a Mononegavirales matrix protein indicate sites of membrane interaction

The matrix protein (M) of respiratory syncytial virus (RSV), the prototype viral member of the Pneumovirinae (family Paramyxoviridae, order Mononegavirales), has been crystallized and the structure determined to a resolution of 1.6 Å. The structure comprises 2 compact β-rich domains connected by a relatively unstructured linker region. Due to the high degree of side-chain order in the structure, an extensive contiguous area of positive surface charge covering ≈600 Å2 can be resolved. This unusually large patch of positive surface potential spans both domains and the linker, and provides a mechanism for driving the interaction of the protein with a negatively-charged membrane surface or other virion components such as the nucleocapsid. This patch is complemented by regions of high hydrophobicity and a striking planar arrangement of tyrosine residues encircling the C-terminal domain. Comparison of the RSV M sequence with other members of the Pneumovirinae shows that regions of divergence correspond to surface exposed loops in the M structure, with the majority of viral species-specific differences occurring in the N-terminal domain.

Quantitative determination of intermolecular interactions with fluorinated aromatic rings.

The chemical double mutant cycle approach has been used to investigate substituent effects on intermolecular interactions between aromatic rings and pentafluorophenyl pi-systems. The complexes have been characterised using 1H and 19F NMR titrations, X-ray crystal structures of model compounds and molecular mechanics calculations. In the molecular zipper system used for these experiments, H-bonds and the geometries of the interacting surfaces favour the approach of the edge of the aromatic ring with the face of the pentafluorophenyl pi-system. The interactions are generally repulsive and this repulsion increases with more electron-withdrawing substituents up to a limit of +2.2 kJ mol(-1), when the complex distorts to minimise the unfavourable interaction. Strongly electron-donating groups cause a change in the geometry of the aromatic interaction and attractive stacking interactions are found (-1.6 kJ mol(-1) for NMe2). These results are generally consistent with an electrostatic model: the polarisation of the pentafluorophenyl ring leads to a partial positive charge located at the centre and this leads to repulsive interactions with the positive charges on the protons on the edge of the aromatic ring; when the aromatic ring has a high pi-electron density there is a large electrostatic driving force in favour of the stacked geometry which places this pi-electron density over the centre of the positive charge on the pentafluorophenyl group.

Characterisation of the interactions of aromatic amino acids with diacetyl phosphatidylcholine

A simple bimolecular system for characterising the interactions of amino acids with diacyl-3-phosphatidylcholines has been developed. The system contains two components: an N-acetyl amino acid N′-alkyl amide and diacetyl-3-sn-phosphatidylcholine (DAPC). Interactions between a series of aromatic amino acids and DAPC have been characterised by 1H NMR techniques. Of the amino acids examined, tryptophan and tyrosine were shown to have particularly favourable interactions with the DAPC choline headgroup. Our observations are consistent with the previously reported tendency for these amino acids to occur preferentially at the lipid-water interface. Using data from ROESY experiments and complexation-induced chemical shift changes, we have been able to generate molecular models for the tryptophan–DAPC adduct that are consistent with the observed results. The adduct is characterised by amide carbonyl–cation interactions, hydrogen bonding and cation–π interactions.

An efficient route to S-N-(9-fluorenylmethoxycarbonyl)-4′-(1-azi-2,2,2-trifluoroethyl)phenylalanine

Abstract An extremely efficient synthesis of optically pure photoactivatable phenylalanine derivative 1 is described. The key step involves a highly diastereoselective alkylation of a chiral glycine equivalent.

A chiral probe for the acute phase proteins alpha-1-acid glycoprotein and alpha-1-antitrypsin based on europium luminescence

Reversible and selective binding of a dynamically racemic europium(III) complex to α(1)-acid glycoprotein and α(1)-antitrypsin is characterised by a significant change in the europium total emission spectral fingerprint and the switching on of a large circularly polarised luminescence (CPL) signal from the metal centre. Observation of an induced CD into the ligand chromophore in the presence of α(1)-AGP allows a structure for the protein-bound complex to be postulated. A direct determination of elevated α(1)-AGP levels in human serum was achieved by monitoring changes in the intensity ratio of Eu emission bands.

Influence of lipids on the interfacial disposition of respiratory syncytical virus matrix protein.

The propensity of a matrix protein from an enveloped virus of the Mononegavirales family to associate with lipids representative of the viral envelope has been determined using label-free methods, including tensiometry and Brewster angle microscopy on lipid films at the air-water interface and atomic force microscopy on monolayers transferred to OTS-treated silicon wafers. This has enabled factors that influence the disposition of the protein with respect to the lipid interface to be characterized. In the absence of sphingomyelin, respiratory syncytial virus matrix protein penetrates monolayers composed of mixtures of phosphocholines with phosphoethanolamines or cholesterol at the air-water interface. In ternary mixtures composed of sphingomyelin, 1,2-dioleoyl-sn-glycero-3-phosphocholine, and cholesterol, the protein exhibits two separate behaviors: (1) peripheral association with the surface of sphingomyelin-rich domains and (2) penetration of sphingomyelin-poor domains. Prolonged incubation of the protein with mixtures of phosphocholines and phosphoethanolamines leads to the formation of helical protein assemblies of uniform diameter that demonstrate an inherent propensity of the protein to assemble into a filamentous form.

Analysis of liposomal membrane composition using Raman tweezers

We have developed a methodology for the analysis of liposomal membranes and their contents using near-IR Raman spectroscopy on liposomes held in an optical trap. We were able to detect a variety of membrane components including lipids, cholesterol, and small molecule solutes such as ethanol, DMSO and hexafluoroisopropanol. The methodology is able to distinguish between solutes that equilibrate across the liposomal membrane from those that partition selectively into the lipid bilayer.

The Synergistic Action of Melittin and Phospholipase A2 with Lipid Membranes: Development of Linear Dichroism for Membrane-Insertion Kinetics

Here we present data on the kinetics of insertion of melittin, a peptide from bee venom, into lipid membranes of different composition. Another component of bee venom is the enzyme phospholipase A2 (PLA₂). We have examined the interaction of melittin and PLA₂ with liposomes both separately and combined and demonstrate that they work synergistically to disrupt the membranes. A dramatic difference in the action of melittin and PLA₂ is observed when the composition of the membrane is altered. Temperature also has a large effect on the kinetics of insertion and membrane disruption. We use a combination of techniques to measure liposome size (dynamic light scattering), peptide secondary structure (circular dichroism spectroscopy), peptide orientation relative to the membrane (linear dichroism spectroscopy) and enzymatic digestion of the lipids (mass spectrometry).

Free-energy relationships for the interactions of tryptophan with phosphocholines

In membrane proteins and peptides, tryptophan exhibits a marked tendency to occur in locations that correspond to the interfacial region of the lipid bilayer. The relative contributions of electrostatic, dipolar, hydrophobic and conformational effects on the interactions of tryptophan with lipids have been the subject of much speculation. In order to elucidate the fundamental properties of tryptophan-phosphocholine interactions in the absence of competing factors such as protein conformation and membrane perturbation, we have determined the binding characteristics of a homologous series of tryptophan analogues to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) in deuterochloroform using NMR titrimetric approaches. The data are analysed using a binding model that includes lipid aggregation and the explicit association of water with the lipid. For a series of substituents (OMe, Me, H, F, Cl, Br, I, NO(2)) at the 5-position of the indole ring, the trends in the free energy of association for the formation of 1 : 1 and 1 : 2 lipid-tryptophan adducts both follow an inverted- relationship as a function of the corresponding para-Hammett parameter, with tryptophan (R = H) exhibiting the weakest binding. These trends are shown to be consistent with participation of the indole side chain in both hydrogen bonds and cation-pi interactions. Molecular dynamics simulations of tryptophan and DMPC in an explicit chloroform solvent model demonstrate that for the formation of lipid-tryptophan adducts, binding is driven predominantly by carbonyl-cation and cation-pi interactions with the choline ammonium group, alongside hydrogen bonding interactions with the lipid phosphate. Some of these interactions operate co-operatively, which may account for the observed trends in free energy.