Synthesis and Characterization of Picket Porphyrin Receptors That Bind Phosphatidylglycerol, an Anionic Phospholipid Found in Bacterial Membranes

Abstract

The lipid binding ability of four urea-picket porphyrins designed to bind to both the phosphate anion portion as well as the glycerol hydroxyl groups of phosphatidylglycerol (PG) has been investigated. Isothermal titration calorimetry (ITC) and (1)H NMR were used to determine the receptor's stoichiometry of binding, association constants, and both the enthalpy and entropy of binding with the PG anion. Spectral evidence shows that the phosphate anion portion of PG is hydrogen bonded to the urea groups of the receptors. This binding interaction orients the PG anion in the receptor pocket such that its glycerol hydroxyl groups can align with a third urea picket, and results are furnished that suggest this multifunctional interaction does occur. The structure of the entire picket was found to influence the enthalpy and entropy of lipid binding. The synthesis of tetrabutlyammonium phosphatidylglycerol (TBAPG), and a detailed spectral characterization of its headgroup, is also presented.

Carbohydrate-based electrochemical biosensor for detection of a cancer biomarker in human plasma

“…Human blood plasma samples were purchased from Helena Biosciences Europe (UK). Polyethylene glycol substituted by an amine terminal group (PEG-NH2), namely 2-[2-(2-ethoxyethoxy)ethoxy] ethylamine (Alliband et al, 2013) and 2-(tert-butoxycarbonyl-aminooxy) acetic acid (Miller et al, 2005) were synthesized according to the reported procedures.…”

Developing a targeting system for bacterial membranes: measuring receptor-phosphatidylglycerol interactions with1H NMR, ITC and fluorescence correlation spectroscopy

“…The de novo design of our receptors was based entirely on modeling analysis, which demonstrated that the four pickets on the meso-phenyl rings of a porphyrin scaffold, appropriately functionalized, formed a complementary binding pocket to the lipid's multifunctional headgroup. 12 A synthetic advantage to the porphyrin structure is the inherent symmetry of the four pickets that are capable of alignment with the two different sets of lipid functional groups. Our initial studies showed that this was true for ureapicket porphyrins 1 when the pickets were relatively bulky and therefore conformationally rigid (only porphyrins with stable all-α atropisomers would bind to PG in a well-behaved 1 : 1 stoichiometry) (Fig.…”

“…20 The average value of the binding constants taken from three titration experiments (each of which used an average determined from the movement of 2 proton resonances) was 4900 M −1 (Table 1). Upon reverse titration of the lipid with 4b the glycerol protons (multiplets between 3.35-3.85 ppm 12 ) moved upfield during complex formation, indicating the glycerol headgroup lies directly above the ring (S-Fig. 19 †).…”

“…We have prepared and characterized the lipid binding ability of several phosphatidylglycerol receptors. 12,13 Armed with an understanding of the structures of bacterial membrane components and the ability to develop selective binding agents, our intent is to link synthetic PG receptors with compounds that exhibit either bacterial membrane disrupting properties (AMP mimic) 14 or compounds able to inhibit bacterial membrane proteins involved in virulence mechanisms 15 to afford new families of antibiotics. Importantly, both type of potential antimicrobials require a targeting system that is able to differentiate between prokaryotic membranes and eukaryotic membranes for it to be used as a systemic therapeutic.…”

“…These initial solution studies clarified the structural requirements for a complementary lipid binding pocket with the determination that four conformationally rigid ureapickets align properly to allow hydrogen bonding with both the phosphate anion portion and the neutral hydroxyl portion of the lipid headgroup. 12 We now report a major advance in the development of our targeting system for bacterial membranes with the demonstration that a similarly configured, water-soluble ammonium-picket porphyrin can selectively bind with high affinity to synthetic membranes that contain PG. The structure of the lipid-receptor complex in solution or at a membrane surface was shown to be quite similar.…”