Methylammonium Groups at the Solid Walls of Nanometer-Sized, Water-Filled Monolayer Gaps as Binding Sites for a Tetraanionic Porphyrin

Abstract

Long-chain hydrosulfides containing two secondary amide functions and either electron-poor or electron-rich carbon-carbon double bonds were self-assembled on gold surfaces around a flat-lying, octaanionic porphyrin. Rigid and reactive surface monolayers with 2 nm-wide, porphyrin-based gaps were thus obtained. The gold electrodes were then immersed in water, and the double bonds on the gaps' surfaces reacted with methylamine. It was added to the double bonds either by Michael addition or by bromination with hypobromite followed by methylamine substitution. Only the double bonds at the border of the gaps were accessible to methylamine dissolved in the bulk water volume and could react. The walls of the rigid membrane gaps now contained methylammonium groups at the sites of the double bonds in defined heights. A tetracationic copper(II) porphyrinate could not diffuse any more into the gap and did not quench the fluorescence of the octaanionic porphyrin on the bottom of the gap. A tetraanionic porphyrin, on the other hand, was fixated by the ring of ammonium groups. The bound porphyrin then acted as molecular cover for the gap with respect to ferricyanide transport from bulk water to the electrode. It was removed by raising the pH to a value of 12, where the methylammonium groups were neutralized to amines. Lowering the pH to 7 again and addition of more of the anionic porphyrin reclosed the gap.The porphyrin "cover" should be localized at distances of 8-10 and 20 A from the bottom porphyrin by multiple charge interactions. The 8-10 A distance is ideal for studies of photoinduced electron transfer between two porphyrin monomers of different redox potential. Furthermore it was found, that redox-active tyrosine could be trapped in the water volume above the porphyrin on gold.

Bolaamphiphiles

“…70 Rigid membranes on smooth surfaces have been reviewed under the title of "Molecular Landscapes". 85 The stiffness may be reached by either two parallelrunning hydrogen-bond chains between two secondary amide groups at the ends of hydrophobic cores made of oligomethylene chains 39,[86][87][88] or stiff hydrophobic units such as bridged carbocycles, stilbene, or biphenylene [89][90][91][92][93][94][95][96] (Figure 21a-d).…”

“…The most useful outside headgroup is oligoethylene (OEG), which allows one to study the nanowells in water as well as in various organic solvents, e.g., chloroform. 69,87,88 The nanowells are the only means to order stacks of different single molecules on top of each other. Both water-and chloroform-soluble molecules can be used.…”

Functionalisation of bolaamphiphiles with mononuclear bis(2,2′-bipyridyl)ruthenium(ii) complexes for application in self assembled monolayers

“…cis 42 tetradec-2-enedioic acid 14-benzyl ester, 8 5-methyl-2,2'-bipyridine [23][24][25] and (2-amino-ethyl)-carbamic acid tert-butyl ester 17 were prepared by literature methods. …”

“…By judicious choice of functional group and conditions only the molecules which form the "picket fence" 16 around the porphyrin or steroid pore will be available to react, thus allowing modification of the pores without disturbance of the bulk structure of the membrane. 17 In an effort to expand on the possible approaches to the closure of these membrane pores, the synthesis of a novel lipid, which should be capable of capping a pore via complexation with a suitable metal ion is reported ( Figure 1). 2,2'-Bipyridine (bipy) is the ligand of choice as it forms stable complexes with a wide range of metals and is relatively easily functionalised.…”

Developments of Yoctowell Cavities as a Nano and Bio-technological Tool

“…Large molecules over 20 Å in diameter cannot enter into the gaps whereas molecules with size proportionate with that of the yoctowell gaps are able to enter and quench the fluorescence of the base porphyrin, whose role is both as a structural element and as a fluorescence reporter. In 2000, fluid and rigid yoctowells had been developed and used as carriers for heterodimers of porphyrins on gold platelets and colloidal gold nanoparticles [4][5][6][7][8]. However, one drawback of gold-based systems is plasmon absorption, which can cause problems (quenching the florescence of the base porphyrin) in photophysical studies.…”