Measuring the Intrinsic Dipole Moments of Oligo-ortho-arylamides

Stephen Bishop, Duoduo Bao, Gokul Upadhyayula, Alex Gerasimenko and Dr. Valentine I. Vullev* Bioengineering Department, University of California Riverside, 92521

*Correspondence email: vullev@ucr.edu

Background

Abstract

AcknowledgmentsJun Wang

Robert BondererNoah Johnson

Dr. Sharad GuptaDr. Diane Marsh

Dr. Daniel Bernier

•Create bio-inspired donor-bridge-acceptor (DBA) systems using oligo-ortho-arylamides

•Measure intrinsic dipole moments of various lengths in oligo-ortho-arylamides

•Compare experimental results to calculated values

Currently, our civilization is highly dependent on fossil fuels for energy. Due to the environmental threats caused by burning of fossil fuels, increasing attention is drawn to alternative energy sources such as solar energy.1 As an alternative, efficient solar-energy-conversion technologies will originate from investigating the fundamentals of photo-induced charge transfer. We aim to develop bio-inspired systems that possess intrinsic dipole moments and will allow us to modulate the charge-transfer processes. We base our macromolecular designs on oligo-ortho-arylamides.2

The Vullev Group is researching new ways to modulate charge transfer: i.e., to accelerate the photo-induced charge separation and suppress the undesirable charge recombination. A principal objective is to create donor-bridge-acceptor (DBA) systems with π conjugated oligo-ortho-arylamides as bridging moieties. My aim in this project is to synthesize and characterize an electron donor molecule, 4-(6-dihexylamino-1,3-dioxo-1H,3H-benzo[de]isoquinolin-2-yl)-benzoic acid. The carboxylate functionality of this donor molecule will allow us to incorporate it in the DBA systems via solid-phase peptide synthesis protocols. This donor molecule will supply the electrons that will travel across the DBA conjugates.

My next aim is synthesizing the bridge molecules. The bridge will comprise oligo-ortho-arylamides with lengths of 2, 4, 8 and 16 residues. Utilizing impedance-spectroscopy and density techniques, we will determine the length-dependence of the dipole moments of the oligomers and compare them with the values we obtained from DFT calculations.

The DBA systems will then be paired (in pairs) based on oligomer lengths. Each system will contain the identical number of residues in their oligomer chains, but one of the DBAs will have the donor and acceptor in opposite positions. These DBA configurations will allow us to determine the differences in the rates of the electrons moving to the positive vs. negative poles of the dipoles intrinsic to the DBA macromolecules.

We will compare the results with theoretical values that we obtain from semi-classical (Marcus transition-state theory) and ab initio (DFT) calculations. This data will help in the development of bio-inspired systems that can facilitate better photo-induced charge transfer and ultimately allow for more efficient harvesting of solar energy.

Objectives

•Polypeptide α-helices have a large intrinsic dipole moment of approximately 4-5 Debye per residue3

Molecular Design Experimental Results

Methods

1

21

1

1

12

11

102

2

31

2

1

aMbM

MPTk

NPPP

B

v33

2

0

002

02

02

•The investigation will concentrate on oligo-ortho-arylamides

Figure 1. Intrinsic dipoles of the polypeptide alpha helix are oriented toward their C termini and the negative poles toward their N termini.

Hedestrand Equation: Debye’s Equation:

21s aXεε 21 bXs

Future Directions

Figure 4. “Balls and sticks” molecular models of aromatic oligo-ortho-amides composed of (a) 2-aminobenzoic acid (anthranilic acid) and (b) 3-amino-!-naphthoic acid. The direction of the dipole moments follows the orientation of the dipoles of the amide bonds and the shift of the electron density from the carbonyl oxygen to the amide hydrogen atoms in the hydrogen-bond network.2

References

Figure 2. Bioinspired aromatic oligo-ortho-arylamides: origin of the intrinsic dipole moment.2

Figure 2. Molecular structure of dimer.

•Extensive π conjugation can facilitate long range charge transfer, thus allowing for a greater dipole moment5

Figure 5. Densitometer used to measure density of solution

a b

Figure 8. Proton NMR confirmation of N2-hexanoylanthranilimde from (a) 7-12 ppm and from (b) 0-4 ppm.

DMAP, DMFIce bath, Ar gas+

•Measure intrinsic dipole moments of oligo-ortho-arylamides

•Compare the results with theoretical values

•The synthesis of N2-hexanoylanthranylamide by combining 2-Aminobenzamide, 4-Dimethylaminopyridine in DMF

•The reaction takes place once Hexanoyl chloride is added under Argon conditions and cold temperature

•Initial purification is done by washing the dissolved compound in a separation flask (figure 6) using acidic and basic water Figure 6.

Separation flask

Figure 7. Büchi Rotovapor R-114

•The rotovap (figure 7) is used to remove excess solvent•Recrystalization is performed as the final method of purification

•Utilize bioinspired macromolecules to enhance the efficiency in harvesting solar energy

2Ashraf, M. K.; Millare, B.; Gerasimenko, A. A.; Bao, D.; Pandey, R. R.; Lake, R. K.; Vullev, V. I. Theoretical Design of Bioinspired Macromolecular Electrets Based on Anthranilamide Derivatives. Biotechnology Progress 2008, in press.

1Lewis, N.S., “Toward Cost-Effective Solar Energy Use,” Science 315, 798-801 (2007).

3Fedorova, A., Chaudhari, A. and Ogawa, M. Y., "Photoinduced electron-transfer along a-helical and coiled-coil," Journal of the American Chemical Society 125, 357-362 (2003). 4Mayo, S. L.; Ellis, W. R., Jr.; Crutchley, R. J.; Gray, H. B. Long-range electron transfer in heme proteins. Science 1986, 233, 948-952.

5Visoly-Fisher, I.; Daie, K.; Terazono, Y.; Herrero, C.; Fungo, F.; Otero, L.; Durantini, E.; Silber, J. J.; Sereno, L.; Gust, D.; Moore, T. A.; Moore, A. L.; Lindsay, S. M. Conductance of a biomolecular wire. Proceedings of the National Academy of Sciences of the United States of America 2006, 103, 8686-8690.

Conclusions•N2-hexanoylanthranylamide was confirmed via proton NMR

•The synthesis procedure of N2-hexanoylanthranylamide was improved by keeping the reaction conditions under argon during the addition of Hexanoyl chloride

•A general radio bridge will be used to measure capacitance and to acquire dielectric measurements

•Optimal function of biological motifs (i.e. polypeptide α-helices) are restricted by properties such as temperature and media4