Conformational Analysis of the Electron-Transfer Kinetics across Oligoproline Peptides Using N,N-Dimethyl-1,4-benzenediamine Donors and Pyrene-1-sulfonyl Acceptors

Abstract

Photoinduced intramolecular charge separation across proline-bridged donor-acceptor complexes of the type Pyr-(Pro)n-DMPD (where Pyr=pyrene-1-sulfonyl and DMPD=N,N-dimethyl-1,4-phenylenediamine) was studied. The steady-state emission spectrum for n=0, 1, 2, 3 showed an increase in emission intensity with the number of proline residues. Time-dependent emission measured by streak camera showed increasing emission signal amplitude with increasing n, along with a decrease in decay rate. In all these studies, Pyr-Pro was used as a control complex for the decay of the excited pyrene acceptor moiety without the donor DMPD. Detailed photon counting experiments carried out in DMF/water, DMF, and toluene showed single-exponential kinetics for n=0, 1 and multiexponential kinetics for n=2, 3. Rate constants observed in DMF are for n=0, k=approximately 5x10(10) s(-1); n=1, k=9.70x10(8) s(-1); n=2, k=35.9x10(8) s(-1) (70%) and 5.58x10(8) s(-1) (30%); and n=3, k=16.6x10(8) s(-1) (55%) and 3.87x10(8) s(-1) (45%). These results show that a significant percentage of the n=2 and n=3 molecules undergo faster electron transfer than for the n=1 case. Conformational analysis for Pyr-(Pro)n-DMPD molecules in water showed that whereas only one conformation is possible for n=1, eight are possible for n=2, and 32 are possible for n=3. Calculation of the free energy and electronic coupling for these conformers in water showed that only a few of these conformations have the appropriate energy and electronic coupling to be observed in the experimental time window from 20 ps to 20 ns. Assignment of the conformers undergoing electron transfer in Pyr-(Pro)n-DMPD for n=2 and 3 was based on the values for the n=1 case, for which the measured rate constant is approximately 10(9) s(-1) and the calculated electronic coupling matrix element Hda is 297 cm(-1). The similarity in ground state energy between the cis and trans conformers for n=2 and 3, their use in aqueous-organic and organic solvents, and the nature of the Pyr and DMPD acceptor and donor groups could be contributing causes for the multiexponential kinetics, which was not observed for the metal ion derivatives of proline peptides studied earlier in aqueous solution.

Insights into the Distance Dependence of Electron Transfer through Conformationally Constrained Peptides

“…This is the case, e. g ., of ET through oligoprolines . Calculations indeed pointed to the importance of changes in backbone dihedral angles as a way to increase the electronic coupling . Several studies provided further arguments in favor of conformationally gated ETs .…”

“…Besides reactions 3,4,14-16, in the mechanism we included the possible effect of the corresponding intermolecular processes. Eqs (17), (18) and (19), (20) are the equivalent of Eqs. (12), (13), which are written for ET from a generic donor D * À .…”

“…A very small effect is only detected on the current of the second peak (at low scan rates) if Eqs. (19), (20) are neglected. In other words, removing Eqs.…”

“…[19] Calculations indeed pointed to the importance of changes in backbone dihedral angles as a way to increase the electronic coupling. [20,21] Several studies provided further arguments in favor of conformationally gated ETs. [22] Flexibility of the peptide backbone is also liable to cause unexpected ET rates thanks to through-space D/A orbital overlap.…”

Relationship between Conformational Dynamics and Electron Transfer in a Desolvated Peptide. Part II. Temperature Dependence

“…16 In each case, the conformer structures are responsible for ensemble averaged electron transfer rates and also average matrix elements. In our measurements these conformer structures arise from large-scale fluctuations of the dye and tryptophan side chain.…”