Time-resolved FT EPR and optical spectroscopy study on photooxidation of aliphatic alpha-amino acids in aqueous solutions; electron transfer from amino vs carboxylate functional group

Determining the Secondary Structure of Membrane Proteins and Peptides Via Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy

Revealing detailed structural and dynamic information of membrane embedded or associated proteins is challenging due to their hydrophobic nature which makes NMR and X-ray crystallographic studies challenging or impossible. Electron paramagnetic resonance (EPR) has emerged as a powerful technique to provide essential structural and dynamic information for membrane proteins with no size limitations in membrane systems which mimic their natural lipid bilayer environment. Therefore, tremendous efforts have been devoted toward the development and application of EPR spectroscopic techniques to study the structure of biological systems such as membrane proteins and peptides. This chapter introduces a novel approach established and developed in the Lorigan lab to investigate membrane protein and peptide local secondary structures utilizing the pulsed EPR technique electron spin echo envelope modulation (ESEEM) spectroscopy. Detailed sample preparation strategies in model membrane protein systems and the experimental setup are described. Also, the ability of this approach to identify local secondary structure of membrane proteins and peptides with unprecedented efficiency is demonstrated in model systems. Finally, applications and further developments of this ESEEM approach for probing larger size membrane proteins produced by overexpression systems are discussed.

Oxidation of Cyclic Dipeptides by Photoinduced H-Atom Abstraction. A Laser Flash FT EPR and Optical Spectroscopy Study

Laser flash photolysis with the Fourier transform electron paramagnetic resonance (FT EPR) and optical spectroscopy detection methods on the nanosecond time scale have been employed in order to investigate the oxidation mechanism of cyclic dipeptides glycine, alanine, and sarcosine anhydrides initiated by SO4*- or 9,10-anthraquinone-2,6-disulfonate (2,6-AQDS) triplet in oxygen free aqueous solutions. A direct hydrogen abstraction from the ring C-H position of an anhydride by both oxidants is proposed as the primary reaction, rather then an electron transfer from nitrogen followed by (alpha)C-H deprotonation. The overall second-order rate constants for the reaction with SO4*- were determined to be 7.2 x 10(7) M(-1) s(-1), 1.2 x 10(8) M(-1) s(-1), and 5.2 x 10(8) M(-1) s(-1) for glycine anhydride, alanine anhydride, and sarcosine anhydride, respectively. The rate constants for 2,6-AQDS triplet as oxidizing species are about two times lower. The radical intermediate products derived from cyclic dipeptides observed on the microsecond time scale were assigned to the general structure of piperazine-2,5-dione-3-yl radical. These are spin polarized by the mechanisms of chemically induced dynamic electron polarization (CIDEP). For SO4*- as the oxidant the spectra are exhibiting an E/A* polarization pattern originating partially from F-pairs of two piperazine-2,5-dione-3-yl radicals.