title: Light-Induced Porphyrin-Based Spectroscopic Ruler for Nanometer Distance Measurements.
abstract: We present a novel pulsed electron paramagnetic resonance (EPR) spectroscopic ruler to test the performance of a recently developed spin-labeling method based on the photoexcited triplet state (S=1). Four-pulse electron double resonance (PELDOR) experiments are carried out on a series of helical peptides, labeled at the N-terminal end with the porphyrin moiety, which can be excited to the triplet state, and with the nitroxide at various sequence positions, spanning distances in the range 1.8-8 nm. The PELDOR traces provide accurate distance measurements for all the ruler series, showing deep envelope modulations at frequencies varying in a progressive way according to the increasing distance between the spin labels. The upper limit is evaluated and found to be around 8 nm. The PELDOR-derived distances are in excellent agreement with theoretical predictions. We demonstrate that high sensitivity is acquired using the triplet state as a spin label by comparison with Cu(II)-porphyrin analogues. The new labeling approach has a high potential for measuring nanometer distances in more complex biological systems due to the properties of the porphyrin triplet state.
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Marilena
authors.initials: M
authors.lastname: Di Valentin
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Albertini
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Maria Giulia
authors.initials: MG
authors.lastname: Dal Farra
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.|Leiden Institute of Physics, Leiden University, Niels Bohrweg 2, 2333, CA Leiden, The Netherlands.
authors.forename: Enrico
authors.initials: E
authors.lastname: Zurlo
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Orian
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Antonino
authors.initials: A
authors.lastname: Polimeno
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Marina
authors.initials: M
authors.lastname: Gobbo
authors.affiliation: Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131, Padova, Italy.
authors.forename: Donatella
authors.initials: D
authors.lastname: Carbonera
keywords: DEER/PELDOR; EPR; ONIOM calculations; distance ruler; peptides; porphyrins