By using attosecond pulses to promptly ionize the amino acid phenylalanine, several European researchers involved within XLIC Action have detected an ultrafast response of purely electronic origin, which is characterized by a charge oscillation with two main frequency components corresponding to periods of 2.8 fs and 4.2 fs. Electron dynamics in biomolecules on a temporal scale preceding nuclear motion is at the heart of many biological processes. This work has been published in Science.
This mixed experimental/theoretical work, undertaken by researchers from Milano, Madrid, Belfast and Trieste, has found clear experimental evidence of ultrafast charge dynamics in the phenylalanine amino acid, after prompt ionization induced by attosecond pulses. Charge migration shows up as oscillations in the yield of a doubly-charged molecular fragment produced from ionization of a second electron by a probe pulse as a function of its delay time. Two main frequencies were measured: 0.24 PHz (corresponding to a period of 4.2 fs) and 0.36 PHz (period of 2.8 fs), thus confirming the electronic origin of the measured dynamics. Numerical simulations of the temporal evolution of the electronic wave packet created by the attosecond pulse strongly support the interpretation of the experimental data in terms of charge migration.
The ability to initiate and observe purely electronic dynamics in the building blocks of life represents a crucial step forward in attosecond science, which is progressively moving towards the investigation of more and more complex systems and can be considered as a first contribution towards attobiology.
Direct measurement of the ultrafast charge dynamics in an amino acid, initiated by attosecond pulses, represents a crucial benchmark for the extension of attosecond methodology to biology. In the same way in which femtosecond pulses have contributed (and still contribute) to the investigation and understanding of important biological processes, attosecond science offers the possibility to elucidate, on a temporal scale preceding nuclear motion, subtle processes ultimately triggering and determining the response of biomolecules. 
For instance, how the ultrafast motion of a hole in DNA created by a high-energy particle might initiate cell necrosis or mutation. The results obtained in the case of phenylalanine can be seen as the first experimental confirmation that attosecond pulses and techniques will be essential tools for understanding of dynamical processes on a temporal scale that is relevant for the evolution of crucial microscopic events at the heart of the macroscopic biological response of molecular complexes.
This study has been conducted by the following XLIC nodes:
- IFN-CNR and Politecnico di Milano (Italy). Groups led by Francesca Calegari and Prof. Mauro Nisoli, PI of the ERC-AdG-ELYCHE project (GA nº 227355)
- Universidad Autonoma de Madrid and IMDEA-Nanociencia (Spain). Group led by Prof. Fernando Martín, PI of the ERC-AdG-XCHEM project (GA nº 290853)
- Center for Plasma Physics, QUB.(Dr. Jason Greenwood)
- Università di Trieste and CNR-IOM. (Dr. Piero Decleva)
Ref. Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses
F. Calegari, D. Ayuso, A. Trabattoni, L. Belshaw, S. De Camillis, S. Anumula, F. Frassetto, A. Palacios, P. Decleva, J. B. Greenwood, F. Martín, M. Nisoli
Science 346, 336-339 (2014). DOI:10.1126/science.1254061
