977 Pure Appl. Chem., Vol. 77, No. 6, pp. 977–993, 2005. DOI: 10.1351/pac200577060977 © 2005 IUPAC Toward a computational photobiology* Adalgisa Sinicropi 1 , Tadeusz Andruniow 1,2 , Luca De Vico 1 , Nicolas Ferré 3 , and Massimo Olivucci 1,‡ 1 Dipartimento di Chimica, Università di Siena, Italy; 2 Department of Chemistry, Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wroclaw, Poland; 3 Laboratoire de Chimie Théoretique et de Modélisation Moléculaire, CNRS Université de Provence, France Abstract: In this paper, we discuss the results of our recent studies on the molecular mecha- nism, which stand at the basis of the photochemical processes occurring in photobiological systems. These results are obtained using modern, robust, and fairly accurate high-level quantum chemical methods. Keywords: photobiology; computational photobiology; quantum chemical; molecular mech- anisms; photoisomerization. INTRODUCTION The aim of this contribution is to show that computers can now be used to understand the molecular mechanism of photochemical processes occurring in photobiological systems. Accordingly, we discuss some recent spectral simulations of the visual pigment rhodopsin (Rh) and of the green fluorescent pro- tein (GFP). We also present work related to the mapping of the photoisomerization path of these com- plex bioorganic molecules. In simple terms, there are two events that may happen when the light energy is absorbed by a mol- ecule. This energy is either wasted or exploited (Scheme 1). The control of these events can be consid- ered as a basic requirement for the rational design of efficient photochemical reactions, artificial photo- synthetic systems, novel materials, molecular devices, and molecular-level machines. In fact, technology often requires molecules where this energy is not wasted, but exploited to achieve specific chemical, conformational, and electronic changes. In contrast, other applications, such as those in the fields of photoprotection or photobiology, need molecules that are structurally unaffected by light ab- sorption (e.g., through light emission and internal conversion). In this respect, during the last few years, computational methods have been successfully applied to explore energy wastage processes such as the quenching of fluorescent probes [1–3] and the mecha- nism of fast internal conversion in the photostabilization of DNA basis [4,5]. Similarly, as an example of process where light is exploited to drive efficient and stereoselective photochemical reactions, we can recall the ultrafast pericyclic reactions [6]. Most interestingly, the same types of processes are known in photobiology. For instance, there are fluorescent proteins, such as GFP, where the energy of the pho- ton can be wasted to produce fluorescence, while there are other photoreceptors, such as the visual pig- ment Rh, where the energy of the photon is exploited to produce a change in the protein structure. *Paper based on a presentation at the XX th IUPAC Symposium on Photochemistry, 17–22 July 2004, Granada, Spain. Other presentations are published in this issue, pp. 925–1085. ‡ Corresponding author