**5. Acknowledgements**

We deeply acknowledge M. Garonzi, M. Zucchelli, A. Pizzolato, C. Grigoli, A. Turati, M. Denitto, A. Atzeni, A. Bazaj, V. Marino, S. Compri, M. I. Muddei and G. Tosadori for the excellent work done for obtaining Figure 1.

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**4. Conclusion** 

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**Part 3** 

**Complimentary Methods** 


**Part 3** 

**Complimentary Methods** 

364 Current Trends in X-Ray Crystallography

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**16** 

*Brazil* 

Cristiano Luis Pinto Oliveira

**Investigating Macromolecular Complexes in** 

*Department of Experimental Physics, Institute of Physics, University of São Paulo* 

Macromolecular complexes have a huge interest in molecular biology. The comprehension of the biological processes in living systems is directly related to the knowledge of the shape and structure of the formed complex and the process of formation. Although X-ray diffraction, Nuclear Magnetic Resonance and cryoEM can provide information on the formed structures, there are several cases where none of those techniques can be applicable. Limitations on molecular weight, the necessity of a well ordered crystal, difficulties on sample preparation etc, are some of the bottle necks of those techniques (Svergun, 2007; Oliveira et al, 2010). Most importantly, in several cases the studies have to be performed directly in solution, with minimum interaction with the studied sample in order to avoid biased results. In this respect, scattering techniques are highly recommended since they allow a study directly in solution in a very non-invasive way. Small angle X-Ray scattering (SAXS) is a standard technique that can be applied to the study of particles in solution, providing information on size, shape, polydispersity, flexibility, oligomerization and aggregation state. Also, it allows real time measurements where the system can be monitored directly in solution, enabling the study of the in situ particle formation (Oliveira et al, 2009). The combination of SAXS and microscopy techniques has been used in several applications due to their complementarity (Oliveira et al, 2010; Andersen et al, 2009). In this chapter some general aspects of Small Angle X-Ray scattering and the state of the art

When a collimated beam (assumed as parallel waves) of X-ray photons strikes a sample, a fraction of the incident beam interacts with the electrons clouds of each molecule, and a possible process is the absorption of this photon by the atoms which excites the electrons of the atom to higher energy levels. When the excited electrons decay to ground state another X-Ray photon is reemitted as a spherical wave. In this way, this process can be viewed as the scattering of the incident photon over the electronic cloud. Depending on the energy of the incident photon several processes can happen: Rayleigh scattering, Resonance scattering, Compton effect, Thomson scattering, pair production, etc. It is beyond the scope of this chapter investigate all these possible processes. However, under certain energy limits (~7- 12KeV), this scattering is well described by the so called first Born approximation, where the

modeling methods will be presented, with several applications.

**2. Small angle X-Ray scattering** 

**1. Introduction**

**Solution by Small Angle X-Ray Scattering** 
