Gold nanoparticles are being increasingly proposed as biotechnological tools for medical diagnosis and therapy purposes. Their safety for human beings and the environment is therefore becoming an emerging issue, which calls for basic research on the interactions between nanostructured gold particles and biological materials, including physicochemical studies of model systems. In this Article, we focus on the "reaction products" of a widely known nanoparticle type, citrate-capped 30 nm gold nanospheres, with a model protein, horse myoglobin. Protein adsorption and partial denaturation were accompanied by the formation of nanoparticle aggregates with strongly distinct optical spectroscopy properties and shapes, as observed by transmission electron microscopy. We singled out the concentration of myoglobin as the determinant of these differences, and verified on this basis that surface-enhanced Raman scattering (SERS) spectra can only be obtained by aggregates with strong interparticle optical coupling, which are obtained at low protein concentration. The results can be useful both in improving the spectroscopy of biomolecules and in understanding the formation of the protein corona in biomedical applications.