**4.1 Albumins**

In this work, the solutions of human serum albumin (HSA) (>96%, Sigma) and of bovine serum albumin (BSA) (>98%, MP Biomedicals) in a phosphate buffer (0.01 M, pH 7.4) have been used. The proteins concentrations were 10–5 (absorption spectra measurement) and 10–9 M (fluorescence measurement at the nanosecond laser fluorimeter). All of the experiments were performed at a temperature of 25±1 0C. The structure and biological functions of HSA and BSA can be found in (Peters, 1996). Tryptophan, tyrosine, and phenylalanine (with relative contents of 1:18:31 in HSA and 2:20:27 in BSA) are the absorption groups in these proteins (as in many other natural proteins). The tyrosine fluorescence in HSA and BSA (as in many other natural proteins) is quenched due to the effect of adjacent peptide bonds, polar groups (such as CO, NH2), and other factors, and phenylalanine has a low fluorescence quantum yield (0.03) (Permyakov, 1992). Therefore, the fluorescence signal in these proteins is determined mainly by tryptophan groups. In that case the fluorescence, registered in nonlinear and kinetic laser fluorimetry measurements, correspond to tryptophan residues (this fact will be used in Section 6.1).

As described in (Peters, 1996), HSA and BSA have similar structure and amino acid sequences that differ insignificantly by location of some certain amino acids. However, HSA contains one tryptophan residue in the protein matrix (Trp-214), and BSA contains two residues (Trp-212 and Trp-134). Trp-212 in BSA and Trp-214 in HSA have a similar microenvironment and, hence, their spectral properties are similar (Eftink et al., 1977). Tryptophans of BSA are not spectrally identical due to the stronger integration of Trp-212 into the protein's structure and the more hydrophobic environment of Trp-212 in comparison with Trp-134. The distance between tryptophans in BSA is about 3.5 nm. This fact makes the intramolecular energy transfer between them using the Forster resonance energy transfer (FRET (Valeur, 2002)) mechanism possible.
