The different mechanisms of generation of rotational strengths from aromatic chromophores are studied theoretically in three beta-lactamases from class A - isolated from Staphylococcus aureus, Bacillus licheniformis and Streptomyces albus. The calculations are performed with the matrix method in its origin independent form based on X-ray structures of these proteins. The participation of the aromatic chromophores in all known mechanisms of generation of rotational strengths is analyzed comparatively. (C) 2004 Elsevier B.V. All rights reserved.