Abstract

Bovine serum albumin, as well as other proteins, produces optoacoustic signals when excited with a laser pulse at wavelengths from 532 to 670 nm, a region where these proteins have no reported absorption. Signals from filtered solutions (obtained at low fluences) exhibited a linear dependence on laser energy and on protein concentration. Hence, they are attributed to radiationless deactivation following weak absorption in the visible region. Molar absorption coefficients calculated for different proteins showed a similar wavelength dependence, albeit with different absolute values—differences that could not be simply correlated to the content of known protein chromophores, such as tryptophan, tyrosine, and peptide bond. Moreover, at 532 nm, such chromophores produced optoacoustic signals only slightly higher than those from the buffer. On the other hand, a synthetic copolymer of tryptophan and lysine produced signals of a magnitude comparable to those from the proteins. Therefore, the weak absorption detected by laser-induced optoacoustics should be attributed to a not-yet-well-established feature of the structure of the protein, probably related to charge transfer interactions between different amino acids. Regardless of their origin, these signals must be taken into account when chromophoric prosthetic groups are studied with the use of laser-induced optoacoustic spectroscopy.

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