Conformational changes of Robinia pseudoacacia lectin related to modifications of the environment: FTIR investigation

The secondary structural characteristics of one of the Robinia pseudoacacia lectins (RPA3) have been investigated by FTIR spectroscopy and have been established from absorption measurements in the amide I,I' frequency range and from the quantitative estimation of the rate of NH---N2H exchange. In an anhydrous state the protein structure consists mainly of antiparallel and parallel beta-structures, which represent 60% of the overall secondary structure of RPA3. Data obtained in different polar media (KBr, 2-chloroethanol, 2H2O, NaCl-2H2O and/or DPPC) reveal that RPA3 is a highly flexible protein. In pure 2H2O a rapid solvation of free peptide units and weak peripheral hydrogen bonds occurs, followed by the solvation of more internal parts of the lectin. The protein precipitates before total unfolding is reached. Increasing the ionic strength modifies the rate of NH---N2H exchange. NaCl concentrations of less than or equal to 0.15 M stabilize RPA3 in a structure close to that of the lyophilized lectin and diminish the rate of exchange, whereas higher NaCl concentrations partially disrupt the original secondary structure and increase the rate of exchange. Furthermore RPA3 was shown to interact with DPPC through polar interactions between the polar heads of the phospholipid and specific peptide units. These interactions appear to favor the NH---N2H exchange.