Time-Resolved CIDNP Study of Native-State Bovine and Human α-Lactalbumins

Abstract

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of two aromatic dyes (2,2‘-dipyridyl, DP, and flavin mononucleotide, FMN) with bovine and human α-lactalbumins (BLA and HLA) in aqueous solution have been studied using the time-resolved CIDNP technique. With DP as the photosensitizer, polarization in BLA is observed for the protons of Trp118, His68, Tyr18, and Tyr103. The latter is not manifested in the CIDNP spectra obtained with FMN. This threshold effect on CIDNP formation indicates that accessibility is a combined property of both the residue and the dye. The nuclear spin−lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics: T1 = 60 μs for H2,4,7 of Trp118, T1 = 53 μs for H3,5 of Tyr103, T1 = 16 μs for H3,5 of Tyr18, and T1 = 10 μs for the H2 and H4 of His68. The correlation times for the side chain motion as determined from the T1 of the radicals are correlated with the accessibility of the side chains in the intact protein.