The effect of several types of BChl interactions of its resonance Raman (RR) spectrum has been examined. In monomeric BChl species, ligation interactions at the central Mg atom produce significant changes in its spectrum and two structure-sensitive bands are identified, which are analogous to those in other porphyrins. Hydrogen-bonding interactions in monomeric BChl affect only the C=0 modes, which are weakly in resonance at the excitation wavelength used here. In aggregated BChl species, large differences are observed in the RR spectra as compared to monomeric BChl for two of the three types of aggregates studied. These are the BChl pyrazine adduct (bifunctional ligand aggregate) and the BChl hydrate (hydrogen-bonded aggregate) absorbing at 845 nm. Both of these aggregates display RR spectra which are distinct from one another as well. The third type of BChl aggregate, that formed through coordination interactions of the acetyl or keto carbonyl group of one molecule with the Mg atom of another (self-aggregate), exhibits a spectrum which differs from monomeric BChl only in relative band intensities; no frequency shifts are observed. Additional results which are discussed include the identification of metal-sensitive and deuteration-sensitive bands. A comparison of the RR spectra of BChl a and its metal-free analogue, BPheo a, shows there are three bands which are sensitive to metalation in the region 1000-1800 cm-1. The fully deuterated spectrum of BChl is substantially different from the fully protonated.
ASJC Scopus subject areas
- Colloid and Surface Chemistry