Charged amino acids as spectroscopic determinants for chlorophyll in vivo

Abstract

In this paper we propose that the large spectroscopic red shifts observed for chlorophyll (Chl) and bacteriochlorophyll (BChl) in vivo may be due to charged amino acids in the binding site. Molecular orbital calculations of the transition energies of Chl in the field of external charges are carried out. The calculated wavelength shifts induced by these charges are comparable in magnitude to those observed in vivo. Moreover the size of the shifts increases in the order BChl b > BChl a > Chl a, which is the observed trend. The ability of the calculations to account for both the absolute and relative magnitudes of the wavelength shifts argues for the validity of the model. Further indirect support comes from the recent demonstration that charged amino acids are responsible for the colors of visual pigments and bacteriorhodopsin. In addition to their effects on spectra the presence of external charges induces large changes in the ionization potential of Chl molecules and thus might explain the in vivo alteration of the oxidation potentials in reaction centers.The difference between the properties of chlorophyll (Chl) molecules in solution and identical molecules in vivo has been a subject of great interest, extensive investigation, and much conjecture. It is clear that its local environment endows Chl in vivo with special physical and chemical properties, yet little is known about the nature of the interactions that produce these effects. Because it now appears that most if not all of the Chl in photosynthetic systems is intimately associated with proteins (1), it seems appropriate to consider how specific amino acids might affect the behavior of bound Chl. Properties that are strongly influenced by the protein environment include redox potentials, photochemically induced electron transfer, and absorption maxima. Of these, the latter are easiest to characterize and interpret because spectroscopic transition energies for conjugated systems may be calculated theoretically with considerable accuracy.Chl in vivo and in isolated lipoprotein complexes absorbs light at longer wavelengths than does monomeric Chl in simple organic solvents (2). The magnitude of the shift varies considerably among species and somewhat less for different pigments within a single organism (comparing, for example, reaction center with antenna Chl). In green plants the magnitude of the wavelength shift is between 300 and 800 cm-'. In the case of photosynthetic bacteria the shifts are often much larger, reaching 1,500 cm-' for bacteriochlorophyll (BChl) a-containing species and as much as 2,700 cm-1 for the antenna complex of the BChl b-containing species Rhodopseudomnonas (Rps.) viridis (2).Although the origin of these shifts is a question of some interest in its own right, its more general relevance owes to the possibility that the shift is due to highly specific and functionally important Chl-protein interactions. Thus, identifying the source of the large in vivo red shifts may be an important step in elucidating the mechanism of elec...

Electron nuclear double resonance evidence supporting a monomeric nature for P700 + in spinach chloroplasts

“…However, recent data obtained by Davis and Pearlstein (23) using well characterized chlorophyll model systems have shown that such effects can also arise as the result of interaction of a monomer with its protein environment. In agreement with this result, Eccles and Honig (24) have recently showed the profound effects caused by neighboring charged amino acid residues on the red shifts observed for neutral Chia, BChla, and BChlb. It is of interest to speculate that the combination of ligation, hydrogen bonding, and charge effects contribute jointly to the lowering of the D1 orbital for P700'.…”

α-Helices direct excitation energy flow in the Fenna–Matthews–Olson protein

“…Single amino acid side chains, either charged or neutral, cause shift magnitudes of at most Ϸ180 cm Ϫ1 and tend to compensate each other (SI Tables 3-5). Accordingly, the charged amino acids do not play the dominant role that was attributed to them in earlier studies (15)(16)(17)28), where large parts of the protein charge distribution, in particular, the backbone, were not modeled, and standard protonation states of titratable residues were assumed (15)(16)(17). In qualitative agreement with experimental results on other antenna systems (29), hydrogen bond donors to BChla cause red shifts of the site energies, but in the present system this shift never exceeds Ϸ130 cm Ϫ1 (SI Table 4).…”

Optical spectrum of myeloperoxidase

“…Charges close to a porphyrin in a proteinporphyrin complex may significantly affect the positions of the absorption maxima of the protein. Theoretical considerations have shown that charges in the vicinity of chlorophyll a may significantly affect the position of the absorption maxima [34]. It has been suggested, in view of these calculations, that a protonatable, negatively charged residue near the chromophore (0.3 -0.35 nm) may be present in myeloperoxidase, which may become protonated at low pH and which affects the electronic structure of the prosthetic group and imposes the red shift in the absorption spectra [27].…”