Bacteriochlorophyll versus chlorophyll: In what key ways do bacteriochlorophylls differ from the chlorophylls of oxygenic phototrophs?

Introduction / Context:
Bacteriochlorophylls power anoxygenic photosynthesis in diverse bacteria. While structurally related to plant chlorophylls, they are tuned for distinct ecological niches and light regimes, which is reflected in both chemical substituents and spectral properties.

Given Data / Assumptions:

• Bacteriochlorophylls and chlorophylls are porphyrin-like (or chlorin/bacteriochlorin) macrocycles with a central metal.
• We compare metal center, side-chain chemistry, and absorption spectra.
• The question seeks the combination that is correct.

Concept / Approach:

Both chlorophylls and bacteriochlorophylls typically coordinate magnesium (Mg) at the center of the macrocycle. However, bacteriochlorophylls differ in hydrogenation of rings and in R-group substituents (e.g., presence of different side chains), which shift electronic transitions. Consequently, bacteriochlorophylls display absorption maxima shifted toward the near-infrared relative to plant chlorophylls. Therefore, differences lie in side-group chemistry and spectra, not in the chelated metal.

Step-by-Step Solution:

Verification / Alternative check:

Spectroscopic data from purple and green bacteria show Qy bands in the near-infrared; structural analyses document side-chain variations (e.g., bacteriochlorin vs chlorin cores).

Why Other Options Are Wrong:

(a) Metal identity is not the distinguishing feature; both commonly use Mg.

Single-statement options omit part of the correct picture; both chemistry and spectra differ.

Common Pitfalls:

Assuming “bacterio-” implies a different central metal; ignoring how subtle side-chain changes produce major spectral shifts.

Final Answer:

Both (b) and (c)