Question

Vincristine in ultraviolet spectrophotometry \(\lambda_{max}\) value.

• A. 217 nm
• B. 245 nm
• C. 290 nm
• D. 360 nm

Hint:

Always use the compound's known $\lambda_{max}$ for most accurate quantitative UV analysis, as this gives the highest molar absorptivity.

Answer 1

The Correct Option is C

Vincristine is a vinca alkaloid used in cancer chemotherapy and exhibits characteristic absorbance in the ultraviolet region due to its conjugated double bond system and aromatic rings. In UV-Visible spectrophotometry, the compound shows a maximum absorbance ($\lambda_{max}$) at around 290 nm, which corresponds to the $\pi \rightarrow \pi^*$ electronic transition.
This wavelength is significant for qualitative and quantitative analysis of vincristine in pharmaceutical preparations, as it allows sensitive and specific detection using UV spectroscopy.
Vincristine’s $\lambda_{max}$ value is a result of its molecular structure, including indole and catharanthine-like moieties that absorb in the UV range. UV absorbance at 290 nm ensures accurate monitoring during analytical and quality control procedures.

Answer 2

The question asks for the wavelength of maximum UV absorbance of vincristine. Let's weigh each option against what is known about the chromophore in vinca alkaloids.

1. 217 nm: This shorter wavelength is typical of isolated, less conjugated chromophores. Vincristine's indole and dihydroindole ring systems are more extensively conjugated than this value suggests.
2. 245 nm: This intermediate value is closer to a simple indole absorbance band but does not capture the full extent of the fused, conjugated ring system found in vincristine's structure.
3. 290 nm: Vincristine's structure combines an indole unit with a dihydroindole (catharanthine-type) unit joined together, giving an extended conjugated system. This level of conjugation is consistent with absorbance shifting out to around 290 nm, which is the reported \( \lambda_{max} \) for this alkaloid.
4. 360 nm: This value is longer than what vincristine's degree of conjugation supports; such a long wavelength would suggest a much more extensively conjugated or highly substituted chromophore than vincristine actually has.

Weighing the extent of conjugation in vincristine's fused indole ring system against the four candidate wavelengths points to the mid-to-higher value as the correct one.

Therefore, the correct answer is 290 nm.