# Study of Calcitriol Interaction with the Vitamin D Receptor Using DFT and TD-DFT Calculations

## TL;DR

This study uses computational methods to understand how calcitriol interacts with the vitamin D receptor, aiming to improve drug design.

## Contribution

The study provides residue-level insights into VDR-calcitriol interactions using DFT and TD-DFT calculations.

## Key findings

- Calcitriol's π → π* transition is influenced by interactions with TRP286 and TYR295.
- The calculated binding energy is consistent with experimental affinity of the VDR construct.
- The study identifies the most stable conformations of VDR-calcitriol interactions.

## Abstract

Calcitriol, the primary
active metabolite of vitamin
D, has garnered
significant research interest due to its role in several pathologies.
However, excessive calcitriol levels or heightened sensitivity of
the vitamin D receptor (VDR) can lead to hypercalcemia, motivating
the search for analogues that preserve therapeutic activity while
reducing adverse effects. Understanding the molecular basis of VDR-calcitriol
recognition is therefore essential for rational ligand design. In
this study, we applied the ONIOM2­(B3LYP/6–31++G­(2d,p):PM7)
hybrid methodology to characterize VDR-calcitriol interactions and
identify the most stable conformations while ensuring computational
efficiency. Additionally, TD-DFT calculations were performed to explore
its electronic properties. We show that calcitriol remains the dominant
chromophore and that its main π → π* transition
is subtly influenced by interactions with TRP286 and TYR295, providing
residue-level insight that is experimentally inaccessible due to the
absence of UV–vis data for the holo complex. Furthermore, the
calculated binding energy (−11.88 kcal/mol) is consistent with
the experimental affinity of the crystallographic VDR construct, supporting
the reliability of the predicted binding mode. This integrated analysis
of structural, energetic, and electronic features offers new mechanistic
insight into VDR-calcitriol recognition and may guide the development
of analogues with improved therapeutic profiles.