Molecular Properties of Bare and Microhydrated Vitamin B5-Calcium Complexes

Abstract

Pantothenic acid, also called vitamin B5, is an essential nutrient involved in several metabolic pathways. It shows a characteristic preference for interacting with Ca(II) ions, which are abundant in the extracellular media and act as secondary mediators in the activation of numerous biological functions. The bare deprotonated form of pantothenic acid, [panto-H]^-, its complex with Ca(II) ion, [Ca(panto-H)]⁺, and singly charged micro-hydrated calcium pantothenate [Ca(panto-H)(H₂O)]⁺ adduct have been obtained in the gas phase by electrospray ionization and assayed by mass spectrometry and IR multiple photon dissociation spectroscopy in the fingerprint spectral range. Quantum chemical calculations at the B3LYP(-D3) and MP2 levels of theory were performed to simulate geometries, thermochemical data, and linear absorption spectra of low-lying isomers, allowing us to assign the experimental absorptions to particular structural motifs. Pantothenate was found to exist in the gas phase as a single isomeric form showing deprotonation on the carboxylic moiety. On the contrary, free and monohydrated calcium complexes of deprotonated pantothenic acid both present at least two isomers participating in the gas-phase population, sharing the deprotonation of pantothenate on the carboxylic group and either a fourfold or fivefold coordination with calcium, thus justifying the strong affinity of pantothenate for the metal.

Keywords: IRMPD spectroscopy; calcium; complexation; molecular structure; pantothenate; quantum chemistry calculations; vitamin B5.

Scheme 1

Schematic representation of (R)-pantothenic acid. Annotation for atom numbering is reported.

Figure 1

Experimental infrared multiple photon dissociation (IRMPD) spectra of: (c) deprotonated pantothenic acid [panto-H]⁻ when the ions are submitted either to 500 ms of free electron laser (FEL) light (blue profile) or to 120 ms using one attenuator (red profile); (b) pantothenate calcium complex [Ca(panto-H)]⁺ irradiated with either 1 s of FEL light (blue profile) or with 500 ms using one attenuator (red profile); (a) and its monohydrated form [Ca(panto-H)(H₂O)]⁺ with 500 ms of FEL light and one attenuator (blue profile) or with 250 ms and three attenuators (red profile).

Figure 2

Structures of [panto-H]⁻ optimized at the B3LYP/6-311++G(d,p) level. Hydrogen bonds are reported with dashed lines together with their length (Å). Relative free energies at 298 K in kJ mol⁻¹ are computed at the B3LYP and MP2 level (in parentheses).

Figure 3

(c) Experimental IRMPD spectrum of deprotonated pantothenic acid [panto-H]⁻ (bottom, colored profiles); (b) computed IR spectrum of Panto_a1 isomer; (a) computed IR spectrum of Panto_b1 isomer. IR spectra and optimized structures are obtained at the B3LYP level. Relative free energies at 298 K in kJ mol⁻¹ are computed at the B3LYP and MP2 level (in parentheses).

Figure 4

Structures of [Ca(panto-H)]⁺ and [Ca(panto-H)(H₂O)]⁺ optimized at the B3LYP/6-311++G(d,p) level. Hydrogen bonds are reported with yellow dashed lines together with their length, while gray dashed lines indicate metal bonds. Relative free energies at 298 K in kJ mol⁻¹ are computed at the B3LYP and MP2 level (in parentheses).

Figure 5

(c) IRMPD spectrum of [Ca(panto-H)]⁺ in the fingerprint range obtained submitting the ions either to 1 s of FEL light (blue profile) or to 500 ms using one attenuator (red profile). The calculated spectra of the lowest lying structures (b) CaPa_a1 and (a) CaPa_a2 are reported in black together with their optimized geometry and relative free energies (298 K) at the B3LYP and MP2 (in bracket) levels in kJ mol⁻¹.

Figure 6

(c) Experimental IRMPD spectrum of [Ca(panto-H)(H₂O)]⁺ (bottom, colored profiles) and computed IR spectra of isomers (b) CaPaW_a1 and (a) CaPaW_a2 obtained at the B3LYP level. Optimized structures at the B3LYP level are reported together with free energies at 298 K in kJ mol⁻¹ computed at the B3LYP and MP2 level (in parentheses).