Conformation and Metal Cation Binding of Zwitterionic Alanine Tripeptide in Saline Solutions by Infrared Vibrational Spectroscopy and Molecular Dynamics Simulations

In this work, linear infrared (IR) spectroscopy and molecular dynamics (MD) simulations were used to examine the interaction of different metal cations (Na+, Ca2+, Mg2+, and Zn2+) with backbone (amide CO) and C-terminal carboxylate (COO–) groups in zwitterionic alanine tripeptide (Ala3) in aqueous solutions with varying saline concentrations. Circular dichroism spectra and MD results suggest that Ala3 is predominantly in polyproline-II (PPII) conformation, whose amide-I and asymmetric carboxylate stretching IR vibration signatures are also supported by quantum-chemistry calculations. The zwitterionic form of Ala3 separates the two amide-I modes in frequency, which are weakly coupled modes, as revealed by two-dimensional IR measurement, and can be used to probe backbone–cation interactions at different scenarios (near charged or neutral chemical groups respectively). Cation concentration-dependent IR frequency red shifts in the amide-I mode are seen for both amide-I modes, whereas blue shifts are also seen in the amide-I mode far from the NH3+ group. The observed spectral changes are discussed from the perspective of the salting-in and salting-out abilities of the cations. In addition, all the metal cations studied here (except Zn2+) can specifically coordinate to the COO– group in bidentate and pseudo-bridging forms simultaneously. For Zn2+, only the pseudo-bridging form exists. Our results shed light on the macroscopic protein salting-in and salting-out phenomena from the perspective of key chemical bonds in peptides

Nitrogen nutrition is a key modulator of the sugar and organic acid content in citrus fruit - Fig 6

Response curves of ‘Huangguogan’ fruit N concentrations versus different organic acid components: (A) pyruvic acid, (B) oxalic acid, (C) fumaric acid, (D) quinic acid, (E) citric acid, (F) malic acid, and (G) lactic acid.</p

PCA analysis of sugars (fructose, Fru; glucose, Glu; sucrose, Suc), vitamin components (ascorbic acid, AsA; total AsA, T-AsA; dehydroascorbic acid, DHA), organic acids (pyruvic acid, Pur; oxalic acid, Oxa; fumaric acid, Fum; quinic acid, Qui; citric acid, Cit; malic acid, Mal; and lactic acid, Lac), and N in ‘Huangguogan’ fruits.

(A) Score plot of PC2 against PC1 (n = 30); days after the first blossom, DAFB. (B) Load plot of PC2 against PC1 (n = 30).</p

The main chemical and physical proprieties of the soil in the study area.

The main chemical and physical proprieties of the soil in the study area.</p

Dynamic changes in the concentration (mg g^-1) of ‘Huangguogan’ organic acids during fruit development (days after the first blossom, DAFB) in response to five N fertilizer levels: 0 (N₁), 1.36 (N₂), 1.81 (N₃), 2.26 (N₄), or 2.72 (N₅) kg y^-1.

Error bars indicate the standard deviations of six replicates.</p

Dynamic changes in N concentration in ‘Huangguogan’ fruit during fruit development in response to five N fertilizer levels: 0 (N₁), 1.36 (N₂), 1.81 (N₃), 2.26 (N₄), or 2.72 (N₅) kg y^-1.

Error bars indicate the standard deviation of six replicates.</p

Nitrogen nutrition is a key modulator of the sugar and organic acid content in citrus fruit - Fig 4

Dynamic changes in enzymes [sucrose synthase, SS (A, B); acid invertase, AI (C); neutral invertase, NI (D); sucrose phosphate synthase, SPS (E); phosphoenolpyruvate carboxylase, PEPC (F); citrate synthase, CS (G); and aconitase, ACO (H)] involved in glycolysis and in the Krebs cycles during fruit development (days after the first blossom, DAFB) in response to five N fertilizer levels: 0 (N1), 1.36 (N2), 1.81 (N3), 2.26 (N4), or 2.72 (N5) kg y-1. Error bars indicate the standard deviations of three replicates. Fw, fresh weight.</p

Nitrogen nutrition is a key modulator of the sugar and organic acid content in citrus fruit - Fig 5

Response curves of ‘Huangguogan’ fruit N concentrations versus different sugars [fructose (A), glucose (B), sucrose (C)] and vitamin components [ascorbic acid, AsA (D); total AsA, T-AsA (E); dehydroascorbic acid, DHA (F)].</p

Nitrogen nutrition is a key modulator of the sugar and organic acid content in citrus fruit - Fig 2

Dynamic changes in the concentration (mg g-1) of ‘Huangguogan’ fruit sugars (A–C) and VCs (D–F) during fruit development (days after the first blossom, DAFB) in response to five N fertilizer levels: 0 (N1), 1.36 (N2), 1.81 (N3), 2.26 (N4), or 2.72 (N5) kg y-1. Error bars indicate the standard deviations of six replicates.</p

Amount of fertilizer (kg/plant) in each growing period.

Amount of fertilizer (kg/plant) in each growing period.</p