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The denatured states of proteins have always attracted our attention due to the fact that the denatured state is the only experimentally achievable state of a protein, which can be taken as initial reference state for considering the in vitro folding and defining the native protein stability. It is known that heat and guanidinium chloride (GdmCl) give structurally different states of RNase-A, lysozyme, α-chymotrypsinogen A and α-lactalbumin. On the contrary, differential scanning calorimetric (DSC) and isothermal titration calorimetric measurements, reported in the literature, led to the conclusion that heat denatured and GdmCl denatured states are thermodynamically and structurally identical. In order to resolve this controversy, we have measured changes in the far-UV CD (circular dichroism) of these heat-denatured proteins on the addition of different concentrations of GdmCl. The observed sigmoidal curve of each protein was analyzed for Gibbs free energy change in the absence of the denaturant (ΔG 0 X→D) associated with the process heat denatured (X) state ↔ GdmCl denatured (D) state. To confirm that this thermodynamic property represents the property of the protein alone and is not a manifestation of salvation effect, we measured urea-induced denaturation curves of these heat denatured proteins under the same experimental condition in which GdmCl-induced denaturation was carried out. In this paper we report that (a) heat denatured proteins contain secondary structure, and GdmCl (or urea) induces a cooperative transition between X and D states, (b) for each protein at a given pH and temperature, thermodynamic cycle connects quantities, ΔG 0 N→X (native (N) state ↔ X state), ΔG 0 X→D and ΔG 0 N→D (N state ↔ D state), and (c) there is not a good enthalpy difference between X and D states, which is the reason for the absence of endothermic peak in DSC scan for the transition, X state ↔ D state.

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In this study, we report the green synthesis of silver nanoparticles (AgNPs) using the aqueous leaf extract of Tridax procumbens (TNP), which acts as the source of the reducing and capping agent. The distinctive absorption at 370 nm suggested synthesis of TNPs, which was confirmed by TEM, with a size in the range of 11.1 nm to 45.4 nm and a spherical shape, having a face-centered cubic structure, analyzed by XRD, and a Zeta potential of -20.7 mV, which indicated a moderate stability of TNP. The FTIR analysis revealed the presence of amines and hydroxyl groups with fluoro compounds over the TNPs. The HRLC-MS analysis of TNPs suggested the presence of a major capping agent such as fosinopril and reducing agents such as peptides (Gln Gly Ala, Ser Pro Asn, and Leu Met), terpenoids (lupanyl acid, tiamulin), polyphenol (peucenin), and alkaloids (8 ′ ,10 ′ -dihydroxydihydroergotamine, carteolol). The synthesized silver nanoparticles exhibited antimicrobial activity against multidrug-resistant (MDR) clinical isolates (Escherichia coli, Shigella spp., Aeromonas spp., Pseudomonas aeruginosa, and Candida tropicalis) and had anticancer activity against A459 (IC50 42.70 μg/ml). The extraction of partially purified aqueous leaf extracts by silica gel column chromatography followed by HPLC to synthesize silver nanoparticles (TNP11) and analyzed by HRLC-MS suggested that dipeptides were involved in the reduction of Ag+ to Ag0. Overall, the results showed that the green silver nanoparticles of T. procumbens could be safe, as they are endowed with potential antimicrobial activity against MDR clinical isolates and human lung carcinoma cells.