Which of the following is/are true of diastereomers?
Differ at one or more chiral center, but not all
Non-superimposable mirror images
Differ in configuration only at one asymmetric center
Differ only at the penultimate carbon
The Correct Answer is A
Diastereomers are a class of stereoisomers that possess multiple stereocenters but are not related as mirror images. Unlike enantiomers, they exhibit distinct physical properties such as different solubilities and melting points. These molecules occur when at least one, but not all, chiral centers undergo a configuration reversal. In biochemistry, understanding these spatial arrangements is vital for predicting how organic molecules interact with enzymatic active sites or receptor proteins.
Rationale:
A. Diastereomers are defined by having at least two chiral centers where the spatial configuration differs at one or more positions, but specifically not at all positions. If all centers were reversed, the molecules would be enantiomers. This partial variation creates unique molecular geometries and physical characteristics. Because they are not mirror images, they do not share the same internal energy or spatial relationships between non-bonded atoms.
B. Non-superimposable mirror images describe enantiomers, not diastereomers. Enantiomers have identical physical properties in an achiral environment but rotate plane-polarized light in opposite directions. Diastereomers are not mirror images of one another because their stereochemical configurations do not match perfectly across all centers. This distinction is fundamental in organic chemistry for separating mixtures of isomers through fractional crystallization.
C. Molecules that differ in configuration at only one asymmetric center among several are a specific sub-type of diastereomers known as epimers. While all epimers are diastereomers, not all diastereomers are restricted to a single center of variation. This choice is too narrow to define the entire class of diastereoisomeric compounds. True diastereomers can vary at multiple asymmetric carbons simultaneously without becoming full mirror reflections.
D. Differing only at the penultimate carbon is a characteristic used to distinguish between D and L isomers in carbohydrate chemistry. This specific structural variation determines the optical series of a sugar but does not encompass the broad definition of diastereomers. Diastereomeric relationships can occur at any chiral center within a molecule, not just the distal carbon relative to the functional group.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
Step 1. Calculate the total weight of active drug.
From 10 g of 10%: 10 g x 0.10 = 1 g
From 40 g of 5%: 40 g x 0.05 = 2 g
Total drug = 1 g + 2 g = 3 g
Step 2. Calculate the total weight of the final ointment.
Total weight = 10 g + 40 g + 10 g = 60 g
Step 3. Calculate ratio strength (1:X).
3 g / 60 g = 1 / X
X = 60 / 3 = 20
Ratio strength = 1:20
Correct Answer is D
Explanation
Alpha-1 receptors are a subtype of adrenergic receptors that play a vital role in the sympathetic nervous system's "fight or flight" response. They are primarily located on vascular smooth muscle and utilize the G-protein coupled signaling pathway. Activation leads to an increase in intracellular calcium, resulting in muscle contraction. Pharmacological agonists are used to increase blood pressure, while antagonists are used to treat hypertension and prostatic hypertrophy.
Rationale:
A. Alpha-1 receptors are not ligand-gated ionotropic channels; they are G-protein coupled receptors (GPCRs). Ionotropic channels allow for the rapid flow of ions like sodium or chloride directly through a pore. Alpha-1 receptors work through a second messenger system involving phospholipase C and inositol trisphosphate (IP3). This metabolic pathway results in a slower but sustained cellular response.
B. Alpha-1 receptors are adrenergic, meaning they respond to norepinephrine and epinephrine, not acetylcholine. Cholinergic receptors are part of the parasympathetic system and include nicotinic and muscarinic types. Furthermore, activation of alpha-1 receptors causes contraction, not relaxation, of smooth muscle. Activation of certain cholinergic receptors would typically promote secretions or bradycardia instead.
C. While some alpha-1 receptors exist in the central nervous system, they are predominantly located in the peripheral vasculature and smooth muscle organs. Their most significant clinical effects are observed in the systemic arteries, where they regulate total peripheral resistance. They are also found in the radial muscle of the eye and the sphincters of the urinary tract.
D. These are adrenergic receptors and their activation causes constriction is the correct statement. As part of the sympathetic system, they bind to catecholamines to trigger vasoconstriction of the arterioles. This increases blood pressure and diverts flow to essential organs. In the eye, they cause mydriasis (pupillary dilation) by constricting the pupillary dilator muscle, facilitating improved vision in low light.
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