diff --git a/README.md b/README.md
index d3baade..80c00d4 100644
--- a/README.md
+++ b/README.md
@@ -32,13 +32,13 @@ julia> propertynames(chain)
 (:id, :atoms, :sequence, :numbering, :ins_codes, :renumbering)
 ```
 
-To store additional properties, `addproperties` can be used to attach persistent chain-level properties or indexable residue-level properties:
+To store additional properties, `addpropertie!s` can be used to attach persistent chain-level properties or indexable residue-level properties:
 
 ```julia
-julia> new_chain = addproperties(chain; taxid=83332)
+julia> addproperties!(chain; taxid=83332)
 256-residue ProteinChain{Float64} (A)
 
-julia> new_chain = addproperties(new_chain; rand3=IndexableProperty(rand(3,256))) # last dimension matches chain length
+julia> addproperties!(new_chain; rand3=IndexableProperty(rand(3,256))) # last dimension matches chain length
 256-residue ProteinChain{Float64} (A)
 
 julia> new_chain[1:100].rand3
diff --git a/src/ProteinChains.jl b/src/ProteinChains.jl
index 000ff24..fe3e2f7 100644
--- a/src/ProteinChains.jl
+++ b/src/ProteinChains.jl
@@ -10,6 +10,7 @@ export Atom
 
 include("properties.jl")
 export AbstractProperty, StandardProperty, IndexableProperty
+export setproperties!, addproperties!, removeproperties!
 export setproperties, addproperties, removeproperties
 
 include("chain.jl")
diff --git a/src/chain.jl b/src/chain.jl
index 3f13444..01e3d2a 100644
--- a/src/chain.jl
+++ b/src/chain.jl
@@ -2,7 +2,7 @@
     ProteinChain{T<:Real}
 
 Represents a protein chain with a basic set of fields from which some other properties might be derived.
-The [`addproperties`](@ref) function can be used to instantiate new chains with additional properties.
+The [`addproperties!`](@ref) function can be used to add additional properties.
 
 ## Fields
 - `id::String`: Identifier for the protein chain.
@@ -12,10 +12,10 @@ The [`addproperties`](@ref) function can be used to instantiate new chains with
 - `numbering::Vector{Int32}`: Residue numbering (author). See [`renumber`](@ref) for renumbering.
 - `properties::ProteinChains.NamedProperties`: Named properties associated with the chain.
 
-See also [`addproperties`](@ref), [`StandardProperty`](@ref), [`IndexableProperty`](@ref).
+See also [`addproperties!`](@ref), [`StandardProperty`](@ref), [`IndexableProperty`](@ref).
 ```
 """
-struct ProteinChain{T<:Real}
+mutable struct ProteinChain{T<:Real}
     id::String
     atoms::Vector{Vector{Atom{T}}}
     sequence::String
@@ -45,7 +45,7 @@ function ProteinChain{T}(id, atoms, sequence, ins_codes, numbering::Vector{<:Int
 end
 
 function ProteinChain(id, atoms::Vector{Vector{Atom{T}}}, sequence, ins_codes=String(fill(0x40, length(sequence))), numbering=collect(1:length(sequence)), properties=(;)) where T
-    ProteinChain{T}(id, atoms, sequence, ins_codes, Int32.(numbering), namedproperties(properties))
+    ProteinChain{T}(id, atoms, sequence, ins_codes, numbering, properties)
 end
 
 Base.convert(::Type{ProteinChain{T}}, chain::ProteinChain) where T =
@@ -60,7 +60,7 @@ Base.length(chain::ProteinChain) = length(chain.atoms)
 
 function Base.getindex(chain::ProteinChain, i::Union{AbstractVector,Colon})
     properties = map(p -> p[i], chain.properties)
-    ProteinChain(chain.id, chain.atoms[i], chain.sequence[i], chain.numbering[i], properties)
+    ProteinChain(chain.id, chain.atoms[i], chain.sequence[i], chain.ins_codes[i], chain.numbering[i], properties)
 end
 
 Base.getproperty(chain::ProteinChain, name::Symbol) =
@@ -68,12 +68,14 @@ Base.getproperty(chain::ProteinChain, name::Symbol) =
 
 Base.propertynames(chain::ProteinChain, private::Bool=false) = (setdiff(fieldnames(ProteinChain), private ? () : (:properties,))..., propertynames(chain.properties)...)
 
-setproperties(chain::ProteinChain, ps::NamedTuple) =
-    ProteinChain(chain.id, chain.atoms, chain.sequence, chain.ins_codes, chain.numbering, setproperties(chain.properties, ps))
+function setproperties!(chain::ProteinChain, ps::NamedTuple)
+    chain.properties = setproperties(chain.properties, ps)
+    chain
+end
 
 """
-    addproperties(chain::ProteinChain, properties::NamedTuple)
-    addproperties(chain::ProteinChain; properties...)
+    addproperties!(chain::ProteinChain, properties::NamedTuple)
+    addproperties!(chain::ProteinChain; properties...)
 
 Creates a new `ProteinChain` instance with the added properties.
 Indexing of property values can be specified with a wrapper type,
@@ -81,17 +83,17 @@ such as `IndexableProperty`.
 
 See also [`removeproperties`](@ref), [`IndexableProperty`](@ref).
 """
-addproperties(chain::ProteinChain, properties::NamedTuple) = setproperties(chain, addproperties(chain.properties, properties))
-addproperties(chain::ProteinChain; properties...) = setproperties(chain, addproperties(chain.properties, NamedTuple(properties)))
+addproperties!(chain::ProteinChain, properties::NamedTuple) = setproperties!(chain, addproperties(chain.properties, properties))
+addproperties!(chain::ProteinChain; properties...) = setproperties!(chain, addproperties(chain.properties, NamedTuple(properties)))
 
 """
-    removeproperties(chain::ProteinChain, names::Symbol...)
+    removeproperties!(chain::ProteinChain, names::Symbol...)
 
 Creates a new `ProteinChain` instance with the property names in `names` removed.
 
-See also [`addproperties`](@ref)
+See also [`addproperties!`](@ref)
 """
-removeproperties(chain::ProteinChain, names::Symbol...) = setproperties(chain, removeproperties(chain.properties, names...))
+removeproperties!(chain::ProteinChain, names::Symbol...) = setproperties!(chain, removeproperties(chain.properties, names...))
 
 Base.summary(chain::ProteinChain) = "$(length(chain))-residue $(typeof(chain)) ($(chain.id))"
 
diff --git a/src/properties.jl b/src/properties.jl
index 3b1bcb5..f56afaa 100644
--- a/src/properties.jl
+++ b/src/properties.jl
@@ -20,6 +20,14 @@ addproperties(properties::NamedProperties, newproperties::NamedTuple) =
 removeproperties(properties::NamedProperties, names::Symbol...) =
     NamedTuple{filter(name -> name ∉ names, propertynames(properties))}(properties)
 
+function setproperties! end
+function addproperties! end
+function removeproperties! end
+
+setproperties(x, args...) = setproperties!(deepcopy(x), args...)
+addproperties(x, args...; kwargs...) = addproperties!(deepcopy(x), args...; kwargs...)
+removeproperties(x, args...) = removeproperties!(deepcopy(x), args...)
+
 checkproperty(::Any, ::AbstractProperty) = nothing
 
 unpack(x) = x
@@ -32,7 +40,7 @@ unpack(p::AbstractProperty) = p.value
 
 A property with arbitrary type. The value is retained as is.
 
-This is the default property type for [`addproperties`](@ref).
+This is the default property type for [`addproperties!`](@ref).
 
 See also [`IndexableProperty`](@ref).
 """
@@ -53,7 +61,7 @@ residue indexing of the chain being propagated to the last dimension of the arra
 ```jldoctest
 julia> chain = pdb"1ASS"A;
 
-julia> chain = addproperties(pdb"1ASS"A; y=IndexableProperty(rand(2,152)));
+julia> addproperties!(pdb"1ASS"A; y=IndexableProperty(rand(2,152)));
 
 julia> chain.y == chain[1:10].y
 false
diff --git a/src/structure.jl b/src/structure.jl
index 808da5b..c408583 100644
--- a/src/structure.jl
+++ b/src/structure.jl
@@ -7,7 +7,7 @@
 - `chains::Vector{ProteinChain{T}}`: a collection of `ProteinChain`s.
 - `properties::NamedProperties`: arbitrary properties.
 """
-struct ProteinStructure{T} <: AbstractVector{ProteinChain{T}}
+mutable struct ProteinStructure{T} <: AbstractVector{ProteinChain{T}}
     name::String
     atoms::Vector{Atom{T}}
     chains::Vector{ProteinChain{T}}
@@ -30,7 +30,11 @@ chainid_to_index(structure::ProteinStructure, id::AbstractString) = findfirst(c
 
 Base.getindex(structure::ProteinStructure, i::Integer) = structure.chains[i]
 Base.getindex(structure::ProteinStructure, id::AbstractString) = structure[chainid_to_index(structure, id)]
-Base.getindex(structure::ProteinStructure, is::AbstractVector) = ProteinStructure(structure.name, structure.atoms, map(i -> structure[i], is))
+
+function Base.getindex(structure::ProteinStructure, inds::AbstractVector{<:Integer})
+    properties = map(p -> p[inds], structure.properties)
+    return ProteinStructure(structure.name, structure.atoms, map(i -> structure[i], inds), properties)
+end
 
 Base.setindex!(structure::ProteinStructure, chain::ProteinChain, i::Integer) = (structure.chains[i] = chain)
 Base.setindex!(structure::ProteinStructure, chain::ProteinChain, id::AbstractString) = (structure.chains[chainid_to_index(structure, id)] = chain)
@@ -68,14 +72,16 @@ Base.getproperty(structure::ProteinStructure, name::Symbol) =
 
 Base.propertynames(structure::ProteinStructure, private::Bool=false) = (setdiff(fieldnames(ProteinStructure), private ? () : (:properties,))..., propertynames(structure.properties)...)
 
-setproperties(structure::ProteinStructure, properties::NamedTuple) =
-    ProteinStructure(structure.name, structure.atoms, structure.chains, setproperties(structure.properties, properties))
+function setproperties!(structure::ProteinStructure, properties::NamedTuple)
+    structure.properties = setproperties(structure.properties, properties)
+    structure
+end
 
-addproperties(structure::ProteinStructure, properties::NamedTuple) =
-    setproperties(structure, addproperties(structure.properties, properties))
+addproperties!(structure::ProteinStructure, properties::NamedTuple) =
+    setproperties!(structure, addproperties(structure.properties, properties))
 
-addproperties(structure::ProteinStructure; properties...) =
-    setproperties(structure, addproperties(structure.properties, NamedTuple(properties)))
+addproperties!(structure::ProteinStructure; properties...) =
+    setproperties!(structure, addproperties(structure.properties, NamedTuple(properties)))
 
-removeproperties(structure::ProteinStructure, names::Symbol...) =
-    setproperties(structure, removeproperties(structure.properties, names...))
+removeproperties!(structure::ProteinStructure, names::Symbol...) =
+    setproperties!(structure, removeproperties(structure.properties, names...))
diff --git a/test/runtests.jl b/test/runtests.jl
index 9cc860c..e92a734 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -3,12 +3,6 @@ using Test
 
 @testset "ProteinChains.jl" begin
 
-    @testset "ideal.jl" begin
-        geometry = BackboneGeometry(N_Ca_length=3, Ca_C_length=3, N_Ca_C_angle=π/2)
-        ideal_residue = IdealResidue{Float64}(geometry)
-        @test ideal_residue == Float64[-2 1 1; -1 -1 2; 0 0 0]
-    end
-
     @testset "atom.jl" begin
 
         @testset "atom name" begin